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THE OKONJEJE IGNEOUS COMPLEX, SOUTH-WEST AFRICA

By E. S. W. Simpson

(PLATES XXIV-XXIX)

ABSTRACT

The Okonjeje complex is one of a series of related Post-Karroo intrusives in theDamaraland region of South West Africa. Exposed over a roughly circular area of about20 square kilometres, the igneous rocks of the complex can be clearly differentiated into(a) an earlier tholeiitic series showing continuous mineral and chemical variation fromolivine gabbro through ferrogabbros to rocks of acid composition, followed by (b) a seriesof alkali rocks ranging from olivine gabbro and essexites to pulaskite, foyaites andtinguaites with an associated radial dyke suite of alkali lamprophyres.

Both series are considered to have been derived from a common parent basic magmaby processes of fractional crystallization involving dominant fractionation of mafic andfelsic constituents respectively. Structures are of the ring type, together with evidencefor faulting and cauldron subsidences accompanied by magmatic injection.

CONTENTS

I. INTRODUOTION 126II. PHYSIOAL FEATURES 127

III. OOUNTRY ROOKS AND THERMAL METAMORPHISM 128(a) DAMARA SYSTEM 128(b) KARROO SYSTEM 128

IV. THE THOLEIITIO SERIES 130(a) FIELD RELATIONS AND PETROGRAPHY 130

(i) Marginal Gabbro-picrite 130(ii) Differentiated Group 131

(iii) Ridge Syenite 135(iv) Marginal Acid Rocks .. 136(v) Granulitic Gabbros 138

(b) NOTES ON MINERALOGY 140(c) STRUCTURE 140(d) PETROLOGY 142

V. THE ALKALI SERIES 146(a) OORE GABBROS 146(b) THE ALKALI ROCKS OF OKONJEJE BERG 147

(i) Pulaskite 148(ii) Andesine-essexite 148

(iii) Oligoclase-essexite 150(c) WITRAND FOYAITES 151(d) MINOR INTRUSIVES 153

(i) Sodalite tingu~ite 153(ii) Bostonite 155

(iii) Oamptonite 155(iv) Alnoite .. 157(v) Melanephelinite 157

(vi) Nepheline monchiquite 157(vii) Pipe breccia 158

(e) RELATIONSHIPS OF THE ALKALI ROCKS 159(i) Order of intrusion 159

(ii) Mineralogy .. 159(iii) Ohemical variation 161(iv) Petrogenesis 161

VI. OONOLUSION 163

VII. REFERENOES 164

I

126 TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA

I. INTRODUCTION

The complex group of igneous rocks at Okonjeje Berg was first recognisedas such by Drs. H. Korn and H. Martin shortly after their discovery of theMessum complex in 1938. In a short account of their field observations (1939,p. 636) they briefly recorded the occurrence of gabbro and syenites on the mainpeak, and indicated the post-Karroo age of these intrusives, thereby relating thecomplex to the other intrusive bodies of similar age in northern Damaraland(Fig. 1). More recently (1953, 1954) Korn and Martin have published theresults of further work on some of these " vulkano-plutone."

IS°E:

°Fransfol1teln

16°£ ...,.

•OKORU5U

10DI

2tfS

FIG. l.Sketch map showing the distribution of post-Karroo igneous complexes (solid black) andKarroo sediments and lavas (stippled) in northern Damaraland, South West Africa.

(Modified after Martin.)

A. more detailed examination of the Okonjeje area carried out by the authorin November-December, 1949, revealed that the intrusive rocks are exposedover a wider area and constitute a more varied assemblage than indicated byKorn and Martin, and a brief description of the principal rock types waspublished shortly after completion of the field work (Simpson, 1950).

The greater part of the laboratory work was carried out in the Department of:Mineralogy and Petrology, Cambridge, under the guidance of Professors C. E.Tilley and W. .A. Deer, and Dr. S. R. Nockolds, to whom the author is greatlyindebted for advice and continued interest during the progress of this work..A further brief visit to the area was made in January, 1953, and the laboratory

THE OKONJEJE IGNEOUS COMPLEX 127

work completed at the University of Oape Town. The author extends his thanksto Professor F. Walker and Dr. M. :Mathias of the University of Oape Town,and Dr. H. Martin of the Geological Survey in Windhoek, for their unfailinghelp and interest; to Messrs. J. A. Mabbutt, IJ. O. Nicolaysen, Z. 1. Dyjas,R. G. Horne and G. Siedner for assistance in the field, and to Mr. and :1\:1rs. T. K.Adams of Omatjette, for their ready kindness and hospitality. Grants madeby the Staff Research Fund, University of Oape Town, to cover travellingexpenses, and by the O.S.1.R. towards the cost of publication are gratefullyacknowledged.

Laboratory JYIethods.Except where otherwise stated, refractive indices were determined with an

accuracy of ± 0·002 and both optic axial angles and extinction angles to ± 2°.Oompositions of plagioclase felspars were determined for the most part by

the Fedorow method (Turner, 1947; Nikitin, 1936), while the optical data forolivines, orthopyroxenes and clinopyroxenes were plotted on the respectivecurves published by Deer and Wager (1939,p. 21), Poldervaart (1947, p. 167)and Hess (1949).

Ohemical analyses of rocks and minerals were made by the author, whilespectrographic determinations of minor constituents were very kindly carriedout by Dr. S. R. Nockolds and Mr. R. S. Allen in the Department of Mineralogyand Petrology, Oambridge.

II. PHYSICAL FEATURES

The Okonjeje complex embraces an area of high relief dominated by theconspicuous Okonjeje Berg (20° 50' S., 15° 20' E.) which lies near the centre ofthe Otjohorongo Native Reserve and about 93 kilometres north-west of the townof Omaruru. The new main road between Omaruru and Fransfontein passeswithin 15 kilometres of Okonjeje at the village of Ozondati, from which apassable track leads to the small Herero settlement (Outjivero) within thecomplex.

The intrusive rocks are exposed over a roughly circular area of about 20square kilometres and are topographically exposed as a group of inselbergswhich rise abruptly from the regional pre-Karroo erosion surface cut acrossDamara rocks and associated granites of pre-Cambrian age. Lying to the southand separated from it by a narrow valley, is the flat-topped Otjongundu plateau,an outlier of coarse horizontally bedded Karroo sediments.

The lofty Okonjeje Berg (1902 metres) lies just within the northern marginof the complex, which is bounded on the south by the elongated Outjivero(1,430 metres). On the eastern margin rises the smooth profile of vVhite Ridge(1,150 metres) from the foot of which local base level drops gradually from1,060 metres to the 1,000 metre level of the regional plain on t:te western side.vVithin this area arcuate gabbro ridges and conical hills with steep talus slopesrise abruptly from the valley floors (Plate XXIV).

Under the prevailing semi-arid climate, mechanical weathering predominates with the production of steep slopes of actively cleaving and exfoliatingigneous rock providing extensive screes which tend to obliterate exposures ofbedrock. The general ruggedness of relief is accentuated by deep incision of

128 TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOU'l'H AFI~ICA

talus slopes by intermittent torrents during localized heavy thunderstorms inlate summer months (January to Thlarch). In valley floors conditions of aggradation and unconfined flow have caused the deposition of broad veneers of silt.Drainage is locally radial, but is consolidated into two main units within thecomplex, both of which have fairly wide channels heavily choked with alluviumand rock debris, and are directed westwards towards the Ugab river about 25kilometres distant.

The sparse vegetation comprises mainly mopane trees and grass on alluvialfloors, and various species of succulents and low scrub on talus slopes.

III. COUNTRY ROCKS AND THERMAL METAMORPHISM

(a) Damara System

Intensely folded sediments of the Damara system and associated syntectonic granites of pre-Cambrian age (Gevers and Frommurze, 1929) build theeroded basement upon which sediments of Karroo age were deposited and intowhich the Okonjeje intrusives were emplaced.

The Damara sediments adjacent to the complex are chiefly yellowweathering dark laminated shales and slates with minor interbedded palebrown felspathic quartzites, probably belonging to the Khoma,s series. Thesediments are isoclinally folded with local contortion of the strata and generallydip at high angles to the east-south-east.

Apart from induration near the contact, no conspicuous metamorphicaureole has been developed in these sediments which usually show only theeffects of low-grade regional metamorphism. In the argillaceous types, smallamounts of andalusite, cordierite, biotite and reconstituted felspar areoccasionally developed near the contact, and quartz veins are conspicuous inthe felspathic quartzites on White Ridge near outcrops of the marginal acid rock.A moderate grade of thermal metamorphism is shown only by dark flintyhypersthene-biotite-orthoclase-cordierite hornfelses in contact with marginalacid rock along the western margin of the complex, particularly to the north ofZebra Kop.

Just north of Black Ridge an extensive outcrop of mottled white andpink quartz-porphyry is traversed by a fine network of quartz-orthoclase veinswhich doubtless represent residual liquids injected during a late stage in the.crystallization of the adjacent marginal acid rocks.

(b) Karroo System

The coarse terrestrial sediments of Stormberg (late Triassic) age whichbuild the highly dissected Otjongundu plateau (Gevers and Frommurze, 1929,p. 46) are structurally continuous with the metamorphosed remnants nowexposed around the margin of the Okonjeje complex (Plate XXIX).

The sediments are more or less horizontally bedded and rest with pronouncedunconformity upon a broadly uneven floor which slopes gently to the south-westand is slightly elevated above the regional plain cut across Salem granite and


folded Damara rocks. The maximum thickness of the Karroo succession isapproximately 150 metres in the south-west, decreasing to about 30 metres onthe eastern side of the plateau. The area covered by this outlier is about 200square kilometres.

Coarse conglomerates at the base are succeeded by coarse arkoses togetherwith ferruginous felspathic grits and sandstones alternating with subordinatelayers of reddish arenaceous shales and mudstones. Rapid facies changes,local disconformities and current bedding with poor sorting point to depositionunder torrential conditions in a semi-arid climate. Granite detritus makes up thebulk of the sediments, but in addition boulders of quartzite, schist and quartztourmaline granulite were found in the conglomerates.

The plateau is capped by a series of transgressive sills of pale pink-brownporphyritic granophyre which produce conspicuous ledges in the profile andvary in thickness from 3 to 10 metres. Vertical dykes of the same rock wereobserved traversing both Damara and Karroo sediments at the northern endof the plateau, and doubtless represent feeders. The granophyres are composedof large flesh-coloured phenocrysts of clouded perthitic orthoclase with occasionalcores of acid plagioclase, set in a micrographic intergrowth of quartz and alkalifelspar, together with skeletal iron ore and a little interstitial pyrophyllite andfluorite, the latter also occurring in small drusy cavities.

Indurated and thermally metamorphosed equivalents of the Otjongunduplateau sediments and granophyres are preserved at several localities along thesouthern, eastern and northern margins of the complex, and a further smalloutcrop was found at the summit of Johannes Berg. East of the wells is anextensive stony outcrop of impure quartzite about 150 metres across, completelysurrounded by ferrogabbros. This body is tentatively correlated with the Karroosediments and may represent a downstoped block from the roof of the intrusivebody.

The marginal Karroo sediments rest unconformably upon Damara rocksand everywhere dip outwards from the complex at 10°-15°. Exposures over avertical distance of about 150 metres are very good on the steep northern face ofOutjivero, at the western end of which the sediments are traversed by a broadeast-west zone of almost vertical breccia-filled fractures.

The effects of thermal metamorphism upon the Karroo sediments are verymuch more pronounced than in the Damara rocks. Particularly near the contact,all have been converted to dark flinty rocks in which rounded fragments ofclastic quartz and felspar are still conspicuous in the case of the coarser arkosicgrits, but these are now set in a recrystallized matrix containing quartz, felsparand cordierite, together with andalusite, occasional biotite and dusty aggregatesof magnetite. Near the intrusive contact andalusite is accompanied by fibresand sheaflike aggregates of sillimanite and occasional small crystals of rutileand blue tourmaline. The metamorphosed granophyres are much darkened incolour through the development of magnetite dust in the groundmass, and grainsof primary iron ore are surrounded by mantles of sphene.

High-grade thermal metamorphic effects are shown by the Karroo sedimentsat the summit of Johannes Berg in the development of dense hornfelses in which


granophyric intergrowths of quartz and felspar are found. Clastic quartz fragnlents are here surrounded or replaced by zones of micropegmatite whereenclosed by felspar. Hawkes (1929) ascribes similar features observed in quartzfelspar rock at Kvosafoss, south-east Iceland, to partial fusion.

IV. THE THOLEIITIC SERIES

The intrusive sequence of the Okonjeje complex began with a series ofrelated rocks characterized by the presence of orthopyroxene and the appearanceof quartz in the n10re acid members. These rocks form a broad interruptedring around the central mass of core gabbro and are exposed chiefly in thesouthern region of the complex. Although not true tholeiites, these rocks havetholeiitic affinities in that they carry orthopyroxene, show iron enrichment andhave produced quartz-bearing late differentiates. The term Tholeiitic series isused both to indicate these affinities and to distinguish these rocks from thelater Alkali series.

(a) Field Relations and Petrography

Five groups can be clearly recognized in the field :(i) Marginal gabbro-picrite.

(ii) Differentiated group.(iii) Ridge syenite.(iv) Marginal acid and hybrid rocks.(v) Granulitic gabbros.

(i) Marginal Gabbro-Picrite.

The gabbro-picrite occurs as a discontinuous dykelike body, mainly alongthe eastern margin of the complex, and forms conspicuous narrow ridges of darkboulders which contrast with the pale colour of adjacent rocks (Plate XXV,Fig. 1). The dyke varies in width between 10 and 25 metres and dips at angles of70°_80° to the east, or outwards from the complex. With several interruptionsthe outcrop strikes south-south-east over Zbigniew and vVhite Ridges, while theBlack Ridge continuation strikes south-south-west and is clearly offset to thewest of the main exposed trend. A further small outcrop, conspicuouslybrecciated by the intrusive marginal acid rock, was found at the north-westernfoot of Outjivero near the Kazombaruru river. Slightly to the west of ZebraKop the marginal acid rock s carry large xenocrysts of clinopyroxene enclosingmagnesian olivine which were probably derived from the intrusive disintegrationof gabbro-picrite, and point indirectly to the sporadic development of theserocks around the greater part of the complex.

The gabbro-picrites are dark massive r:ocks of medium grain size, becomingsomewhat finer in grain towards the margins of the dyke. The analyzed specimen(Ok. 110), collected from the Black Ridge outcrop, contains 46 per cent. ofolivine accompanied by two pyroxenes, basic plagioclase and iron ore, withoccasional flakes of biotite. The olivine occurs as irregular or subidiomorphiccrystals which may exceed 2 mm. in size, characteristieally showing prominentfractures and trails of minute black inclusions. The crystals are colourless orfinely dusted with iron ore and the composition averages Fa22 tbough some are

THE OKON.JE.JE IGNEOUS COMPLEX 131

as magnesian as Fa17. Olinopyroxene builds almost colourless subophiticcrystals moulded on olivine and has a composition approximating toOa4ol\lg47Fe13. Subordinate orthopyroxene (Of22 ) is generally subophitic butlater than clinopyroxene. Small tabular crystals and stumpy laths of basicplagioclase often show faint metamorphic clouding (MacGregor, 1931) and areinvariably strongly zoned from .An 73 to .An54. .A small amount of orange-brownbiotite is associated with the iron ore, most of which crystallized late as prominentinterstitial crystals.

Most of the gabbro-picrite outcrops are traversed by narrow veins from theadjacent marginal acid rocks or pulaskite, which have given rise to localserpentinization of olivine, slight granulation of pyroxene, sericitization ofplagioclase and the development of biotite aggregates. These effects are veryprominent in specimens collected from the intrusion breccia on the north-westfoot of Outjivero.

(ii) Differentioted Group.

The dark gabbroic rocks of the differentiated group are exposed chieflyin the eastern and southern areas of the complex where they form a broad outcropwhich partly surrounds the central mass of core gabbro and extends almostbut not quite to the outer margin. When traced outwards from the inner marginthe rocks of this group show continuous variation in the compositions of themajor constituent minerals which corresponds to the cryptic layering of theSkaergaard intrusion (V\rager and Deer, 1939, p. 64), and is represented diagrammatically in Figs. 2 and 3.

The inner and more basic rocks of the group haye been termed the ridgegabbros, since they are topographically expressed as a series of arcuate ridgesand bouldery kopjes which are most conspicuous between the wells and Stanley(Plate XXV, Fig. 2). The contrast in appearance between the very dark, massiveridge gabbros and the coarse, felspathic core gabbros pro"vided the main criterionin mapping the inner margin of the differentiated group which was traced withsome difficulty parallel to the trend of the arcuate ridges. Distinct ridges ofridge gabbros again appear on the south-eastern shoulder of Johannes Berg andcontinue intermittently with arcuate strike to Swartrand where they disappearbeneath alluvium and talus. Field relations on Johannes Berg are highly obscuresince the gabbros are exposed mainly as dark bouldery patches heavily veinedand surrounded by marginal acid rock, and outcrops shown on the map(Plate XXIX) are diagrammatic only. In the eastern type area the form of thegabbro ridges clearly suggests a pseudostratification of the ridge gabbros withgeneral inward inclination estimated at 15°-30°. On the south-eastern shoulderof Johannes Berg the inward inclination is considerably steeper (Plate XXIV)and approaches 45°. The frequently observed tendency toward parallel orientation of tabular felspars in these rocks conforms with the inferred dip of pseudostratification.

The rocks which succeed the ridge gabbros outwards have been namedferrogabbros on the basis of criteria established by Wager and Deer (1939,pp. 9R-99), which apply to all these rocks except the outermost types in thedifferentiated group. North of Korn the ferrogabbro zone is thin or absent andoutcrops in the eastern alluvial flats are poor; the best exposures of ferrogabbro

TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA132

900 0

a:: III

cD..JC

800 cD c

• ~

'-'700 0

a: a:

a: III

z600 z

500

0

400 II:

cD a:cD III

300 • I-

'-' ::>

0

200'-'0

100~a:~

E 22

5 10

ORTHOPYROXENE

10 20 30

5 10

ALK. FELSPAR QUARTZ BIOTITE

FIG. 2.Mineral compositions and modal variation in rocks of the differentiated group, plotted

against distance from the inner margin.

En Fs

Fe Fd\\

\\

An Ab10 20 30 40 SO 60 70 80 90

FIG. 3.Compositions of co-existing phases of the clinopyroxene, orthopyroxene, olivine andplagioclase mineral series in rocks of the differentiated group. Corresponding data for'

the ridge syenite (66, broken joins) included for comparison.


occur to the north and west of Black Ridge, while scattered boulders were foundlow down on the south-western slopes of Johannes Berg and among screes east ofZebra Kop. The inner ferrogabbros closely resemble the dark ridge gabbros inhand specimen, but to"\yards the outer margin they show a variable increase ingrain size, together with the development of increasing proportions of a dull-greenopalescent alkali felspar. Some outer ferrogabbros are megascopically similarto the coarse, dull-green ridge syenite. At a short but variable distance from theeastern and southern margins of the complex the outer ferrogabbros pass bygradation into the marginal acid rock which carries numerous small xenoliths offerrogabbro. The outer margin of the differentiated group as shown on the map istherefore somewhat arbitrary and is based mainly on petrographic evidence inconjunction with field observations. Parallel orientation of tabular felspars isoften evident in all but the outer ferrogabbros, causing some exposures to have asheeted appearance which roughly conforms with the inward inclination of theridge gabbro pseudostratification.

Chemical analyses have been made of four representative specimens(7, 1, 10(~, 109) collected on a traverse across the strike of the differentiated group,and are presented together with spectrographic, normative and modal data inTable 1. The following brief descriptive notes are supplemented by the datagiven in Figs. 2 and 3, and in the section on mineralogy.

All the rocks of the ridge gabbro zone are normal olivine gabbros in whichthe major constituents are zoned plagioclase, olivine, clinopyroxene and subordinate orthopyroxene. Biotite is invariably present as reddish-brown flakesusually associated with patches of late iron ore. Apatite is a minor accessory,together with occasional small grains of pyrrhotite and chalcopyrite, and a littleinterstitial orthoclase. The very dark aspect of these rocks in hand specimen isdue to strong metamorphic clouding in the abundant plagioclase, for which thelater emplacement of the core gabbro is undoubtedly responsible.

The inner ridge gabbros carry nearly 70 per cent. of zoned plagioclase andup to 15 per cent. of magnesian olivine. Clinopyroxene, which is often denselyschillerized, is sporadically developed as large ophitic crystals up to 5 mm. insize, so that in some thin sections it may predominate over olivine, while inothers it is often absent. Orthopyroxene forms narrow reaction rims aroundmany olivine crystals, and sometimes enters into vermicular intergrowths withiron ore which clearly replace large areas of olivine (Plate XXVI, Fig. 2). Thesetroctolitic and coarsely ophitic inner ridge gabbros are restricted to a narrowinner marginal sub-zone of the differentiated group, apparently not exceeding50-100 metres in width.

More extensively developed and characteristic of the zone are the outerridge gabbros which rarely exhibit ophitic texture and are more even-grainedthan the rocks of the inner sub-zone. Clinopyroxene crystals have a tendencytowards a prismatic habit and are often heavily schillerized or are crowded withsmall inclusions of olivine, iron ore, biotite 01' plagioclase. Orthopyroxeneoccurs both as rims around olivine when the vermicular intergrowth with ironore is fairly common, and as independent columnar crystals moulded on plagioclase and clinopyroxene. Biotite is a major constituent and forms large Bakf'swhich are conspicuous even in hand specimen.


An arbitrary subdivision of the ferrogabbro zone into three sub-zones hasbeen made to facilitate reference, the middle ferrogabbros being those whichstraddle the break in crystallization of olivine. The rocks of the outer sub-zonedo not conform with the definition of ferrogabbro as given by Wager and Deer(Zoe. cit.), but the term is retained in order to emphasize the close relationshipwith the other rocks of the differentiated group.

The most characteristic feature of the ferrogabbros is the increasing tenorof ferrous iron in the ferromagnesian minerals, leading to a geochemical culmination of iron in the analyzed middle ferrogabbro (Ok. 106). This is accompaniedby progressive enrichment in alkalis and silica which becomes most pronouncedin the outer ferrogabbros, i.e. after the stage of maximum iron enrichment.

On account of the persistence of metamorphic clouding in plagioclase, theinner and middle ferrogabbros are very similar in megascopic appearance, thoughsomewhat finer in grain than the outer ridge gabbros, and nlost specimens show amarked parallel orientation of tabular plagioclase crystals. Olivine is absentfrom several specimens of the middle ferrogabbros, but when present it clearlycrystallized early as pale yellow, irregular or rounded crystals usually associatedwith orthopyroxene, though showing a distinct tendency toward independentdevelopment after the break in crystallization. Orthopyroxene builds largecolumnar crystals which frequently enclose rounded, optically continuousfragments of olivine and irregular grains of iron ore (Plate XXVI, Fig. 3). Inthe olivine-free middle ferrogabbros the orthopyroxene crystals invariablyenclose numerous irregular granules of iron ore, the latter very occasionallyforming vermicular intergrowths with the host mineral. By analogy with similarstructures in the ridge gabbros this suggests that primary olivine did in factcrystallize but was subsequently replaced by orthopyroxene accompanied by thesimultaneous ex-solution of iron ore. In this connection it is noteworthy thatorthopyroxene reaches its greatest proportion of about 10 per cent. by weightin the olivine-free middle ferrogabbros. Parallelism of clouded plagioclase lathsis evident in suitably oriented thin sections and some laths show slight bendingand wavy extinction. Independent orthoclase increases gradually in amountand invariably occurs as clear interstitial patches, against which plagioclase lathsshow slight corrosion and embayment. Quartz makes its first appearance in theolivine-free middle ferrogabbros in small interstitial amounts, increasing progressively in amount thereafter. Primary iron ore is more abundant than in theridge gabbros, reaching a maximum in the iron-rich nliddle ferrogabbros. Aconsiderable proportion of it appears to have crystallized early, together withsmall scattered grains of yellow sulphides. Apatite is a prominent accessory,as stout idiomorphic prisms, and together with reddish-brown biotite, reachesits maximum proportion in the iron-rich middle ferrogabbros.

The outer ferrogabbros show considerable variation in megascopic appearance. Some are dark and relatively fine-grained, closely resembling the middleferrogabbros, while others are medium to coarse-grained rocks in which crystalsof dull-green opalescent alkali felspar are easily recognized. Plagioclase is stillthe most abundant constituent, forming corroded laths and tabular crystalspoikilitically enclosed by extensive patches of potash felspar. The latter oftenshows fine parallel striae near cry stal margins which are resolved under highmagnifications into extremely narrow perthitic lamellae of twinned plagioclase.


Myrmekitic intergrowths of qllartz and plagioclase seen in some sections projectinto potash felspar and appear to replace it. A dactylitic intergrowth of the twofelspars, which is very similar in appearance to myrmekite, also occurs withpatchy distribution within areas of potash felspar. The plagioclase componentsometimes shows fine twin-lamellae under high magnification, while the potashfelspar component may in addition show fine parallel striae which appear to beexsolution lamellae of plagioclase. In one case only the dactylitic intergrowthwas found to surround a small remnant of plagioclase, and it seems probablethat the intergrowth arises from incomplete corrosion of early formed plagioclase by a late magmatic fluid from which potash felspar was crystallizing (cf.Oftedahl, 1948, p. 30).

Green hornblende is usually the most abundant ferromagnesian mineralin the outer ferrogabbros, and builds irregular or prismatic crystals enclosingrounded cores of pale pink-green clinopyroxene. The optical properties of thehornblende are 2Vex = 67°, y = 1,696, y: c = 17°; pleochroism, ex = yellowgreen, (3 = olive-green, y = deep brown-green. The small amount of pleochroicorthopyroxene present is sometimes associated with crystals of olivine, but alsoforms independent crystals. Olivine is distinctly yellowish and occurs as subidiomorphic independent crystals of early crystallization. Interstitial quartzis a major constituent and shows slight undulatory extinction. Small amounts ofiron ore are accompanied by yellow sulphide ores and a little brownish-redbiotite. Apatite forms numerous idiomorphic and skeletal prisms. Idiomorphicprisms and small radiating tufts of blue pleochroic tourmaline have been observedin some thin sections. Interstitial micropegmatite is present in one specimenonly.

(iii) Ridge Syenite.

Typically exposed on the northern slopes of Outjivero, the ridge syeniteoutcrops as an arcuate dyke, concave to the north and some 40 metres wide.The outcrop ends abruptly in broad wall-like features at the two northernextremities where the vertical attitude of the dyke· is clearly shown, and belowwhich only loose boulders of ridge syenite extend down to the· Kazombarururiver.

The ridge syenite has a characteristic and handsome dull-green appearance,being composed essentially of large glistening crystals of alkali felspar togetherwith sparsely distributed spots of ferromagnesian minerals. Along the northern(concave) margin against core gabbro the normally massive ridge syenitedevelops a coarse foliation due to a parallel orientation of tabular felsparcrystals, which suggests that the ridge syenite was injected shortly after theemplacement of the core gabbros, though the actual contact was not observed.Along the southern or convex margin, however, the ridge syenite is chilledagainst ferrogabbros, while at the highest and most southerly point to which theoutcrop extends, it is chilled against Karroo sediments. Although both the outerferrogabbros and marginal acid rock extend upward and outward beneath theKarroo cover in this area (Plate XXIX), the upward extent of the ridge syenitedyke before erosion can no longer be established.

In addition to this type occurrence, outcrops of ridge syenite, gradingimperceptibly into the surrounding marginal acid rock, were found at several


localities on Johannes Berg and near Zebra Kop. The field evidence suggeststhat the two rock types are closely connected, the peculiar characteristics of themarginal acid rock being due to contamination.

A specimen of the typical coarse-grained ridge syenite (Ok. 66) has beenanalysed. Large (3 mm.) irregular crystals of very pale green schillerized clinopyroxene make up nearly 10 per cent. by weight and show slight alteration tobrowniSh-green hornblende (2Va = 63°, y = 1,703). The pyroxene wasseparated for analysis (Table III) and found to lie in the ferroaugite field ofcomposition. Fayalitic olivine (Fa S7 ) also builds large rounded crystals sparselyscattered through the rock, and is usually altered along fractures to greenishbrown serpentinous material. Hare orthopyroxene (Of65 ) is associated withiron ore and always occurs adjacent to olivine crystals. Small amounts of deepbrown biotite are associated with iron ore which occurs in accessory amountstogether with idiomorphic prisms of apatite and an occasional crystal of zircon.Very coarse intergrowths of orthoclase microperthite and sodic plagioclase occuras broad tabular crystals and make up 80 per cent. by weight of the rock. Thetwinned plagioclase of the coarse intergrowth is rarely zoned and has the averagecomposition An s, varying between An15 and An4 • The orthoclase microperthitecomponent is often optically continuous over areas up to 4 mm. across, but themargins are highly irregular and enclose small patches belonging to adjacentcrystals. Small petaloid bodies of myrmekite and the dactylitic two-felsparintergrowth are abundant and can be distinguished only with difficulty.Occasional independent crystals of twinned oligoclase are variable in compositionand often as calcic as An25 , but show only very slight zoning. Small amounts ofquartz occur as irregular interstitial patches.

(iv) Margina,z.A cid Rock.

Xenolithic and contaminated rocks of acid composition form a narrow outcrop of variable width up to 100 metres between the outer ferrogabbros andcountry rock around the eastern and southern margins of the complex. In thewestern part of the complex the outcrop is very much more extensive, occupyingthe greater part of Johannes Berg and extending to the north of Zebra Kop andAdams Shoulder, though in the latter area field relations are much obscuredby screes and are undoubtedly more complex than has been indicated on the map.Dark hybrid rocks of intermediate composition and doubtful parentage arecommonly observed here and on the north-eastern slopes of Johannes Berg.

The acid rocks are intrusive into core gabbros along an almost straightcontact on the western slopes of :Martin and into the Karroo sediments at thesummit of Johannes Berg, with much veining of the intruded rocks. On thewestern slopes of Johannes Berg extensive bouldery outcrops and patches ofdark rocks of the differentiated group apparently in situ are conspicuous amongthe intrusive lighter acid rocks.

In hand specimen the acid rocks are grey or greenish rocks of medium tofine grain, characteristically showing dark ferromagnesian clots and roundeddark xenoliths representing all degrees of incorporation of foreign material pickedup during emplacement. Sedimentary xenoliths are less often seen, probablybecause of the greater ease with which they were assimilated. The texture ofmost of the acid rocks is granitic or microgranitic which immediatel;y distinguishes


them in thin section from some outer ferrogabbros in which the same mineralphases are present. However, considerable areas of the western outcrop compriserocks which are macroscopically and microscopically identical with, or at leastrelated to the ridge syenite. These are not normally xenolithic and gradeimperceptibly into the adjacent contaminated acid rocks.

The analyzed specimen (Ok. 185), collected from Zebra Kop, is fairly typicalof these heterogeneous rocks and is free from recognizable xenoliths apart fromthe characteristic ferromagnesian clots. The normal plagioclase of the rockoccurs as clear laths and tablets zoned outwards from An20 to AnIO- 5 and usuallysurrounded by broad rims of clouded microperthite. Large subidiomorphicxenocrysts of more basic plagioclase (An25- 34 ) make up about 7 per cent. of therock by weight; they show faint metamorphic clouding and are surrounded bysecondary mantles of more sodic plagioclase. Orthoclase microperthite isslightly in excess of the normal plagioclase of the rock and occurs as mantlesaround plagioclase and clouded allotriomorphic crystals. Small lobate patchesof myrmekite are commonly observed. Subidiomorphic crystals of green hornblende (analyzed-Table III) often show simple twinning on (100) and arescattered through the rock or gathered together in glomeroporphyritic clots upto 3 mm. in size. The larger crystals surround cores of pale pink sehillerizedferroaugite. Quartz is interstitial, and small amounts of pale brown biotite andgranules of iron ore are commonly associated with the hornblende clots.Accessory minerals are prisms and needles of apatite, and occasional idiomorphiccrystals of zircon.

Other specimens of the marginal acid rocks vary according to the mineralspresent and in their relative proportions. In addition to the above minerals,many carry small crystals of fayalitic olivine (Fa S7 ) or iddingsite, and iron-richorthopyroxene, while some show small radiating tufts of blue tourmaline andoccasional patches of micropegmatite. All these rocks show the dactylitic intergrowths between potash felspar and plagioclase found in the outer ferrogabbrosand ridge syenites.

Both xenocrysts and xenoliths in these contaminated acid rocks are of morebasic composition than the enclosing rock, and the following transformationsin the direction of attainment of equilibrium (cf. Nockolds, 1935, pp. 308-315 ;Hurlbut, 1935, pp. 627-630) can usually be observed: Olivine (--+orthopyroxene)--+cummingtonite--+green hornblende. Orthopyroxene (--+cummingtonite)--+green hornblende. Olinopyroxene--+green hornblende (--+biotite).Intermediate plagioclase corroded and/or--+more acid plagioclase.

These transformations have rarely proceeded far, which accounts for thedose approach in chemical composition between the analyzed xenolith (Ok. 112,Table I) and its parental outer ferrogabbro, although alkalis are significantlyhigher in the former. The frequent presence of olivine, clinopyroxene and-orthopyroxene in the acid rock immediately surrounding many xenoliths mustbe due to mecl!anical disintegration of incorporated material. Olivine and.clinopyroxene usually show the above transformations which tended to restorephasal equilibrium in the crystallizing hydrous acid magma, but orthopyroxeneis rather more stable in the acid environment over an extended range of composition. Apatite needles, often in great abundance, are commonly developedin the xenoliths.


It is noteworthy that the marginal acid rocks adjacent to Damara quartziteson White Ridge are considerably more quartr,-rich (up to 25 per cent.) than thosecollected elsewhere, which clearly points to assimilation of country rock eitherduring emplacement or in situ,. It appears probable that the uncontaminatedmagma approximated in composition to the ridge s:y"enites, which are closelyrelated to the marginal acid rocks in the western 1)art of the outcrop.

Dykes of acid composition are particularly numerous in the south-easternsector of the complex, varying up to 2 metres in width and traversing the ferrogabbros and marginal acid rocks, though some have been traced in to the ridgegabbro zone and others continue out into the country rocks. :Most show apronounced radial orientation with respect to the centre of the complex, thougha few are sill-like, having both dip and strike directions concordant with thepseudostratification of the differentiated group.

No evidence of eontamination has been found in any of these dyke rockswhich seem to represent an acid aplitic facies of the marginal acid rocks andhave the same mineralogy, though olivine is always absent and orthopyroxeneis rarely seen.

(v) Granulitic Gabbros.

In the area bounded on the north by the Kazombaruru river and on thesouth by a line joining the extremities of the ridge syenite dyke, the arcuatetrend of the ridge gabbro ridges is continued by a series of bouldery ridgescomposed of very dark gabbroic rocks (Plate XXIV, foreground) which formelongated bodies surrounded and traversed by core gabbro. Detailed mappingwas made impossible by the nature of the exposures, and their representationon thc geological map is largely diagrammatic.

Microscopic examination has revealed that these dark gabbroic rocks havebeen granulitized to varying degrees. The xenolithic rocks adjaeent to intrusiveeore gabbro are very fine-grained types with subeonchoidal fra,cture, hornfelsicappearanee, and small lenticular felspathie streaks. vVhen traced towards theeentre of each xenolithic mass, the rocks lose the eharaeteristic felspathic streaksand heeome more coarse-grairied.

Similarly granulitir,ed gabbroic rocks were found in ferrogabbros as a Slllalldark knoll north of Black Ridge, in core gabbro at the southern end of AuasValley, on Ada,ms Shoulder, and among ridge gabbro xenoliths in marginalacid rock on .Johannes Berg.

Of the coarse-grained specimens which show little or no granulation, not onecan be matched with the ridge gabbros as might be expected from the neldevidenee. :Many are strongly reminiseent of the marginal gabbro-pierite, exceptfor a greater variation in the relative proportions of the constituent minerals,which are invariably fresh and unaltered. Colourless magnesian olivine(Fa17-Fa25 ) is always abundant as large irregular crystals up to"4 mill. in size,aecompanied by zoned laths of plagioclase (An 72-An56). Clinopyroxene(Ca39Mg46Fe15) and orthopyroxene (Of18-Of23) are always present, the clinopyroxene forming large irregular crystals up to 5 mm. in size and occasionallyexceeding olivine in amount. Small amounts of water-clear orthoclase arealmost invariably present as small interstitial patches, against which plagioclase


shows embayment. Accessory minerals are apatite and reddish-brown biotiteassociated with iron ore. The chemical composition of one of these rocks(Ok. 100) shows a close approach to that of the analyzed marginal gabbropicrite (Ok. 110).

From a study of the material collected, it has been possible to trace roughlythe sequence of changes accompanying the progressive granulitization of theserocks. One of the first minerals to respond is clinopyroxene which developsprominent schiller inclusions, often zonally distributed parallel to potentialcrystal boundaries (Plate XXVII, Fig. 1); at the same time metamorphicclouding of plagioclase develops into the separation of tiny, but recognizablegranules of iron ore within the felspar host. Incipient granulation of olivinefollows, and is generally accompanied by very dense schillerization of clinopyroxene, with a tendency towards aggregation of schiller rods to form discretegranules of iron ore as the pyroxene host itself commences to granulate. Duringthe advanced stages of granulation of olivine and pyroxene, plagioclase isrecrystallized to form small unzoned grains and the rocks develop a thoroughlyfine-grained granular texture. Near the margin of each xenolithic body, aggregates of twinned plagioclase crystals show a pronounced tendency to segregateinto lenticular patches (Plate XXVII, Fig. 2) which are conspicuous even in handspecimen.

Some representatives of all these stages carry small amounts of brownhornblende which replaces pyroxene and often poikilitically encloses granules ofoliVIne. The formation of hornblende thus appears to post-date the granulation,and the same applies to flakes of reddish-brown biotite associated with iron ore(ci. Bichey and Thomas, 1930, p. 232). Orthoclase, too, commonly occurs asclear" pools" enclosing corroded remnants of plagioclase and was probablyderived from the destruction of original biotite.

The chemical composition of a typical, completely granulated specimen(Ok. 93) collected from the type area is in many respects intermediate betweenthose of the gabbro-picrite and ridge gabbros. It is suggested that thesexenolithic masses represent more basic representatives of the ridge gabbroswhich are apparently no longer preserved in their original structural positionthrough faulting (Plate XXIX).

A fine-grained granulitic gabbro (Ok. 168) collected among ridge gabbroxenoliths in marginal acid rock on the western ridge of .Johannes Berg, and asimilar rock from xenoliths in core gabbro on Adams Shoulder differ from thenormal types in the ophitic habit of the constituent clino- and orthopyroxene.One thin section (Ok. 168A) is traversed by a narrow (0,5 mm.) vein of subophitic orthopyroxene. Moreover, all the orthopyroxene cry-stals in both l'ockand veins enclose numerous small exsolution blebs of clinopyroxene(2Vy = 50° ±J uniformly oriented within the orthopyroxene host. The originof these pyroxene veins is puzzling, since the evidence for initial formation of acalciferous pyroxene followed by exsolution rules out the ingenious explanationoffered by Bowen and Tuttle (1949, p. 460). The ridge gabbro parentage of theanalyzed specimen (Ok. 168) of this type is proved by the close approach inchemical composition to the analyzed outer ridge gabbro (Ok. 1).


In general the granulitic gabbros are closely comparable with similarxenolithic basic granulites found within the hypersthene gabbro of Centre 2,Ardnamurchan (Richey and Thomas, 1930, pp. 229-233), but which are regardedby Wells (1951, pp. 727-735) as the products of metamorphism and metasomatism of sedimentary inclusions. The chemical and microscopical evidence,however, leaves no doubt as to the obvious igneous parentage of the Okonjejegranulitic gabbros.

(b) Notes on Mineralogy

A notable feature of the course of clinopyroxene crystallization (Fig. 3) isthe remarkably small variation in lime, in this respect differing considerablyfrom the trends established for pyroxenes of basaltic magmas by Walker andPoldervaart (1949, p. 631), vVager and Deer (1939, p. 79), Hess (1949, p. 634),and :Muir (1951, p. 701).

Orthopyroxenes are never schillerized and never exhibit the patchy extinction or lamellar intergrowth of orthopyroxenes of the Bushveld type ('videPoldervaart, 1947). Measured values of 2V and y were found to agree well withthe curves published by Poldervaart (1947, p. 167), except in the range Of4o-Of65

where 2V falls as low as 48° (y = 1,728) at the composition Of5 0 ((j. Hess, 1952).

The break in crystallization in the olivine series occurs between Fa53 andFa60 and is thus narrower than the corresponding break in the Skaergaardintrusion (vVager and Deer, 1939, pp. 74, 131, 239) and the New Amalfi sheet(Poldervaart, 1944, p. 94).

Although independent crystals of olivine and orthopyroxene occur in theserocks, and this is more especially the case with the iron-rich members, the twominerals are usually closely associated in rather a special way which suggests areaction relationship over an extended range of composition. The replacementprocess is accompanied by the exsolution of iron ore (Plate XXVI, Fig. 2) andthe orthopyroxene is consequently always somewhat more magnesian than theassociated olivine (Fig. 3) in rocks of the differentiated group (ct. W~ager andDeer, 1939, p. 251; Ramberg and De Vore, 1951, pp. 195-196). The developmentof intergrown iron ore and orthopyroxene at the expense of olivine moremagnesian than Fa50 implies a reaction relation over a more extended range ofcomposition than the experimentally determined limit of about Fa30 in theartificial system MgO-FeO-Si02 (Bowen and Schairer, 1935). No doubt thepresence of lime-bearing plagioclase is responsible for this apparent discrepancy,as shown by Andersen (1915). The textural evidence in the middle ferrogabbros(Plate XXVI, Fig. 3) might be interpreted as replacement of orthopyroxene byolivine or vice versa. and the former alternative is supported by experimentalevidence of the opposite type of incongruency at the iron-rich end of the pyroxeneseries of solid solutions (Bowen and Schairer, 1935, pp. 211-212).

(c) Structure

The outward-dipping marginal dyke of gabbro-picrite in many respectsrecalls the ring-dykes of the British Tertiary igneous province (although thelatter are generally wider), even to the extent of asymmetrical development(Richey, 1932, p. 118) and discontinuity of outcrop due to upward tapering as


in the Glen More ring-dyke (op. cit., p. 67). According to Anderson (1936,pp. 136-137) ring-dyke formation follows as a result of subsidence, but theapparently undisturbed Karroo beds on either side of the gabbro-picrite north ofZbigniew Bidge (Plate, XXV Fig. 1) indicate little or no differential movement.The local updoming of the marginal Karroo sediments was probably effectedduring an initial upward pressure of the magma reservoir.

It has already been shown that the differentiated group constitutes aconformable pseudostratified series. The inner and outer margins of the groupare arcuate and parallel both to the trend of the ridge gabbro ridges and to theouter margin of the complex (except to the north), although the latter has beenmodified in detail by the marginal acid rock. Extrapolation of the mappedtrace of the inner margin of the group shows that it encloses an elliptical areaelongated in an east-west direction and centred slightly to the north of Martin.The western half of the ellipse is, however, slightly offset to the north along anorth-north-east-south-south-west line passing roughly through the centre.Although there is no sign of the inward increase in dip characteristic of conesheet intrusions, the differentiated group shows many structural similarities tothe Cuillin gabbro in Skye (Richey, 1932, pp. 71-74), and is considered to form asaucer-shaped multiple intrusion (Plate XXIX), elliptical in plan, and wasprobably emplaced by the successive injection of a series of thin sheets, the orderof injection being from above downwards. The space problem may be relievedby appeal to either progressive subsidence of the floor or updoming of the roof,and the former alternative seems the more probable in view of the concordantdips observed. Perhaps the initial fracture was of the cone-sheet type andcontrolled thc attitude of subsequent sheetlike injections in sequence beneath it.Such a mechanism offers a satisfactory explanation for the inversion of the twounits of the differentiated group as compared with similar units in layereddifferentiated complexes such as the Bushveld, Skaergaard and Stillwater.

The emplacement of the core gabbro was apparently effected after thedifferentiated group had solidified, and was followed by the intrusion of themarginal acid rock and ridge syenite dyke, in that order. On the western slopes ofMartin the intrusive contact between core gabbro and marginal acid rockfollows a remarkably linear north-north-east-south-south-west trend which,when extrapolated to the south, intersects the zone of breccia-filled verticalfractures in Karroo sediments at the western end of Outjivero. This line moreovercoincides with that along which the elliptical trace of the differentiated group isoffset. The most obvious explanation of these observations is that hinge-faultinghad taken place along this north-north-east-south-south-west trending linebefore the emplacement of the marginal acid rock (Plate XXIX). The fracturezone in Karroo sediments on Outjivero is regarded as the hinge-line, withdifferential downward tilting of the western half with respect to the eastern alongthe fault plane. The discrepancy between dips of the ridge gabbro pseudostratification in the eastern and western areas suggests that the angle of tiltingwas approximately 25°.

According to this interpretation the outlier of highly metamorphosed Karroosediments at the summit of Johannes Berg is regarded as a remnant of thedmvnwarped roof (Plate XXIX). That the marginal acid rock is of no great

J


thickness in this area is proved by the abundant scattered outcrops of ridgegabbro and ferrogabbro. The marginal acid rock therefore seems to have anentirely marginal distribution and apparently intervenes between thedifferentiated group and the country rocks forming the walls and roof of thecomplex as indicated in Plate XXIX. It is probable that the emplacementof the marginal acid rock was closely connected with the hinge-fault movements.

Although they apparently continue the arcuate trend of the ridge gabbroridges, the granulitic gabbros of the type area south-east of JYlartin seern torepresent rather more basic accumulative types which may have formed anupper zone of the differentiated group. On this basis it is tentatively suggestedthat, during or before the emplacement of the core gabbro, a roughly circularfissure developed, causing the enclosed central block to subside and bringinginto their present position the rocks which were granulitized by the intrusivecore gabbro. The arcuate dyke of ridge syenite is believed to be closely relatedto this hypothetical ring-fracture, along at least part of which the syenite magmawas injected at some time after the emplacement of both the core gabbro andmarginal acid rock.

(d) Petrology

Ohemical and spectrographic data for rocks of the Tholeiitic Series arepresented in Table 1. In the linear variation diagram (Fig. 4) the chemical data

(FeO + Fe20 S) X 100have been plotted only against the mafic index MgO + FeO + Fe

20

S(Simpson,

1954), with a consequent narrowing of the field occupied by the more acid rocks,although serious displacement is apparent only in the points representing themarginal acid rock (Ok. 185).

With few exceptions, the oxides show regular variations which can berepresented by smooth curves passing through points representing the outerridge gabbro, ferrogabbros and ridge syenite. The variation trends of the majorelements are consistent with the derivation of the Tholeiitic Series from a singleparent magma by processes involving fractional crystallization, while thevariation of individual trace elements follows the trends established by Nockoldsand Mitchell (1948) and Wager and Mitchell (1951) for rocks derived by thesame process.

There are, however, significant differences in the manner of this variation.As in the Skaergaard intrusion, the trace elements characteristically associatedwith the ferromagnesian minerals (chromium, nickel, vanadium and cobalt) areconcentrated in the more basic rocks, and decrease (rapidly in the case ofchromium and nickel) to values below the limits of sensitivity in the intermediate rocks. The behaviour of these elements in particular contrasts with themore regular variation of trace elements in the Oaledonian rocks, and is regardedby both Nockolds and Mitchell (1948, p. 545) and Wager and Mitchell (1951,pp. 200-201) as indicative of a high degree of fractionation. On the other hand,those trace elements present in felspars (gallium, strontium, barium andrubidium) show a much more regular variation up to a late stage in thedifferentiation series, and thus resemble more closely the variation trends ofthe Oaledonian rocks in which fractionation was apparently not so pronounced.


110100 93 7 168 I 10966

3

2• • •I

.. K20~2

• • • C>- • 0-

No2 O )(

~-4~

2 • 0 • 0• •

20><~•

10 • AI2 03•x

60 Si02~O

50 • • 0

• 0 0•15 •10 • •5 FeO + F~203

•15 MgO10 •5

10 • • ••5 CaO

40 50 60 70 80 90MAFIC INDEX

FIG. 4.Mafic variation diagram for the Tholeiitic Series. Open circles-normal differentiationseries (ridge gabbros, ferrogabbros, ridge syenite): black circles-accumulative andgranulitized basic rocks: crosses-marginal acid rock. For index to rock numbers, see

explanation of Table I.


As discussed elsewhere (Simpson, 1954) it appears that both mafic and felsicfractionation have operated simultaneously to influence the trend of differentatwn, 1jhe former being dominant up to the stage ot maXImum iron enrichmentand felsic fractionation during the later stages. The effect of early and steadilyincreasing felsic fractionation is clearly reflected in the lower degree of ironenrichment (Figs. 5 and 6) and pronounced zoning of plagioclase crystals inthis series as compared with the Skaergaard rocks. Thus the middle hortonoliteferrogabbro Ok. 106, which is probably the most iron-rich rock in the TholeiiticSeries, is comparable in composition with both the middle gabbro 3661 and thehortonolite ferrogabbro 1907 in the layered series of the Skaergaard intrusion.Although the outer ferrogabbro Ok. 109 is ferrohortonolite-bearing it is by nomeans comparable with the Skaergaard ferrohortonolite ferrogabbro 4145 whichapproaches the limit of iron enrichment in that series, but is more similar tothe Skaergaard basic hedenbergite-granophyre 4137, which bears an analogousrelation to the more basic rocks of the layered series.

The outer ridge gabbro (Ok. 1) is certainly the most typical basic memberof the Tholeiitic Series and is regarded as representing a close approach to thecomposition of the parent magma. It furthermore shows a close correspondencewith the Skaergaard average chilled marginal olivine gabbro (op. cit., p. 141)and the average of 43 analysed Karroo dolerites (vValker and Poldervaart, 1949,p. 649). The most significant differences are the higher silica in the Karroodolerites and low potash in the Skaergaard rocks. The trace elements show onlylimited correspondence (cf. Walker and Poldervaart, 1949, p. 644; Wager and:Mitchell, 1951, Table A), and suggest that the original Tholeiitic magma wascomparatively rich in lithium, scandium, yttrium and rubidium, very rich instrontium and barium, but poorer in chromium than the other two. Theseinherent characteristics (reflected in the presence of biotite and developmentof alkali felspar at a comparatively early stage in the differentiation series),associated with early and sustained felsic fractionation, are believed to accountfor the earlier preponderance of alkali enrichment than is the case in theSkaergaard intrusion (cf. Wa~er and Deer, 1939, pp. 310 and 312).

The scattering of points on the variation diagram representing the marginalgabbro-picrite (Ok. 110) and granulitic gabbros of similar composition (Ok. 100and 93) suggests an accurllulative origin for these rocks. The inner ridge gabbro(Ok. 7), on the other hand, is relatively enriched in alumina, lime, gallium andstrontium and impoverished in magnesia, iron, titania, chromium, vanadiumcobalt, etc., which strongly suggests relative enrichment in plagioclase(ef. Nockolds and Mitchell, 1948, p. 537), and these rocks probably bear a.complementary relation to the gabbro-picrites which are enriched in olivine:and pyroxene.

As explained previously, the ridge syenite is regarded as the normal latedifferentiate of the Tholeiitic Series, which gave rise to the marginal acid rocksby the incorporation of siliceous sedimentary material, followed by contaminationand hybridization with more basic rocks during emplacement. The ridge syeniteis to a certain extent anomalous in that it carries considerably less quartz thanthe outer ferrogabbros. It may well be that this is the result of less strongfractionation during the later stages of crystallization, whereas the abnormal


Fig. 5

-_--- i1'".... -

b

•

/._~---

/ .-----------/

//

/I

//

//

/iIII

o50

90

,...2:-SO

><I.&Joz70

~o • Fig. 6•

20 30 40 50FELSIC

60 10INDEX (F)

80 90

FIGs. 5 and 6.FMA and M-F diagrams for the Tholeiitic Series (symbols as for Fig. 4). Data forSkaergaard liquids and Daly's average basalt, andesite, dacite, rhyolite (broken curves)

included for comparison.


increase of quartz in the outer ferrogabbros represents the excess silica storedup in the liquid during the preceding stages of strong fractionation which ledto a break in the crystallization sequence of olivine.

V. THE ALKALI SERIES

The intrusive bodies of alkali rock are for the most markedly transgressiveto the earlier Tholeiitic Series, and moreover show little continuous variationin chemical composition and mineralogy. The alkali rocks fall naturally intofour groups which are described separately: (a) core gabbros, (b) alkali rocksof Okonjeje Berg, (c) vVitrand foyaites, and (d) minor intrusives.

(a) Core Gabbro

Relatively undifferentiated gabbros are exposed over an area of high reliefin the centre of the complex, and are easily distinguished from the adjacentdark ridge gabbros by their lighter colour and felspathic conlposition. The coregabbro is chilled against ridge gabbros in the east, but maintains its coarsegrain size right up to the contacts against both ridge syenite and marginal acidrock. Somewhat similar, but more melanocratic and consistently nephelinebearing gabbros are exposed around Adams Shoulder adjacent to the intrusivepulaskite. Field relations of these gabbros on the lower slopes of Okonjeje Bergare largely obscured by extensive screes.

The core gabbros are generally light grey or mottled rocks of variable grainsize in which the individual mineral components can be easily identified in thefield. 'l'he coarse-grained members are maSSive, and variation in the plagioclasecontent gives rise to occasional troctolitic, pyroxenitic and anorthositic facieswhich occur in discontinuous lenticular bands rarely more than a few metresthick. The medium-grained rocks usually exhibit a rude lamination which isparticularly well developed on Martin. Here it passes locally into a zone ofrhythmically banded gabbro (Plate XXVI, Fig. 1). In these laminated rocksthe plagioclase crystals tend to lie with (010) faces parallel to the plane oflamination, and prismatic crystals of pyroxene are oriented with their longeraxes in this plane but without preferred linear arrangement.

The low inward dips of these planar structures, together with the similarattitude of the major joint planes and the lenticular bands of contrasted mineralcomposition, suggest that the core gabbros of the central area form a pseudostratified succession which is apparently concordant with the saucer-shapedstructure of the differentiated group. The inclinations of the planar structuresconverge towards a centre situated slightly to the north of lVIartin and closeto the hinge fault along which the western half of the core gabbro mass hasbeen downfaulted (Plate XXIX).

Petrographically the core gabbros are remarkably uniform except for theirregular minor development of orthoclase and nepheline, and the compositionsof the constituent minerals show little variation apart from the effects of zoning.Plagioclase is generally the most abundant constituent, and occurs as stronglyzoned tabular crystals up to 5 mm. across. The composition of the calcic coresvaries between An so and An 7o, while the margins are richer in the albite moleculeby about 10 per cent. near the bottom of the succession, increasing to about30 per cent. in the Martin gabbros.

THE OKO~JEJE IGNEOUS COMPLEX 147

Clinopyroxene builds granular crystals in the laminated rocks but ischaracteristically ophitic in the coarse massive gabbros, and the crystals almostinvariably show schiller structure. It is particularly abundant in the moremelanocratic Adams Shoulder gabbros. Chemical analyses of two pyroxenes(Table III) separated fronl core gabbros indicate that they are diopsidic typesas opposed to the augites of the Tholeiitic Series, and that they are moreovermoderately rich in titania and sesquioxides. In the Adams Shoulder gabbrosand the laminated gabbros on Martin, the pyroxenes often show alteration tobrown amphibole with the following optical properties: 2Vex = 74°, y = 1·697,y:c = 14°; pleochroism ex = pale yellow; 13 = nut brown; y = deep brown.

Olivine is always present but rarely exceeds 10 per cent. by weight in mostcore gabbros, where it varies in composition between Fa27 and Fa35 • Olivine is,however, more iron-rich (up to Fa46 ) in the dark nepheline-bearing AdamsShoulder gabbros. Alteration to serpentine, bowlingite or celadonite alongfractures is occasionally observed.

Iron ore invariably occurs in two generations, the earlier crystals beingsman octahedra accompanied by sparse grains of yellow sulphide ores, whilethe later generation builds interstitial sub-poikilitic crystals which oft€n snowsegregations or fine lamellae of exsolved ilmenite in reflected light. Biotite isan Important accessory in all these rocks and forms large independent flakesand reaction rims around iron ore. Small amounts of apatite are always present.Interstitial and occasionally antiperthitic orthoclase is sometimes present insmall amounts, while the Adams Shoulder gabbros usually carry interstitialnepheline, which is only rarely observed in the core gabbros of the central area.

The analysed chilled marginal core gabbro (Ok. 36, Table II), althoughfine-grained and porphyritic, is mineralogically typical of the core gabbros ofthe central area. The analysed Adams Shoulder gabbro (Ok. 158) is considerablyricher in alkalis and approaches rnore closely the composition of average alkaligabbros in general and the Oslo-essexites in particular (Barth, 1944).

On chemical evidence it seems probable that the Adams Shoulder gabbrosrepresent a more advanced stage in the differentiation of the core gabbro magmatowards enrichment in alkalis and titania, but not significantly in iron. Thepossibility cannot be completely excluded that these dark alkali gabbros owetheir peculiar characteristics to a mild degree of fenitization by the adjacentintrusive pulaskite. As against this view it can be stated that there is nosystematic development of nepheline and orthoclase in the core gabbros adjacentto the Witrand foyaites, the pyroxenes of the Adams Shoulder gabbros showno enrichment in alkalis, and there is no trace whatsoever of metasomatic effectsin the ridge gabbros adjacent to the intrusive pulaskite north of Korn.

(b) The Alkali Rocks of Okonjeje Berg

The upper slopes of the lofty Okonjeje Berg at the northern end of thecomplex are made up of three bodies of alkali rock which are exposed withroughly concentric arrangement, as shown on Plate XXIX. The outcrop of theouter ring of coarse-grained leucocratic pulaskite is readily distinguished in thefield, but the structural relations between the inner bodies of andesine- andoligoclase-essexite are undoubtedly more complex than, though essentiallysimilar to, the simple arrangement indicated on the map.


The observed intrusive contacts, which markedly transgress the contoursin this area of very high relief, suggest that the three bodies of alkali rock areroughly cylindrical in form, though outwardly inclined at a steep angle. Thewhole structure is strongly reminiscent of the well-known Ben Nevis, Glen Coeand Etive complexes in Scotland, and the Sande cauldron of the Oslo region(Oftedahl, 1948, p. 210), in which the operation of multiple cauldron-subsidencesaccompanied by magmatic intrusion has been clearly demonstrated.

(i) PvlaskiteThe pulaskite of the outer ring is a uniformly coarse-grained massive

leucocratic rock with occasional pegmatitic patches, which characteristicallyweathers into large rounded boulders and extensive exfoliation surfaces (PlateXXV, Fig. 1). Observed contacts with Damara sediments are rarely sharp, andoccasional sedimentary xenoliths are sometimes seen in the marginal facies whichis almost devoid of dark minerals and generally somewhat less coarse-grained.Contacts against gabbros and gabbro-picrite are sharply defined with occasionallocal brecciation of the basic rorks. The pulaskite maintains its coarse-grain onthe inner side right up to the chilled marginal facies of the andesine-essexite.

When fresh the pulaskite is a pale felspathic rock with an irregular spottedappearance due to a tendency of the sparse dark minerals to occur in crystalaggregates. The bulk of the rock is composed of large subidiomorphic or irregularcrystals of slightly clouded soda-potash felspar which show considerable variationfrom optically homogeneous or cryptoperthitic material to coarse perthiticintergrowths of albite and orthoclase in which the twinned soda felspar maypredominate. The latter is never more calcic than AniO' Nepheline occurs assporadic small irregular crystals enclosed by felspar and is often altered tomuscovite. Small amounts of sodalite are usually present.

Pyroxene and amphibole tend to occur together as scattered groups ofallotriomorphic crystals, together with accessory iron ore and an occasionalcrystal of sphene. The pyroxene is a very pale green diopsidic type with2Vy = 57°, y = 1,719, y: c = 42°, which corresponds approximately with thecomposition Di62He38• The brown amphibole usually occurs replacing pyroxeneand has the following optical properties: 2Yex = 47°, y = 1,704, y: c = 12° ;pleochroism ex = pale yellow-brown, ,3 = reddish-brown, y = deep brown;optic axial plane parallel to (010). Deep reddish-brown biotite forms largescattered flakes which sometimes poikilitieally enclose the other dark minerals.

AnguJa,r interstitial cavities between felspar crystals are filled with lathsof twinned albite, analcite and calcite, in that order of formation. Apatite occursin accessory alllOunts as prominent idiomorphic prisms.

The chemical composition of a typical specimen of lmlaskite (Ok. 143) isgiven in Table II.

(ii) Andesi1/e-essexiteThis intermediate member of the Okonjeje Berg ring structure is con

spicuous in the field, being darker than the rocks of the 130re and outer ring.The andesine-essexites are llledium to coarse grained massive rocks, very similarin appearance to some of the more felspathic core gabbros except for recognisablecrystals of nepheline which give rise to pitting on weathered surfaces. :Many of


these rocks show rounded white felspathic spots up to 8 mm. in diameter whichare quite conspicuous in the field. The andesine-essexite maintains its coarsegrain right up to the inner contact with oligoclase-essexite. The outer marginis chilled against the enclosing pulaskite.

Felspars and felspathoids make up approximately 70 per cent. of the typicalc03Jrse-grained andesine-essexites. Plagioclase occurs as numerous twinnedzonal laths which sometimes form clusters of sub-parallel crystals. Zoning overthe limits of An55 to AnI5 has been measured, but the bulk of the plagioclaselies within the andesine range of composition. Crystals with highly irregular,mutually interfering margins and undulose extinction are conlmon. Orthoclaseforms mantles round plagioclase, but occurs for the most part as large crystals,nsually untwinned, which often enclose irregular optically-continuous crystalsof nepheline. Dactylitic intergrowths between the two felspars are not uncommon.

:Nepheline is usually perfectly fresh and occurs as large allotriomorphiccrystals moulded on dark minerals and plagioclase. Sodalite occasionallyreplaces nepheline, but generally occurs independently. About 10 per cent. oftjhe analysed specimen (Ok. Hl5, Table II) consists of a curious dactyliticintergrowth between orthoclase or plagioclase on the one hand and nephelineor sodalite on the other (Plate XXVII, Fig. 3). The term fingerprint intergrowthdescribes it very effectively (cf. Allan, 1914, pp. 133, 285). The intergrowthoccupies patches of variable extent and it usually terminates abruptly againstpotash felspar and calcic plagioclase, but the margins are sometimes lobate andthe felspar component optically-continuous with an adjacent crystal. Somewhatsimilar though plagioclase-free intergrowths in borolanite have been describedby Shand (1910, p. 387), and the absence of any uniform orientation suggeststhat such structures have a replacement origin. The rounded white spots insome specimens are composed essentially of clouded orthoclase, nepheline andsodalite '" hich poikilitically enclose twinned laths of plagioclase An4 o. Thepresence of lime-bearing plagioclase distinguishes them from the otherwisesimilar spots in borolanite which ,,-ere thought by Shand (1909, pp. 203-205)to be pseudoleucite.

Clinopyroxene is evenly distributed as clusters of independent prismaticcrystals. The optical properties differ only very slightly from those of the coregabbro pyroxenes, but the chemical composition (Table III) shows significantincreases in iron and alkalis, and lower magnesia. Brown amphibole varies inamount, but always bears a replacement relation to clinopyroxene. Opticalproperties are: 2VO' = 71°, y =-= 1·701, y: c = 13°; pleochroism a = paleyellow, (3 = nut brown or greenish-brown, y =: deep brown or deep greenishbrown. Olivine is always present as rounded crystals which vary in compositionbetween Fa64 and Fan.

Accessory minerals are early iron ore and associated deep reddish-brownbiotite, together with stout prisms of apatite and small amounts of interstitialcalcite.

Towards the outer margin a gradual diminution of grain size is noticeableuntil the andesine-essexite passes into a dull grey porphyritic chilled facies afew metres from the contact. Here the chief ferromagnesian is a greenish brown


amphibole which forms prismatic phenocrysts and numerous smaller crystalsin the groundmass. Simple twinning on (100) is common and the phenocrystsshow a zonal variation in optical properties as follows :-

2Vexyy:c

pleochroismrL

Core

48°1·700 ± ·004

11°ex pale yellow-brownf3 browny deep reddish-brown

Margin

36°1·720 ± ·004

gopale yellow-greenolive greendeep brown-green

The groundmass amphiboles have optical properties approximating tothose of the phenocryst margins. Pale green pyroxene (2Vy = 63°, y = 1,725)is not abundant and usually shows alteration to amphibole.

Zoned plagioclase (An38-AnIS ) builds twinned phenocrysts which are oftenmantled by sodic plagioclase or potash felspar which belong to the groundmassperiod of crystallization. The groundmass shows flow structure and consistsof numerous crystals of nepheline, sodalite and tabular orthoclase, and accessoryiron ore, sphene and apatite.

The dominance of amphibole over pyroxene and the less calcic compositionof the plagioclase in these rnarginal rocks is probably due to contamination ofthe andesine-essexite magma by assimilation of pulaskite during ascent andemplacement. Somewhat similar rocks are found in a well-marked zone ofdykes, veins and schlieren which traverse the pulaskite on the lower southernslope of Okonjeje Berg and originate from the chilled marginal facies of theandesine-essexite.

(iii) Oligoclase-Essc:riteThe central position in the Okonjeje Berg ring structure is occupied by a

body of oligocla,se-essexite which is exposed in the summit region of the mountain.The areal extent of the outcrop as indicated on the map is necessarily somewhatconjectural owing to precipitous slopes and the abundance of fallen blocks.

The oligoclase-essexites are medium-grained mesotype rocks with scattereddark rounded xenoliths which become more numerous as the andesine-essexiteoutcrop is approached. "Vith little variation in texture and mineralogy, theoligoclase-essexite is composed essentially of plagioclase, orthoclase, nephelineand brown alkali amphibole.

Plagioclase (An20-AnIS ) builds broad irregular laths, the larger of whichoften show slight zoning within the limits An26-AniO' Q.uite common are irregularbranching fractures, usually filled with analcite, which resenlble shatter-cracks,but optical continuity of the host crystals is undisturbed. Orthoclase formsirregular independent crystals and nlantles around plagioclase laths, and isgenerally microperthitic. Nepheline builds large crystals, but also occurs asoptically continuous rounded patches within plagioclase, suggesting contemporaneous crystallization. In some specimens sodalite is prominent, yet in othersit is subordinate to nepheline or completely absent.


The most prominent ferromagnesian is a greenish-brown alkali amphibolewhich occurs as large subidiomorphic crystals, frequently twinned on (100),and occasionally showing remnant cores of pale green clinopyroxene. Theehemical composition and structural formula of this amphibole (Table III) showit to be a member of the hastingsite group (Berman and Larsen, 1931), corresponding to femagha~tingsite in the classification adopted by Billings (1928).

Interstitial cavities in the rock fabrie are usually filled with twinned albitelaths, analeite and occasionally c8Jlcite, in that order of crystallization. Deepbrown biotite forms independent flakes which are often enclosed by amphibolebut neyer closely associated with iron ore. Titaniferous iron ore and yellow-greypleochroic sphene are prominent aceessory minerals, together with prisms andneedles of apatite. Iron ore grains are oecasionally surrounded by narrow rimsof sphene.

Some of the larger xenoliths enclosed by the oligoclase-essexite are verysimilar in texture and mineralogy to the contaminated marginal facies of theandesine-essexite, with the addition of innumerable acicular crystals of apatite.They show all stages of mechanical disruption by the enclosing rock, with whichthey are in phasal equilibrium, requiring no mineral transformations duringassimilation. The field evidenee suggests that the xenoliths represent shatteredfragments of andesine-essexite and this interpretation is accepted as being themost probable.

(c) Witrand Foyaites

Two almost circular outcrops of foyaite form conspicuous, pale colouredbouldery knolls in the core gabbro terrain at the northern end of the Auas valley.

The rock of the smaller western outcrop is a light grey porphyritic foyaitein which subidiomorphic phenocrysts of orthoclase and nepheline up to 15 mm.in size are set in a fine-grained groundmass composeu of turbid orthoclase andalbite laths together with much sodalite and nepheline, and minor amounts ofanalcite, calcite, cancrinite, muscovite and apatite. Sparse nlicrophenocrystsof zoned greenish-purple titanaugite, zoned brown-green ferrohastingsite, spheneand iron ore are scattered throngh the rock. The orthoclase phenocrysts almostinvariably show Carlsbad twinning and zonally arranged inclusions of sodalitenear crystal margins. The larger nepheline phenocrysts are usually composite,and in seetions stained with methylene blue show a slight zonal variation indepth of stain parallel to potential crystal boundaries. The average modalcomposition of the porphyritic foyaite is as follows: nepheline 18 '0, orthoclasephenoerysts 16,3, sodalite 14,2, pyroxene and amphibole 8,6, iron ore 1,9,sphene 0,6, remainder (chiefly groundmass felspar) 40·4.

Towards the surrounding gabbros the foyaite tends to become streakedout parallel to the contact, with the development of dark streaks and clots ofmelteigite, and occasional patches of coarse-grained foyaite pegmatite. Thelatter is considerably enriched in nepheline at the expense of felspar and carrieslarge zoned cr3Tstals of titanaugite, ferrohastingsite, and flakes of deep brownbiotite. The melteigite patches are composed of irregular poikilitic phenocrystsof zoned hastingsite amphibole set in a dark fine-grained groundmass of roundednepheline crystals separated by, or poikilitically enclosing clusters and small


single crystals of greenish-purple titanaugite, together with abundant acicularapatite and accessory amounts of iron ore, sphene, albite, analcite, calcite andmuscovite. The average modal composition is: titanaugite 50,7, amphibole23,2, nepheline 24,3, accessories 1·8.

The normal rock of the larger eastern outcrop is a medium-grainedleucocratic foyaite in which vitreous crystals of nepheline, grey orthoclase, andnumerous prisms of pyroxene and amphibole are easily recognised. Sinuousvertical bands and streaks of a more melanocratic facies are sporadicallydeveloped and trend roughly north-east-south-west parallel to the longer axisof the outcrop. The darker bands are rarely more than a few centimetres wideand occur singly or in groups which may coalesce or anastomose. On the westernside where exposures are good, the normal banded or massive foyaite passesinto a marginal porphyritic facies identical with that of the western knoll.Elsewhere near the margin this transition is not so pronounced but occasionalstreaks and patches of coarse-grained foyaite and dark melteigite are developednear the gabbro contact.

On the field evidence it appears probable that these two outcrops of foyaiterepresent cupolas (the eastern eroded more deeply to reveal the non-porphyriticcore) of an intrusive body, possibly small and stock-like. A few narrow dykesand stringers of microfoyaite (one specimen of which carries accessory eudialyte)were found traversing the core gabbros up to a distance of about 50 metres fromthe foyaite bodies.

The mineralogical characters of the eastern foyaite body are very constant,though the proportion of felspathoid varies considerably in different thinsections and the dark bands are enriched in ferromagnesian minerals. The rockconsists essentially of orthoclase, nepheline, sodalite, green pyroxene andhastingsite amphibole, with accessory amounts of albite, analcite, sphene, ironore, apatite and calcite.

Orthoclase is the most abundant constituent by weight, forming largecrystals with average 2V0: = 52° and often showing fine parallel striae near themargins which seem to be fine lamellae of exsolved 31bite. A second generationfills extensive interstitial areas and is usually perthitic with 2Vo: = 63°. Latealbite forms radiating laths or broad tabular crystals in interstitial cavitiestogether with weakly birefringent analcite and calcite, in that order ofcrystallization. Nepheline builds large subidiomorphic crystals up to 5 mm. insize and generally makes up about 25 per cent. of the rock by weight. Thecrystals commonly show prominent (1010) cleavage cracks, and incipientalteration to muscovite flakes. Sodalite is prominent as large interstitial crystals.

Aegirine-augite builds irregular prismatic crystals, many of which havecores of greenish-purple titanaugite. Average optical properties follow:-

2Vyyy:c

pleochroism {

Core

59°1·747 ± ·003

51°a greenish-purplef3 pale purpley pale yellow-purple

Margin

75°1·760 ± ·003

66°grass greenpale yellow-greengreenish-yellow


Some smaller crystals approach the composition range of aegirine, havingoptic axial angles as high as 90° (Sabine, 1950) and others show slight alterationto deep green ferrohastingsite.

Alkali amphibole occurs as large zoned idiomorphic prisms, often twinnedon (100). Strong absorption reduces the accuracy of optical measurements :-

Core Margin

2Vex 76° ± 3° 35° ± 5°a 1·682 ± ·002 1·705 ± ·003y 1·705 ± ·003 1·725 ± ·004y:c 20° 19°

r ex pale yellow yellow-greenpleochroism ~ f3 deep reddish-brown deep olive-green

l y deep reddish-brown deep brown-green

The optic axial angle drops as low as 24° in the green margins of somecrystals, and the optic axial plane is always parallel to (010). The amphiboleis thus a lllember of the hastingsite group as defined by Billings (1928), thezoning extending from brown femaghastingsite to green ferrohastingsite(op. cit., p. 294).

Idiomorphic sphene is a constant and prominent accessory lllineral, some-crystals showing advanced alteration to ilmenite or leucoxene accompanied bycalcite. Primary iron ore is not abundant.

(d) Minor Intrusives

The intrusive rocks described above are traversed by a swarm of narrowdykes which are well exposed in the south-eastern sector of the complex, andtheir presence elsewhere can be inferred from the abundant boulders ofcharacteristic rock types in screes. vVith the exception of the tinguaites, whichseem to be confined to the vicinity of vVitrand and Okonjeje Berg, the dykesshow a pronounced radial distribution within the complex. Most of the radialdykes are dark alkali lamprophyres, comprising camptonites, monchiquites,alnoites, melanephelinites and allied types. The remainder are pale colouredbostonites, one of which was observed to cut a monchiquite dyke south ofStanley, and the acid dykes already described as being genetically related to theTholeiitic Series. The mutual age rela.tionships of the other dykes is unknown,but all appear to postdate the lllain intrusive bodies.

In addition to the dyke swarm there occurs a particularly interestingstock-like body of differentiated camptonite in the Auas valley, and a volcanicpipe filled with brecciated igneous rock which pierces the ridge gabbros southof Stanley.

(i) SodaUte :l'ingvaitc

Two types of tinguaite can be recognised in the field-a dull green aphaniticvariety which breaks with subconchoidal fracture and carries small phenocrystsof nepheline, felspar and acicular amphibole; and a holocrystalline type, lightgrey in colour and fine-grained with occasional felspar phenocrysts. The lattercan be further classified as trachytoidal or stellate according to texture.


The trachytoidal variety of the holocrystalline tinguaites is the more commonand a representative specimen (Ok. 42) has been analysed (Table II). IJargetabular phenocrysts of perthitic orthoclase up to 5 mm. in length are set in agroundmass composed chiefly of alkali felspar laths which have a pronouncedfluxional arrangement. Albite builds occasional small twinned laths, but mostof the albite present occurs interstitially and as prominent vein-like bodies inpotash felspar crystals. Sodalite exceeds nepheline in amount, and both formirregular crystals scattered through the groundmass. Aegirine-augite (2Yy = 80°,y = 1'758, y: c = 70°) builds small allotriomorphic crystals which occasionallyenclose cores of pale pink titanaugite. Deep green ferrohastingsite exceedsaegirine-augite in amount and forms irregular poikilitic crystals nloulded ongroundmass felspar. Optical properties as follows: 2Va = 20° :1::, a = 1·726 ±'004, y: c = 16° ± 3°; pleochroism a = pale yellow-brown, f3 = deep browngreen, y = deep (bluish) green; dispersion r>v (very strong). Prominentaccessory minerals are rounded grains of iron ore, sphene and apatite, togetherwith occasional interstitial fluorite and calcite. Always present in the groundmass, and often poikilitically enclosing felspar laths, are skeletal crystals of acolourless to very pale yellow mineral with low birefringence and showingmultiple twinning on (100). The crystals are commonly elongated parallel tothe c axis and have a well-defined (100) cleavage. The optic axial plane isparallel to (010) with 2Vy = 70° ± 5°, a: C = 11°, a =1,655, y = 1,667, andmoderate axial dispersion r> v. These optical properties compare closely withthose given by Fersman (1926, pp. 295--':299) for the cerium-rich mineral rinkolite,but the identification is not regarded as positive.

The stellate variety of the holocrystalline tinguaites is represented by onlyone specimen (Ok. 189) collected from a dyke traversing marginal acid rocknorth of Adams Shoulder. The chief element of the rock fabric is albite, whichbuilds slender twinned laths arranged in criss-cross fashion and forming roughlystellate groups (Plate XXVII, Fig. 4), with a slight tendency toward fluidalarrangement around the rare phenocrysts of orthoclase. Orthoclase cryptoperthite, together with sodalite and subordinate nepheline, is always interstitialto the albite laths. Aegirine is the dominant ferromagnesian as numerous smallcrystals with the following optical properties: 2Va = 85° ±:, a = 1'728,y = 1,770, y: c = 79°; pleochroisnl a = f3 = deep green, y = yellow-brown.Intensely pleochroic biotite (a =, yellow-brown, f3 = )' =: very deep brown,almost opaque) accompanies aegirine to the exclusion of alkali amphibole.Accessory minerals are skeletal iron ore, sphene, apatite, and inconspicuouscrystals of the mineral referred to rinkolite.

In spite of differences in mineralogy and texture, the two types of holocry stalline tinguaite are very closely similar in chemical composition (Table II),and provide an excellent example of a heteromorphic pair. The contrastedfelspar phase relationships in the two rocks are of interest when considered inthe light of recent work on the equilibrium relations of the alkali felspars(Bowen and Tuttle, 1950; Tuttle, 1952).

The green aphanitic tinguaites are porphyritic with occasional idiomorphicphenocrysts of nepheline, orthoclase and deep green ferrohastingsite, and raremicrophenocrysts of sphene. The groundmass is composed of a felted mass of


tiny aegirine-augite needles and sparse octahedra of iron ore embedded in alk81ifelspar, nepheline and sodalite. The chemical composition of a typical specimen(Ok. 41) is given in Table II.

(ii) Bostonite

Dykes of bostonitic character are not common and were found only in thesouth-eastern sector of the complex. The rocks are light grey or pale brown incolour and usually porphyritic. The felspar phenocrysts occur singly or inglomeroporphyritic groups and generally show clear cores of sodic plagioclasemantled by clouded orthoclase. The groundmass is trachytic and composeddominantly of small laths of clouded orthoclase and sometimes a little clearalbite. Interstitial quartz is usually present in small amounts to the exclusionof felspathoids. One specimen carries rare clinopyroxene phenocrysts withnarrow green margins, together with granular aegirine-augite and rathernumerous small fibrous crystals of blue pleochroic riebeckite in the groundmass.Although quartz-bearing, these rocks clearly have alkaline affinities.

In the Korn-Stanley area the core gabbros and ridge gabbros are traversedby an irregular network of veins and stringers of a fine-grained pink-weatheringrock. It is composed of small twinned crystals of orthoclase microperthite,scattered grains of aegirine-augite, spongy iron ore and a little sphene. Bothquartz and felspathoids are absent and the rock is strongly reminiscent of theOslo solvsbergites, the name applied by Bragger (1894 a) to the intermediatemembers of his tinguaite-grorudite series.

(iii) Carnptonite

Narrow dykes of melanocratic eamptonite are of common occurrence. Mostcarry idiomorphic phenocrysts of brown alkali amphibole and zoned titanaugite,both of which recur in the groundmass together with iron ore, yellow sulphidesand apatite. These dark constituents are embedded in an altered mass of zonedplagioclase laths (Anao-An30), alkali felspar, and usually small anlOunts ofnepheline, sodalite, analcite and calcite. Small ocellar patches of variable shapeare generally filled with albite laths, analcite, calcite and occasionally epidote,in that order of crystallization.

The most interesting occurrence of camptonite is the small stock-likedifferentiated body which outcrops as a bouldery ridge at the northern end ofAuas valley. The two low summits of the ridge are composed of pitted bouldersof a pale greyish rock, while angular blocks of melanocratic camptonite form theoutcrop around the base and between the two summits, covering an area ofapproximately 100 by 150 metres. The two rock types are very strongly contrasted both in the field and in their microscopic and chemical characteristics.Gradational types are rare, but occasional veins and schlieren of the pale rockare found in the camptonite. Unfortunately no outcrops of rock in sit'u, werefound, but the camptonite carries occasional small xenoliths of core gabbro,and both field and microscopic evidence suggest the differentiation of an intrusivebody of originally homogeneous camptonitic magma by crystal settling in situ.

The dark basal rocks have the typicallamprophyric panidiomorphic texture(Plate XXVIII, Fig. 1), but differ from normal camptonites in carrying up to10 per cent. of felspathoids. The most prominent constituent is brown alkali


amphibole (probably femaghastingsite) as idiomorphic zoned crystals withgreen margins and commonly twinned 011 (100). The optic axial plane is parallelto (010) and measured optical properties follow:-

2Vaayy:c

pleochroism }

Core

79°1·6741·702

12°a very pale yellow13, y yellow-brown

Margin

16°very pale yellowgreenish-brown

Colourless to pale green titanaugite crystals show prominent zoning andvery strong inclined dispersion. :Marginal corrosion and alteration to amphiboleor biotite is often evident. Scattered irregular phenocrysts of magnesian olivine(Fa12) are sometimes zoned with more iron-rich narrow margins (Fa20). Biotiteis evenly distributed as small independent flakes which are very stronglypleochroic from -very pale yellow to deep brown. Rounded grains of iron oreand brassy pyrrhotite are present in accessory amounts.

The light constituents rnake up about 30 per cent. of the rock which ise,'idently nmch enriched in dark minerals compared with the correspon,dingdyke rocks. Plagioclase is the chief light mineral and builds strongly zonedlaths (An65--An30), sometimes mantled by more sodic plagioclase (An20~Anl0)

and orthoclase. Occasional rounded patches in the rock are filled with smalldisoriented laths of zoned labradorite. Nepheline, sodalite and orthoclase occuras interstitial crystals moulded on plagioclase and dark minerals. Small amountsof interstitial calcite and analcite are usually present, together with numerousslender prisms of a]Jatite.

The overlying greyish rock of contrasted composition (Plate XXVIII,Fig. 2) carries a maximnm of only 25 per cent. of dark rninerals, which consistof rare phenocrysts of olivine, together with ragged cry stals of brown anlphiboleand titanaugite with green margins. The groundmass is composed largely ofaltered alkali felspar and twinned laths of albite-oligoclase. Sodalite is rare andinterstitial nepheline shmvs advanced alteration to aggregates of muscovite andhydronepheline. Irregular flakes of muscovite are quite abundantly scatteredthrough the rock. Analcite and calcite are prominent in interstitial areas, thelatter occurring also as a COITtlllOn alteration product. Apatite is rresent onlyin small amounts.

These greyish rocks resemble the gauteites deseribed and named by Bibsch(1897, pp. 84--86) from the Bohemian l'.fittelgehirge where they appear to beara complementary relation to dykes of camptonite. Similar conclusions werearrived at by Bragger (1894 b, pp. 23-31) who observed the close associationof carnptonite and bostonite (later renamed lime-bostonite or nmenite) in theGran district of Nonvay, and deduced that both ,vere derived from 9J parentolivine-gabbro-diabase magma by differentiation. The direct association ofcamptonite and gauteite at Okonjeje, together with their strongly contrastedchemical compositions (Table II) affords a striking confirmation of the suggestionsput forward by Hibsch and Bragger.


(it') Alnoite

Several dykes of dark grey alnoite with numerous biotite }Jhenocrysts werefound, though some differ from the analysed specimen (Ok. 85) in not carryingmelilite or garnet.

The analysed rock (Table II) is a typical alnoite (Plate XXVIII, Fig. 4).Biotite phenocrysts are abundant and show the weak pleochroism (almostcolourless to pale orange) and very small optic axial angle consistent with a highlymagnesian phlogopitic composition. Irregular crystal margins show even weakerabsorption and belong to the groundmass I)eriod of crystallization. Irregularphenocrysts of wagnesian olivine (Fa13) and zoned titanaugite are sparselydistributed.

The earliest groundmass minerals are numerous small octahedra of iron oreand perovskite, the latter occasionally showing anomalous birefringence. l\felilitecrystallized before groundmass biotite and builds small tabular crystals sometimes showing peg-structnre and alteration in patches to highly birefringentfibrous cebollite associated with carbonate aggregates. The crystals are stronglyzoned, and the discrepancy between modal and normative nepheline in thisrock Ruggests the presence of a certain amount of soda-melilite in solid solution.Nepheline is present as small clear crystals with a tendency towardsidiomorphism. Sparse irregular crystals of wollastonite are characteristicallytwinned on (100). Pale yellow or colourless garnet is a prominent constituentof late crystallization as subidiomorphic spongy crystals showing weak birefringence and sector twinning, and also as scattered granular aggregatesassociated with calcite which are possibly derived from the alteration of meliliteas suggested by von Eckermann (1948, p. 96). Abundant carbonate as aggregatesand interstitial patches nlake up the remainder of the rock.

(v) .J..7J:1. elanepheliniteThree specimens collected from dykes in the Stanley area are Inegascopieally

similar to some of the weathered alnoites, but correspond more closely to themelanephelinites as defined by Johannsen (1938, p. 363) and described byKing (1949, pp. 26-33).

Pale golden-yellow phenocrysts of mica are abundant and are accompaniedby numerous subidiomorphic crystals of pale green titanaugite ,vith colourlessor pale pink margins. Phenocrysts of magnesian olivine and rounded nephelineare occasionally seen in thin section. These, together with scattered microphenocrysts of iron ore and apatite, are set in a groundmass consisting essentiallyof small subidiomorphic nepheline crystals (which contain acicular inclusionsof pyroxene and apatite, and much of which is altered to analcite, fibrouszeGlites, calcite and occasionally white mica) separated by fine aggregates ofbiotite, apatite and acicular pyroxene (Plate XXVIII, Fig. 3). Intt'Tstitial andsecondary calcite is abundant, and sphene has been recognisell as a rare aceessory.

The analysis of a typical specimen (Ok. 54) is given in Table II.

(vi) N epheUne rnonchiquiteThe dyke rocks most frequently encountered in the ICorn-Stanley a,rea are

dark compact types which usually carry phenocr~vsts of olivine, pyroxene andbiotite, and chara,cteristic white ocelli.

K


Phenocrysts of colourless magnesian olivine (Fa10-Fa1S) are rounded,irregular or sharply idiomorphic, and are generally surrounded by a narrowfelt of biotite flakes. Colourless or pale lilac titanaugite phenocrysts tendtowards idiomorphism, though the margins of some are highly irregular andbelong to the groundmass period of crystallization. Zoning and the effects ofstrong dispersion (r>v) a.re very prominent. The occash,nal laJrge crystals ofbiotite may well be xenocrysts since they invariably have rounded dark browncores surrounded by narrow orange-brown nlantles similar to groundmassbiotite. J\ficroplJenocrysts of olivine, titanaugite, biotite and sometimes iron oreand apatite are genera,lly present.

The dense fine-grained groundmass is seen in very thin sections to consistof innumerable interlocking needles of pyroxene, tiny octahedra of iron ore andscattered poikilitic flakes of biotite embedded in a colourless base rich in calcite,isotropic analcite and weakly birefringent nepheline.

Hounded or irregular ocelli are associated with patches in the groundmasswhich are somewhat coarser in grain than usual. Acicular crystals of greenishpyroxene project from these coarser patches into the ocelli which are largelyfilled with calcite and fibrous zeolites. The ocelli thus seem to represent pegmatoidsegregations and are by no means as sharply defined as those generally describedfrom lamprophyres (cf. Knopf, 1936, pp. 1740-1741). In the rocks under discussion they rarely exceed 5 mm. in size and often appear as microscopic schlierenwhich grade into the gronndmass.

Follo\\dng vVilliams (1936, p. 153) the term nepheline monchiquite has beenadopted for these rocks and the analysis of a typical specimen (Ok. 119) appearsin Table II.

(1:ii) Pipe Breccia,About 700 metres south of Stanley near the Kazombaruru river bed is a

circular outcrop, some 75 metres in diameter, of dark tuffaceous materialcontaining abundant rounded xenoliths of gabbro and xenocrysts of olivine,pyroxene, amphibole and biotite. No contacts are exposed, but the field evidenceis plain enough to suggest that this outcrop is the eroded remnant of a volcanicpipe drilled through ridge gabbros and underlying rocks.

The gabbro xenoliths are all remarkably fresh and each one examinedmicroscopically can be matched with the core gabbros, none being recognisedas of ridge gabbro origin. A small xenolith of melteigite was found in one thinsection, and consists of titanaugite crystals rimmed with aegirine-augite set ina base of sodaIite with minor amounts of nepheline, calcite and granular ironore. One dark xenolith of brown alkali amphibolite was found, and mostabundant of the microscopic xenocrysts is a sjmilar pale brown ,pleochroic alkaliamphibole, like that found in the Okonjeje Berg pulaskite. Particularly conspicuous in the field are numerous xenocrysts of pyroxene which vary in sizeup to about 7 cms. The pyroxene is pleochroic in shades of green and is COIIt

pletely free from zoning, alteration and inclusions, except for occasional irregularmasses of associated black spinel which is virtually opaque in thin section butgives an X-ray powder pattern identical with that of the iron-aluminium spinelhercynite. The chemical composition and optical -properties of the green pyroxeneare given in Table III.

THE OKON.JEJE IGNEOUS COMPLEX 159

The tuffaceous matrix in which these foreign fragments are embedded iscomposed largely of sericite, serpentine and carbonate aggregates dusted withiron ore and comminuted fragments of altered ferromagnesian minerals.Interstitial analcite and zeolites can often be recognised.

(e) Relationships of the Alkali Rocks

(i) OreIer of Tnfr~(sion

It is quite clear that the core gabbros are the oldest exposed rocks of theAlkali Series and possibly the Adams Shoulder gabbros represent a separatebasic intrusion prior to the emplacement of the alkali rocks of Okonjeje Berg.Of the la,tter, the outer ring of pnla,skite is clearly the oldest, followed in turnby the andesine-essexite and oligoclase-essexite. The 'Vitrand foyaiteR areyounger than the core gabbros, but their age relative to the Okonjeje Berg rocksis unknown. Fin~Jly the intrusion sequence was terminated by injection of thedyke rocks.

(if) Mineralogy

Olivine is relatively magnesian (Fa27-Fa35) in the normal core gabbros,becoming more iron-rich in the nepheline-bearing varieties (Fa37-Fa46) and inthe andesine-essexite (Fa6cFa73L It is absent from all later rocks excel't thelamprophyres in which it is highly magnesian (Fa10-Fa20).

Clinopyroxcnes are titania-rich diopsidic types showing a steady inCl'easein the Fe/Mg and LiIMg ratios in the series core gabbro-nepheline gabbroandesine-essexite, which suggests that these rocks represent successive stagesof differentiation (Strock, 1936, p. 196). Aegirine-augite makes its first appearancein the foyaites and passes into the aegirine range of composition in the tinguaites.In general the pyroxenes show a crystallization trend as follows: diopsidicpyroxene --+ "titanaugite" --+ "soda-diopside" --+ aegirine-augite--+aegirine. A tendency toward alteration to hastingsitic amphibole of appropriatecomposition is evident at each stage. Orthopyroxene is consistently absent.

Brown magnesian hastingsite occurs as the alteration product of pyroxenein the gabbroic rocks and andesine-essexite, but becomes the dominant ferromagnesian in the oligoclase-essexite where the higher refractive indices, loweroptic axial angle and greenish tinge indicate enrichment in iron. In the foyaitesthe amphibole is zoned to deep green ferrohastingsite, becoming even moreiron-rich in the tinguaites.

Calcic plagioclase is strongly zoned in all the gabbroic rocks, becomingprogressively more sodic in the essexites. I-lime-bearing plagioclase is hardlypresent in the pulaskites and absent from the foyaite. Independent alkalifelspar first appears in the Adams Shoulder gabbros and thereafter increasesprogressively in amount as the plagioclase becomes more sodic, reaching itsmaximum proportion in the pulaskite, but dominant also in the foyaite- tinguaitesuite.

Nepheline, usually accompanied and sometimes exceeded by sodalite, makesits first appearance in the Adams Shoulder gabbros, thereafter increasing inall later rocks except the pulaskite and lamprophyres. Analcite is always


interstitial and, together with late albite and a little calcite, probably representsthe hydrothermal stage of crystallization in each rock type when volatileconstituents appear to have been relatively abundant, particularly in theoligoclase-essexite, pulaskite and foyaites.

i lSi is IIi Sj i8 liS Ij2 57 47 li3 f 4,1I I 189,

I ISJ(

0

~" " P2 Os0 ~-.--...o • •

J

)C. Ti02

~XI(

-<>---..I 0

,,-.~ -K

20 • • •><

)( 0 ~

x "x-

•No2 O •-0 '''------0

,-0--

~x-xl

~ • •~

~X----a -5 I(X.

A120~x

• •~ x •

"x

" Si02x

..~,,---x xl( 0- •F¢O + Ff:2 03

0 x--e--o •>< MgO

0 o l( xx

0-~ X --.--0 .. -

x

0---CoO

0

" "x_n

I ~x~ • ·10

10

5

10

5

50

o

64

2

2

20I

8

6

4

2

20

40

20 30 40 50 60

FELSIC INDEX70 80 90

FIG. 7.

Felsic variation diagram for the Alkali Series. Open circles-" normal" differentiationseries (core gabbros, essexites, pulaskite): black circles-foyaite-tinguaite suite: crosses

lamprophyres and gauteite. For index to rock numbers, see explanation of Table II.


(iii) Chetnical Fariation

In the linear variation diagram (Fig. 7) the major oxides have been plotted. .. (Na20 + K 20) X 100

agamst the felsIC mdex CON' 0 K 0 (Simpson, 1954) and show a veryJa + a2 + 2

nearly linear variation through all the rock types except the lamprophyres.'Vhen plotted against the mafic index, however, the oxides show no approachto regular variation. Assuming then, that crystallization differentiation hasbeen the main operative factor, it may be inferred with reasonable certaintythat felsic fractionation has dominated over mafic in the development of analkaline line of descent. Both the FMA and l\f-F diagrams (Figs. 8 and 10)emphasize the fact that the variation trend of the Alkali Series is not one ofsignificant iron-enrichment, especially when compared with the Tholeiitic Series.

Both major and trace constituents show a rough trend of increasingalkalinity and decreasing basicity which can be traced from the core gabbrothrough the Adams Shoulder gabbro, andesine-essexite, oligoclase-essexite tothe pulaskite. The same trend is apparent in the chief ferromagnesian mineralsin this rock series which, moreover, produces smooth variation curves on bothFig. 8 and Fig. 10. The foyaite-tinguaite suite, however, show a marked departurefrom this trend (Figs. 8 and 10) but nevertheless make up a related series at tilehigh alkali end. The sudden alkali increase in the foyaite is accompanied byapparently anomalous increases in vanadium, strontium and rubidium, and asudden decrease in barium in spite of the high potash content. These differencesare to some extent accentuated by the tinguaites in which barium and strontiumare very low, and rubidium, zirconium, yttrium and lanthanum very high,suggesting that these rocks represent residual magmas (cf. von Engelhardt,1936, pp. 214-215; Nockolds and J\.Iitchell, 1948).

vVhen the alkali rocks with more than 70 per cent. normative salic mineralsless anorthite (oligoclase-essexite, pulaskHe, foyaite, tinguaites and gauteite)are plotted on the nepheline-kaliophiJjte-silica diagram (Fig. 9), all but thefoyaite fall within the low temperature trough towards which all liquids in thissimple system must move as differentiation proceeds (Bmven, 1937, pp. 11-21).With the possible exception of the foyaite the Flalic members of the alkali seriest,hus conform remarkably ,yell "ith the chemical requirements of a series derivedhy dominant fractionation of alkali felspars.

In spite of their very basic character, the felspar-free lamprophyres areehal:aeterizeil by high alkalis, barium and rubidium, and the failure of theserocks to fit the linear variation trends of the alkali series as a whole (Fig. 7)su~gests an accumulative or extraneous origin. The contrast in chemicalcomposition between the camptonite and its gauteite differentiate is verymarked, both in major and trace constituents. Allowing for accumulativeferromagnesians, however, both these rocks fit in remarkably well with t,helinear variation of the alkali rocks as shown in Fig. 7.

(iv) Petrogenesis

The mineralogical and chemical evidence available suggests that the alkaliseries has been developed by proeesses of differentiation involving dominantfelsic. fractionation) and leading to the following genetic series: core gabbros--3>-


90

50

...-./

//

//

//

II

,II

I/

//

!

------

Fig. 10

Q

/-_.. /',/

"/

,,''"'"'"'"

20 30 50 60 70FELSIC INDEX (F)

80 90

FIGs. 8, 9 and 10..FMA, " residua system" and M-F diagrams for the Alkali Series (symbols as for Fig. 7).

Data for the Tholeiitic Series (upper broken curve) included for comparison.

:andesine-essexite-~oIigoclase-essexite-~pulaskite. The nlechanism of the·differentiation is not known, but fractionation of fe]spars, however strong, fail sto account for the very evident degree of undersaturation leading to the developrnent of felspathoids. Admittedly, the core gabbros (whieh presumably approachthe parent basic magma in composition) arf~ undersaturated, but not significantlymore so than the basic members of the Tholeiitic Series in whieh differentiation-has produced oversaturated rocks.


It is clear then, that considerable desilication has been effected during themiddle and later stages of differentiation of the alkali series and the effects arepronounced in all these rocks except the pulaskite. Of all the theories putforward to explain rHagmatic desilication with the consequent production offelspathoids, only two are pertinent to the present discussion-the incongruentnlelting of orthoclase (Bowen, 192R, pp. 240-257), and limestone assimilation.

Although there is no direct evidence in favour of the latter mechanism, theIJOssibility cannot be entirely dismissed since an adequate supply of crystallinelimestone is available in the surrounding rocks of the Damara System, thenearest outcrop lying some 15 kilometres to the south-east. The nlelilitenepheline-wollastonite mineral assemblage of the alnoites may well be regarded:as evidence for limestone assimilation since these are l)recisely the significantndnerals produced by surh a process at Scawt Hill (Tilley and Harwood, 1931),Iron Hill, rolorado (Larsen, 1942) and elsewhere (Shand, 1947, p. 301)~ and soadmitted by Ro" en (19M>, p. 86).

In considering the incongruent melting of orthoclase and reaction of earlyformed leucite with liquid to produce undersatnration, it is interesting to recallthe peculiar rounded white spots in the andesine-essexites. These are at first:sight strongly reminiscent of pseudoleucite, but the presenre of plagioclase inaddition to orthoclase and nepheline within them is puzzling. Furthermore itis now well known that the pseudoleucite reaction is inhibited by pressure andconsiderable doubt has been thrown on this mechanism as applied to the development of intrusive undersaturated rocks.

The foyaite and tinguaites differ geochemica,1ly and mineralogicall;r fromthe well-defined gabbro-essexite-pulaskite series; whkh suggests that otherprocesses have influenced the derivation of this suit,e. ~rhe occurrenre of localizedpegmatitic facies in the foyaite and the alnmdance of zirconium and lanthanidesin these rOl3ks suggest the presence of a high proportion of vola,tile constituents,and it may be significant that the extrapolated trend line of the foyaite suitein Fig. 10 joins up with the rnain trend at a position corresponding with theandesine-essexite, i.e. at a stage in the nlain series when undersaturation seemsto have reached a maximum.

It appears that the nlain differentiation trend has split into two branchesat about this point, leading in the one ease to highly undersaturated residualliquids corresponding to the foyaite-tinguaite suite owing to an abundance ofvolatiles, and in the other to a " normal" differenti8Jtion series ending in theproduction of pulaskite which reflects the compoRition of the parent basic magmain its low degree of undersaturation.

In the absence of direct evidenee it thus appears that the development ofan alkali undersaturated trend of differentiation has been influenced by initialundersaturation of the parent basic magma and abundant volatile constituents,.aided perhaps by long-continued fractionation.

VI. CONCLUSION

A comparison between the chilled core gabbro (Ok. 36) and the basicrepresentatives (Ok. 7 and Ok. 1) of the Tholeiitic Series reveals a close correspondence in chemical composition, accolllpanied by significant under- and


oversaturation in the forrrter and the outer ridge gfl"bbro (Ok. 1) respectiYely.These two rock types are the typical basic repreHentatives of their respectivedifferentiation series, throughout each of ,Yhich the initial under- and oversaturation is characteristically rilaintained. The same close correspondence inchemical composition and characteristic degrees of saturation are evident inthe pulaskite (Ok. 143) and ridg(j syenite (Ok. 66), and it appears that the twocontrasted lines of liquid descent fronl similar parent basie magmn,s have yieldedcorresponding normal late differentiates. In terms of the simple quaternarysystem nepheline-anorthite-silica--olivine -+- diopside (Barth, 1952, pp. 123-127)it is probable that the two contrasted series have deyeloped from basic magmaswhich differed in compo~itiononly to the extent that they fell on opposite sidesof the sa,turation houndary surface.

lt cannot be doubted that the two basic magmas originated frorn a commonparent, and the two are probably related in much the same way as the oversaturated outer ridge gabbro and the undersaturated inner ridge gabbro whichhas been relatively enriched in plagioclase and in this respeet bears a complementary relation to the gabbro-picrites as discussed earlier.

Analogous close associations of contrasted acid and alkaline series of relatedrocks possibly derived frolll. a common parent basic magma are well known inthe "Midland Valley of Rcutland (Carhoniferons-Permian), the Oslo region(Permian), and the Tertiary igneous provinces of Central l\'fontana, Britain andEast Greenland. As in the Damaraland provinee the rocks are of sub-volcaniccharacter, and t.he tectonic environrnents are characterised hy bloek-faulting,sometimes aceompanied hy local cauldron-subsidences, and a general abseneeof orogenic movements.

Elsew"here in the Damaraland petrographic province (see Fig. 1) the coexistence of alkaline and tholeiitic or acid rocL types has been established in thecomplexes at Cape Oross (Cevers, 1933), Meesunl (Rorn and :Martin, 1954) andto a lesser extent in the lavas of the Paresis 1\Tonntains. The same characteristicis exhibited by the province as a whole Eince somc of the complexes, Huch as theBrandbprg (Oloos and Ohudoba, 1931) and Erongo (Cloos, 1919), consistessentiaJIy of alkali granite, '''''hile alkali and ultra-alkali roeks predominate atOkorusu and Kalkfeld-Eisenberg.

J\lnny of the problems presented by this study of the Okonjeje complexmllst be considered. in the light of evidenee from these related centres wheninvestigations have been completed. It should then be possible to integratethe observ-ations made on the whole Damaraland petrographic province into asignificant contribution to the developn~ent of petrogenetic theory.

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LARSEN, E. S. (1942). Alkalic rocks of Iron Hill, Gunnison County, Colorado. U.S. Geol.Surv., Prof. Paper 197A.

MACGREGOR, A. G. (1931). Clouded felspars and thermal metamorphism. Min. Mag., 22,524-538.

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16, 140-144.BILLINGS, M. P. (1928). The chemistry, optics and genesis of the hastingsite group of

amphiboles. Amer. Min., 13, 287-296.BOWEN, N. L. (1928). The evolution of the igneous rocks. Princeton Univ. Press.------ (1937). Recent high-temperature research on silicates and its significance

in igneous geology. Amer. J. Sci., 233, 1-21.------- (1945). Phase equilibria bearing on the origin and differentiation of

alkaline rocks. Amer. J. Sci., 243A, 75-89.------ and SCHAIRER, J. F. (1935). The system MgO-FeO-Si02. Amer. J. Sci.,

230, 151-217.------ and TUTTLE, 0. F. (1949). The system MgO-Si02-H20. Bull. Geol. Soc.

Amer., 60, 439-460.------------- (1950). The system NaAlSiaOs-KAlSiaOs-H20. J. Geol.,

58, 489-511.BROGGER, W. C. (1894a). Die eruptivgesteine des Kristianagebietes. I-Die gesteine der

Grorudit-Tinguaite Serie. Skr. Videns.-Selsk. Kristiana, I Mat.-Naturv. Kl., No.4..------ (1894b). The basic eruptive rocks of Gran. Quart. J. Geol. Soc. Lond., 50,

15-38.CLOOS, H. (1919). Der Erongo. Beitr. Geol. Erf. Deutsch. Schutzgeb., 17.---- and CHUDOBA, K. (1931). Der Brandberg. Bau, bildung und gestalt der jungen

plutone in Sudwestafrika. (Beil.) N. Jb. Min. Geol. Palaont., 66B.DEER, W. A. and WAGER, L. R. (1939). Olivines from the Skaergaard intrusion,

Kangerdlugssuak, East Greenland. Amer. Min., 24, 18-25.ECKERMANN, H. VON (1948). The alkaline district of Alno Island. Sver. Geol. Und.,

Ser. Ca No. 36.ENGELHARDT, W. VON (1936). Die geochemie des bariums. Chemie der Erde, 10, 187-246.FERSMAN, A. E. (1926). Minerals of the Kola Peninsula. Amer. Min., 11, 289-299.GEVERS, T. W. (1932). Kaoko-eruptives and alkali rocks at Cape Cross. Trans. Geol. Soc.

S. Afr., 35, 85-96.------ and FROMMURZE, H. F. (1929). The geology of north-western Damaraland,

in South West Africa. Trans. Geol. Soc. S. Afr., 32, 31-56.HALLIMOND, A. F. (1943). On the graphical representation of the calciferous amphiboles.

Amer. Min., 28, 65-89.HAWKES, L. (1929). On a partially fused quartz-felspar rock and on glomerogranular

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pyroxenes, Part I. Amer. Min., 34, 621-666.----- (1952). Orthopyroxenes of the Bushveld type, ion substitutions and changes

in unit cell dimensions. Amer. J. Sci., 250A, 173-187.HIBSCH, J. E. (1897). Erlauterungen zur geologischen karte des bohmischen Mittel

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RAMBERG, H. and DE YORE, G. (1951). The distribution of Fe++ and Mg++ in coexistingolivines and pyroxenes. J. Geol., 59, 193-210.

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SABINE, P. A. (1950). The optical properties and composition of the acmitic pyroxenes.Min. Mag., 29, 113-125.

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TUTTLE, O. F. (1952). Origin of the contrasting mineralogy of extrusive and plutonicsodic rocks. J. Geol., 60, 107-124.

WAGER, L. R. and DEER, W. A. (1939). Geological investigations in East Greenland.Part III-The petrology of the Skaergaard intrusion, Kangerdlugssuak, East Greenland. Medd. om Gronland, 105, No.4.

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WALKER, F. and POLDERVAART, A. (1949). Karroo dolerites of the Union of SouthAfrica. Bull. Geol. Soc. Amer., 60, 591-706. .

WELLS, M. K. (1951). Sedimentary inclusions in the hypersthene-gabbro, Ardnamurchan,Argyllshire. Min. Mag., 29, 715-736.

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DEPARTMENT OF GEOLOGY,

UNIVERSITY OF CAPE TOWN,

CAPE TOWN, C.P.Accepted for publication by the Society

on 7th August 1954.

THE OKONJEJE IGNEOUS COMPLEX

EXPLANATION OF TABLES

167

s Sensitivity.* Present in amounts below sensitivity of the method used.

Chemical analyses by author and expressed as weight percentages of the oxides;spectrographic analyses by S. R. Nockolds and R. S. Allen and expressed as parts permillion of the elements.

The following elements are not present in amounts above the sensitivity limit whichis indicated in brackets: Sn (10), In (2), Cs (10).

The values for Cl are for acid-soluble chlorine only. Values for F were determinedspectrographically.

TABLE I.

110 Marginal gabbro-picrite, Black Ridge.100 Ungranulitized equivalent of granulitic gabbros from centre of a xenolithic body,

200 metres north-west of wells.93 Granulitic gabbro near margin of a xenolithic body, 700 metres south-west of wells.

168 Ophitic granulitized gabbro from ridge gabbro xenoliths in marginal acid rock onwestern slopes of Johannes Berg.

7 Inner ridge gabbro, 500 metres north of wells, adjacent to core gabbros.1 Outer ridge gabbro, 100 metres east of wells.

106 Middle ferrogabbro, 400 metres west of Black Ridge.109 Outer ferrogabbro, 250 metres west of Black Ridge.

66 Ridge syenite, centre of arcuate dyke.185 Marginal acid rock, Zebra Kop.112 Ferrogabbro xenolith in marginal acid rock, 300 metres south-west of Black Ridge.

TABLE II

36 Chilled marginal facies of core gabbro, Stanley.158 Core gabbro (Adams Shoulder type), Adams Shoulder.195 Andesine-essexite, east slope of Okonjeje Berg.122 Oligoclase-essexite, summit of Okonjeje Berg.143 Pulaskite near centre of outer ring, south slope of Okonjeje Berg.

47 Foyaite from eastern outcrop, Witrand.189 Stellate sodalite-tinguaite from dyke in marginal acid rock north of Adams Shoulder.

42 Trachytoidal sodalite-tinguaite from dyke in core gabbro, 400 metres north ofWitrand.

41 Aphanitic sodalite-tinguaite from dyke in core gabbro, 500 metres north of Witrand.58 Camptonite from exposed base of intrusive body, Auas valley.57 Gauteite from exposed summit of intrusive body, Auas valley.85 Alnoite from dyke in ferrogabbros, 400 metres east of wells.54 Melanephelinite from dyke in core gabbro, Auas valley.

119 Nepheline monchiquite from dyke on south-east slope of Stanley.

TABLE III.

A Clinopyroxene (ferroaugite) from ridge syenite (Ok. 175).B Hornblende from analysed marginal acid rock (Ok. 185).C Clinopyroxene from normal core gabbro (Ok. 147).D Clinopyroxene from analysed Adams Shoulder gabbro (Ok. 158).E Clinopyroxene from analysed andesine-essexite (Ok. 195).F Amphibole (femaghastingsite) from oligoclase-essexite (Ok. 145).G Pyroxene xenocryst from pipe-breccia.

TABLE I (a) THOLEIITIO SERIES-OHEMIOAL AND SPEOTROGRAPHIC DATA

s 110 100 93 168 7 1 106 109 66 185 112

Si02 - 45·86 44·37 49·17 49·83 46·02 47·85 45·78 58·02 57·46 63·78 58·08 Si02Ti02 - 1·10 1·00 1·29 1·18 0·85 1·47 3·24 1·50 1·38 0·98 1·78 Ti02Al20 3 - 10·27 9·12 13·95 17·75 22·30 16·84 14·29 15·88 18·50 15·05 15·12 Al20 3Fe20 3 - 0·47 1·15 3·81 0·59 0·97 2·70 2·74 0·95 3·12 1·29 1·30 Fe20 3FeO - 11·30 12·81 6·50 10·14 6·43 8·50 13·35 8·51 4·80 3·67 7·73 FeOMnO - 0·16 0·19 0·17 0·10 0·11 0·19 0·27 0·25 0·04 0·06 0·11 MnOMgO - 18·69 20·21 11·48 7·34 6·84 6·66 4·41 1·50 0·99 1·29 1·78 MgOOaO - 8·74 9·08 10·12 10·16 13·27 11·97 8·74 4·70 3·68 3·18 4·68 OaONa20 - 1·91 1·42 2·48 2·13 2·37 2·54 3·08 3·99 4·86 4·91 4·52 Na20K 20 - 0·48 0·35 0·77 0·77 0·42 0·75 1·66 3·25 4·63 4·50 3·71 K 20H 2O+ - 0·39 0·15 0·26 0·11 0·28 0·10 0·37 0·19 0·10 0·44 0·25 H 2O+H 2O- - 0·12 0·30 0·10 0·05 0·14 0·36 0·08 0·41 0·22 0·12 0·13 H 2O-P 20 5 - 0·31 0·08 0·16 0·16 0·25 0·27 2·22 0·79 0·38 0·69 0·98 P 20 5

- 99·80 100·13 100·26 100·31 100·25 100·20 100·23 99·94 100·16 99·96 100·17

Ga+3 5 20 25 20 25 30 25 25 30 25 20 30 Ga+3Or+3 2 1,500 2,100 1,250 250 30 100 * * * 15 5 Ort-3V+3 5 250 250 300 250 100 200 250 20 * 70 100 V+3MoH 5 * * * * * * * * * * * MO+4LiH 1 25 25 40 15 10 20 50 100 30 80 75 Li+1

Ni+2 3 1,100 1,000 500 250 125 100 3 * * 10 7 Ni+200-t-2 5 120 110 80 100 60 70 30 * * 5 10 00+2Sc+3 10 * 10 30 20 * 40 * 20 * * * Sc+3Zr+ 4 10 90 60 80 200 30 60 120 200 90 250 200 Zr+ 4y+3 15 15 * 25 25 * 15 25 55 25 40 45 y+3La+3 50 * * * * * * * 50 50 100 50 La+3Sr+2 10 250 300 250 400 1,000 800 700 700 400 500 500 Sr+2Pb+2 10 * * * * * * * 10 * 10 * PbHBa+2 10 50 50 200 350 300 600 1,000 2,000 5,000 1,500 2,000 Ba+2Rb+1 10 20 * 30 25 * 25 150 300 250 250 400 Rb+1

TIH 1 * * * * * * * * * * * TI+l

Sp. Gr. - 3·20 3·13 3·04 3 ·0,9 2·92 3·02 3·09 2·83 2·76 2·70 2·84 Sp. Gr.

(FeO +Fe20 3) X10038·6 40·8 47·3 59·4 52·0 62·7 78·5 86·4 88·9 79·4 83·5

(FeO +Fe20 3) X100MgO + FeO + Fe20 3 MgO + FeO +Fe203(Na20 +K20) X100

21·5 16·3 24.31 22·2 17·4 21·6 35·0 60·7 72·3 74·8 63·9(Na20 + K 20) X100

OaO + Na20 + K 20i

OaO + Na20 + K 20

TABLE I (b) THOLEIITIO SERIES-NORMATIVE AND MODAL DATA

110 100 93 168 7 1 106 109 66 185 112

Q · . ·. · . - - - - - - - 6·66 1·80 10·50 4·44or · . · . · . 2·78 1·67 4·45 4·45 2·22 4·45 10·01 19·46 27·24 26·69 21·68ab · . · . · . 15·72 9·43 20·96 17·82 13·62 21·48 25·68 34·06 40·87 41·39 38·25an · . · . · . 18·07 17·51 24·46 36·97 49·21 32·25 20·29 15·57 15·01 5·56 10·01ne · . · . · . 0·28 1·42 - - 3·41 - - - - - -ra · . · . · . 9·86 11·14 10·32 5·22 6·73 10·79 4·41 1·16 0·35 2·55 3·13

di en · . · . · . 6·50 7·30 7·50 2·70 4·00 6·10 1·70 0·30 0·10 1·10 0·90fs · . · . · . 2·64 3·04 1·85 2·38 2·38 4·22 2·77 0.92 0·26 1·45 2·24

h fll · . · . · . - - 9·00 11·10 - 1·00 3·40 3·40 2·40 2·10 3·60y fs · . · . · . - - 2·11 9·77 - 0·53 5·28 11·88 3·70 2·77 7·92I fo · . · . · . 28·14 30·24 8·54 3·22 9·10 6·58 4·08 - - - -

o fa · . · . · . 12·44 13·87 2·24 3·26 5·71 5·10 7·24 - - - -mt · . · . · . 0·70 1·86 5·57 0·93 1·39 3·94 3·94 1·39 4·41 1·86 1·86il · . · . · . 2·13 1·98 2·43 2·28 1·52 2·89 6·08 2·89 2·74 1·82 3·50ap · . · . · . 0·67 0·34 0·34 0·34 0·34 0·67 5·04 2·02 1·01 1·68 2·35

water · . · . · . 0·51 0·45 0·36 0·16 0·42 0·46 0·45 0·60 0·32 0·56 0·38

Quartz ·. · . · . - - - - - - 0·1 9·1 0·8 9·3 4·1Orthoclase · . · . - - 0·7 1·0 tr. tr. 8·5 26·2

}80'034·7 27·7

Plagioclase · . · . 34·0 31·6 41·3 50·7 67·4 56·8 43·4 40·9 37·2 46·1Olinopyroxene · . · . 16·8 20·5 }47 04 }34'4

12·8 20·4 10·3 1·3 8·9 1·1 4·5Orthopyroxene · . · . 1·5 2·3 0·7 4·2 9·3 1·0 0·4 - 4·9Olivine ·. · . · . 46·2 44·1 9·0 14·0 7·5 8·7 4·9 3·6 - 1·1Hornblende · . · . - - - - - - - 13·6 0·9 13·4 7·5Biotite ·. · . · . tr. tr. 3·1 1·4 2·3 5·6 7·1 0·1 0·3 1·7 0·4Iron Ore · . · . · . 1·2 1·0 7·5 3·5 2·8 4·8 7·6 1·2 4·3 1·6 2·5Apatite · . · . · . tr. 0·5 tr. tr. tr. 0·7 5·4 1·7 0·8 1·0 1·2

~o~o~

~~H

~t.r:1oqw.oo~t-et"It.r:1~

}--l0:>~

TABLE II (a) ALKALI SERIES-CHEMICAL AND SPECTROGRAPHIC DATA

I I I I I Is 36 158 195 122 143 47 189 42 41 58 57 54 119 85

Si02 .-. ·. - 48·48 48·21 I 49·32 53·98 59·50 49·96 58·35 58·35 56·32 45·40 50·29 37·42 38·59 34·52 Si02·. ·.Ti02 ·. ·. - 0·45 1·45 1·30 0·95 0·70 0·80 0·40 0·49 0·08 1·63 0·70 1·72 3·17 2·22 ·. ·. Ti02AI20 s ·. ·. - 18·31 13·29 18·87 20·70 19·29 22·30 17·91 18·10 22·09 15·38 20·51 14·78 12·06 10·62 ·. ·. Al20 SFe20 S ·. · . - 0·69 1·87 1·90 1·88 1·65 2·54 3·48 3·27 1·86 2·48 1·04 5·26 4·02 5·28 ·. ·. Fe20 SFeO ·. ·. - 6·40 7·46 6·39 3·31 2·32 2·25 3·44 3·51 0·83 6·64 3·83 4·23 6·00 4·12 ·. ·. FeOMnO ·. ·. - 0·15 0·17 0·15 0·10 0·03 0·03 0·03 0·04 tr. 0·13 0·05 0·13 0·15 0·10 ·. ·. MnOMgO ·. ·. - 8·68 8·51 3·15 1·51 1·10 0·68 0·47 0·50 0·02 9·00 1·90 6·20 12·30 9·55 ·. ·. MgOCaO ·. ·. - 14·33 13·96 7·43 4·94 2·53 3·86 2·10 2·23 1·50 9·90 3·80 13·17 12·99 21·32 ·. ·. CaONa20 ·. ·. - 2·08 2·55 5·53 6·62 5·97 8·11 6·49 6·79 9·72 4·32 6·26 4·02 3·46 2·49 ·. ·. Na20K 20 ·. ·. - 0·35 1·34 4·14 3·93 5·78 6·04 5·93 6·07 6·27 2·77 5·46 5·01 2·37 4·01 ·. ·. K 20H 20 + ·. ·. - 0·13 0·53 0·60 1·01 0·84 1·27 0·81 0·30 0·85 1·05 3·56 2·19 1·81 2·97 ·. ·. H 20 +H 2O- ·. ·. - 0·12 10 .24 0·03 0·07 0·12 6·10 0·15 0·04 0·12 0·08 0·16 0·34 0·22 0·08 ·. ·. H 2O-P 20 5 ·. ·. - tr. 0·48 0·86 0·39 0·34 0·27 0·25 0·21 tr. 0·50 0·46 1·15 0·98 0·45 ·. ·. P 20 5CO2 ·. ·. - n.d. n.d. 0·25 0·86 0·28 1·88 0·37 0·17 0·26 0·88 2·21 4·16 1·33 2·20 ·. ·. CO2CI ·. ·. - n.d. n.d. 0·28 tr. tr. 0·40 0·17 0·30 0-45 0·18 tr. 0·21 0·38 0·11 ·. ·. CIF ·. ·. - n.d. n.d. 0·10 0·07 0·04 0·07 0·10 0·07 0·05 0·04 0·06 0·20 0·15 0·40 ·. .. F

100·17 100·06 100·32 100·32 100·49 100·56 100·45 100·44 100·42 100·38 100·29 100·19 100·03 100·44

less 0=01, F ·. 0·10 0·03 0·02 0·12 0·08 0·10 0·12 0·06 0·03 0·13 0·15 0·19 less O=Cl, F

100·22 100·29 100·47 100·44 100·37 100·34 100·30 100·32 100·26 100·06 99·88 100·25

GaH ·. ·. 5 20 20 20 20 15 10 25 20 15 20 15 10 10 10 ·. ·. Ga+a:CrH ·. ·. 2 160 275 15 * * * * * * 800 80 80 500 300 ·. ·. Cr+s.V+S ·. ·. 5 120 500 120 70 15 50 * * 10 200 80 150 300 150 ·. ·. VHMoH ·. 5 * * * * * 5 * * * * * * * * ·. ·. Mo+&·.LiH ·. · . 1 15 20 40 50 30 45 35 10 25 15 30 15 20 5 ·. ·. LiHNiH ·. ·. 3 125 125 20 4 * 3 * * * 150 15 25 150 50 ·. ·. NiHOoH · . ·. 5 60 70 30 5 * 5 * * * 40 8 20 40 30 ·. .. Co+:!ScH ·. ·. 10 40 30 * * * * * * * n.d. n.d. n.d. n.d. n.d. ·. ·. Sc+s

Zr+ 4 ·. · . 10 30 175 200 200 70 100 550 300 225 100 120 100 150 20 ·. .. Zr+4

y+s ·. 15 * 30 30 20 20 20 60 60 20 * * 20 * * ·. ·. yH·.LaH ·. ·. 50 * * * 50 80 100 100 100 80 * 50 50 50 * ·. ·. La+88rH ·. ·. 10 700 1,000 1,350 1,500 500 1,500 10 25 20 700 500 1,250 1,000 1,250 ·. .. 81'+2PbH ·. 10 * * * * * * * * * * * * * * ·. ·. Pb+2·.BaH ·. ·. 10 175 800 2,000 1,500 3,000 1,000 75 300 10 700 700 1,750 1,000 1,250 ·. ·. BaHRbH ·. ·. 10 * 100 300 300 200 500 350 350 550 100 350 300 100 300 ·. ·. Rb+lTIH ·. ·. 1 * * * * * 2 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. ·. ·. TI+l

8p. Gr. ·. - 3·04 3·03 2·84 2·68 2·61 2·68 2·65 2·62 2·55 2·95 2·55 2·86 3·05 3·03 ·. 8p. Gr.

(FeO +Fe20 S ) x 100 - 44·9 52·3 72·3 77·2 78·4 87·7 93·8 93·2 99·4 50·3 72·0 60·3 44·8 49·6 (FeO +Fe20 S ) x 100

MgO + FeO + Fe20 SMgO + FeO + Fe20 ~

(Na20 +K20) x 10014·6 21·8 56·5 68·1 82·2 78·4 85·8 85·3 91·4 41·7 75·6 40·7 30·9 23·4

(Na20+K20) X ]00

CaO + Na20 +K2O - I CaO + Na20 +K20I

TABLE II (b) ALKALI SERIES-NORMATIVE AND MODAL DATA.-.

36 158 195 122 143 47 189 42 41 58 57 54 119 85

C · . · . - - - - - 1·12 - - - - 3·67 - - -or ·. · . 2·22 7·78 24·46 23·35 34·47 35·58 35·03 36·14 37·25 16·68 32·25 5·56 - -ab ·. · . 16·24 13·10 14·67 37·73 45·06 19·39 42·97 39·30 28·30 7·86 30·29 - - -an · . · . 39·20 20·85 15·29 15·29 8·62 5·28 2·50 1·67 - 15·29 1·67 8·34 12·23 6·12lc ·. · . - - - - - - - - - - - 18·75 10·90 18·75ne · . · . 0·57 4·54 16·19 9·66 2·84 24·99 5·96 8·80 27·26 14·48 12·21 17·61 14·20 11·09hI · . · . - - 0·47 - - 0·70 0·23 0·47 0'70 0·35 - 0·35 0·70 0·12ro · . · . 13·34 18·91 7·19 0·58 - - 1·86 3·02 - 10·44 - 9·86 14·50 1·86

di en · . · . 8·30 12·00 3·40 0·30 - - 0·60 0·90 - 7·20 - 8·20 11·70 1·60fs · . · . 4·22 5·68 3·70 0·26 - - 1·32 2·24 - 2·38 - 0·40 1·06 -wo · . · . - - - - - - - - 2·32 - - - - -

01 {fo · . · . 9·08 6·24 3·08 2·38 1·82 1·12 0·42 0·21 - 10·50 3·36 5·11 13·30 15·59,fa · . · . 5·10 3·47 3·47 2·24 1·43 0·61 1·02 0·51 - 3·98 3·88 0·30 1·22 -cs · . · . - - - - - - - - - - - - 0·78 23·56mt · . · . 0·93 2·78 2·78 2·78 2·32 3·71 5·10 4·87 2·55 3·71 1·39 7·66 5·80 6·96hm .. · . - - - - - - - - 0·16 - - - - 0·48il · . · . 0·76 2·74 2·43 1·82 1·37 1·52 0·76 0·91 0·15 3·04 1·37 3·19 6·08 4·26ap · . · . tr. 1·34 2·02 1·01 0·67 0·67 0·34 0·34 - 1·34 1·34 2·69 2·35 1·34fr · . · . - - 0·23 0·16 0·07 0·16 0·23 0·16 0·07 0·07 0·16 0·39 0·31 0·78cc · . · . - - 0·60 1·90 0·70 4·30 0·80 0·30 0·60 1·90 5·00 9·40 3·10 5·00

water · . · . 0·25 0·77 0·63 1·08 0·96 1·37 0·96 0·34 0·97 1·13 3·72 2·53 2·03 3·05

Alk. Felspar · . tr. 2·4 12·9 21·3 83·9 39·3 40·4 71·3 -}20'6 }57'6

- - -Plagioclase · . · . 56·2 37·8 24·8 48·3 2·1 8·0 27·0 pro - - - -Intergrowth · . - - 10·3 - - - - - - - - - - -Nepheline · . · . - 3·1 19·3 8·1 1·8 26·8 3·7 3·2 - 7·5 4·5 2 pro 5·2Sodalite · . · . - - 1·0 0·5 - 5·3 5·6 5·2 - 3·4 tr. - - -Analcite · . · . - - - 1·4 0·6 0·9 - - - 2·0 9·3 pro 3 -Olivine · . · . 12·2 7·0 4·8 - - - - - - 9·2 0·5 pro 5 3·6Pyroxene · . · . 29·5 41·0 16·9 0·5 1·5 4·7 14·6 2·7 - 19·1 8·4 28 10 1·4Amphibole · . · . - tr. 2·7 15·0 3·4 6·6 - 13·2 -. 31·8 9·5 pro - -Muscovite · . · . - - - 1·4 1·7 1·4 - - - - 4·0 pro - -Biotite · . · . 0·6 4·9 2·7 1·2 2·4 - 3·8 - - 2·9 - 10 4 27·7Sphene · . · . - - - 0·7 0·5 1·4 0·7 1·7 - - - - - -Iron Ore · . · . 1·5 3·2 3·6 0·6 1·6 3·8 2·6 1·6 - 2·1 2·1 2 1 6·7Calcite ·. ·. - - tr. 0·5 tr. 2·0 - - - 1·1 4·3 pro pro 2·6Apatite · . · . tr. 0·6 1·0 0·5 0·5 tr. 0·4 0·4 - 0·3 tr. 2 pro -Rinkolite(?) .. · . - - - - - - 1·2 1·3 - - - - - -Perovskite ·. · . - - - - - - - - - - - - - 4·0Garnet · . · . - - - - - - - - - - - - - 16·5Melilite ·. · . - - - - - - - - - ....- - - - 11·8Wollastonite · . - - - - - - - - - - - - - 2·1Groundmass .. · . - - - - - - - - - - - 56 77 -Alteration · . ·. - - - - - - - - - - - pro - 18·4

""

TABLE II (b) ALKALI SERIES-NORMATIVE AND MODAL DATA-_.

36 158 195 122 143 47 189 42 41 58 57 54 119 85

C · . · . - - - - - 1·12 - - - - 3·67 - - -or · . · . 2·22 7·78 24·46 23·35 34·47 35·58 35·03 36·14 37·25 16·68 32·25 5·56 - -ab · . ·. 16·24 13·10 14·67 37·73 45·06 19·39 42·97 39·30 28·30 7·86 30·29 - - -an · . · . 39·20 20·85 15·29 15·29 8·62 5·28 2·50 1·67 - 15·29 1·67 8·34 12·23 6·12lc · . · . - - - - - - - - - - - 18·75 10·90 18·75ne · . · . 0·57 4·54 16·19 9·66 2·84 24·99 5·96 8·80 27·26 14·48 12·21 17·61 14·20 11·09hI · . ·. - - 0·47 - - 0·70 0·23 0·47 0·70 0·35 - 0·35 0·70 0·12ro · . · . 13·34 18·91 7·19 0·58 - - 1·86 3·02 - 10·44 - 9·86 14·50 1·86

di en · . ·. 8·30 12·00 3·40 0·30 - - 0·60 0·90 - 7·20 - 8·20 11·70 1·60fs · . · . 4·22 5·68 3·70 0·26 - - 1·32 2·24 - 2·38 - 0·40 1·06 -wo · . · . - - - - - - - - 2·32 - - - - -

01 {fo · . ·. 9·08 6·24 3·08 2·38 1·82 1·12 0·42 0·21 - 10·50 3·36 5·11 13·30 15·59Ja ·. · . 5·10 3·47 3·47 2·24 1·43 0·61 1·02 0·51 - 3·98 3·88 0·30 1·22 -cs · . · . - - - - - - - - - - - - 0·78 23·56mt · . · . 0·93 2·78 2·78 2·78 2·32 3·71 5·10 4·87 2·55 3·71 1·39 7·66 5·80 6·96hm .. ·. - - - - - - - - 0·16 - - - - 0·48il · . · . 0·76 2·74 2·43 1·82 1·37 1·52 0·76 0·91 0·15 3·04 1·37 3·19 6·08 4·26ap · . · . tr. 1·34 2·02 1·01 0·67 0·67 0·34 0·34 - 1·34 1·34 2·69 2·35 1·34fr · . · . - - 0·23 0·16 0·07 0·16 0·23 0·16 0·07 0·07 0·16 0·39 0·31 0·78cc · . · . - - 0·60 1·90 0·70 4·30 0·80 0·30 0·60 1·90 5·00 9·40 3·10 5·00

water · . ·. 0·25 0·77 0·63 1·08 0·96 1·37 0·96 0·34 0·97 1·13 3·72 2·53 2·03 3·05

Alk. Felspar · . tr. 2·4 12·9 21·3 83·9 39·3 40·4 71·3 -}20'6 }57'6

- - -Plagioclase · . · . 56·2 37·8 24·8 48·3 2·1 8·0 27·0 pro - - - -Intergrowth · . - - 10·3 - - - - - - - - - - -Nepheline · . · . - 3·1 19·3 8·1 1·8 26·8 3·7 3·2 - 7·5 4·5 2 pro 5·2Sodalite · . · . - - 1·0 0·5 - 5·3 5·6 5·2 - 3·4 tr. - - -Analcite ·. · . - - - 1·4 0·6 0·9 - - - 2·0 9·3 pro 3 -Olivine · . · . 12·2 7·0 4·8 - - - - - - 9·2 0·5 pro 5 3·6Pyroxene ·. · . 29·5 41·0 16·9 0·5 1·5 4·7 14·6 2·7 - 19·1 8·4 28 10 1·4Amphibole · . · . - tr. 2·7 15·0 3·4 6·6 - 13·2 -- 31·8 9·5 pro - -Muscovite · . · . - - - 1·4 1·7 1·4 - - - - 4·0 pro - -Biotite · . · . 0·6 4·9 2·7 1·2 2·4 - 3·8 - - 2·9 - 10 4 27·7Sphene ·. · . - - - 0·7 0·5 1·4 0·7 1·7 - - - - - -Iron Ore · . ·. 1·5 3·2 3·6 0·6 1·6 3·8 2·6 1·6 - 2·1 2·1 2 1 6·7Calcite · . · . - - tr. 0·5 tr. 2·0 - - - 1·1 4·3 pro pro 2·6Apatite · . · . tr. 0·6 1·0 0·5 0·5 tr. 0·4 0·4 - 0·3 tr. 2 pro -Rinkolite(?) .. ·. - - - - - - 1·2 1·3 - - - - - -Perovskite · . · . - - - - - - - - - - - - - 4·0Garnet · . · . - - - - - - - - - - - - - 16·5Melilite ·. · . - - - - - - - - - ...- - - - 11·8Wollastonite · . - - - - - - - - - - - - - 2·1Groundmass .. · . - - - - - - - - - - - 56 77 -Alteration ·. ·. - - - - - - - - - - - pro - 18·4

~

172 THE OKONJEJE IGNEOUS COMPLEX

TABLE III (a) MINERALS-CHEMICAL AND SPECTROGRAPHIC DATA

s A B C D E F I G•

Si02 ·. · . · . - 46·61 43·75 47·51 49·77 48·61 38·45 50·09Ti02 ·. ·. ·. - 1·18 1·95 1·25 1·18 1·63 3·62 1·15Al20 3 · . · . - 3·47 9·32 5·11 4·28 3·77 12·42 5·34Fe20 3 ·. · . - 0·90 3·15 3·57 2·33 1·54 5·38 4·23FeO ·. · . · . - 20·18 17·36 6·53 6·76 11·48 14·67 6·13MnO ·. · . - 1·11 0·61 0·14 0·21 0·42 0·54 0·15MgO · . ·. - 7·27 8·39 14·51 12·96 10·79 7'78 10·49OaO · . · . · . - 17·24 10·96 20·99 21·89 20·73 11·61 19·17Na20 · . · . - 1·04 1·72 0·67 0·64 0·97 3·07 2·78K 20 ·. · . · . - 0·27 0·84 0·07 0·05 0·25 1·68 0·04H 2O+ · . · . - 0·42 1·66 0·02 0·16 0·10 1'16 0·07H 2O- · . · . - 0·04 0·09 0·00 0·08 0·04 0·01 0·09F · . ·. · . - n.d. 0·8 n.d. n.d. n.d. 0·9 n.d.

-----99·73 100·61 100·37 100·31 100 ·33 101·29 99·73

----less °=F ·. ·. 0·34 0·4

100·27 100·89

GaH · . · . 5 5 20 20 10 20 15 20CrH · . ·. · . 2 * 70 500 400 90 * *VH · . · . 5 10 100 600 450 300 300 300MoH .. · . · . 5 * * * * * * *LiH ·. · . · . 1 50 80 10 25 15 45 10Ni+2 ·. · . · . 3 * 30 125 125 30 10 300+ 2 · . · . · . 5 * 35 100 90 65 30 50SO+3 ·. · . · . 10 140 30 120 70 20 * 30Zr+ 4 ·. · . · . 10 125 200 100 350 550 800 500yH ·. ·. · . 15 80 250 25 60 100 100 *La+3 · . · . · . 50 * 100 n.d. n.d. * * n.d.SrH · . · . 10 * 12 10 * 50 70 300PbH · . · . 10 * * * * * * *Ba+2 · . ·. 10 25 25 * * 30 150 30RbH .. · . · . 10 * 40 * * * 20 *TIH · . · . 1 n.d. n.d. n.d. n.d. n.d. n.d. n.d.

TABLE III (b) :MINERALS-STRUCTURAL FORMULAE AND OPTICAL PROPERTIES

IA I B I C

1D

IE I F G

I I

I

II

Z {:·. ·. 1·859 6·615 1·778 1·858 I 1·852 5·898 1·876

·. ·. 0·141 1·385 0·222 0·142 0·148 2·102 0·124

Al ·. ·. 0·022 0·277 0·004 0·046 0·021 0·144 0·112

Ti ·. ·. 0·035 0·222 0·035 0·033 0·047 0·417 0·032

Fe+3 ·. ·. 0·027 0·358 0·100 0·065 0·044 0·621 0·119Y

FeH ·. ·. 0·673 2·194 0·204 0·211 0·366 1·881 0·192

Mn ·. ·. 0·038 0·078 0·004 0·007 0·014 0·070 0·005

:Mg ·. ·. Q->432 1·890 0·809 0·721 0·613 1·778 0·585

ra ·. ·. 0·736 1·775 0·841 0·875 0·846 1·907 0·769

X Na ·. ·. 0·080 0·504 0·048 0·046 0·072 0·912 0·202

K ·. ·. 0·014 0·162 0·004 0·002 0·012 0·328 0·002-

(OH) .. ·. - 1·673 - - - 1·186 -F ·. ·. - 0·382 - - - 0·436 -

Z ·. ·. 2·00 8·00 2·00 2·00I

2·00 8·00 2·00

y ·. ·.} 2·06

5·02} 2·05 } 2·01 }2.03

4·91}2.02

X ·. ·. 2·44 3·15

(OH),F ·. - 2·05 I - - - 1·62 -I

I core margin core margin1

2V ·. ·. 52°( +) 65°( -) 46tO( +) 52°(+) 55°(+) 52°( +) 63°( -) ±3° 40 0( -) ±5° 63°( +)

oc ·. ·. 1·710 1·674 1·693 1·694 1·696 I 1·694 1·687 1·690 1·705

fJ ·. ·. 1·716 1·688 1·697 1·699 1·701

I1·700 I 1·705 1·710 1·711

y ·. ·. 1·736 1·695 1·719 1·722 1·725 1·724

I1·711 1·713 1·729

Iy-oc ·. ·. 0·026 0·021 0·026

I 0·028 0·029 0·028

I0·024 0·023 0·024

y:c ·. ·. 45° 16° 42° 39° 39° 41° 11° 10° 50°

Pleochroism {t · . {Shades of very Pale yellow-green { Shades of { Shades of pale r Shades of

I

Pale yellow Pale greenish-yellow Pale yellow-green· . pale green, with Olive green very pale pink and green, l pale pink Reddish-brown Deep olive green Bottle green·. pale pink cores Dark bottle green brown margins darker and green Deep brown Deep brown-green Bottle green

Sp. Gr. ·. 3·49 3·21 3·37 3·37 3·38I - 3·38

EXPLANATION OF PLATES

(Magnification X 25 in all photomicrographs)

PLATE XXIV

Composite photograph of the Okonjeje complex from the eastern extremity of theridge syenite outcrop.

1, talus and alluvium: 2, Rarroo sediments: 3, Damara sediments: 4, differentiatedgroup: 5, marginal acid rock: 6, ridge syenite: 7, core gabbro: 8, pulaskite: 9,andesine-essexite: 10, oligoclase-essexite: 11, granulitic gabbros invaded by core gabbro:12, pipe-breccia: 13, foyaite: 14, camptonite-gauteite: solid black, marginal gabbropicrite.

PLATE XXV

Fig. I-Structural relations north of Zbigniew Ridge, facing south. Left and centre,Rarroo sediments overlying Dmnara slates and traversed by outward-dipping dyke ofmarginal gabbro-picrite. Exfoliation surfaces of pale pulaskite on right.

Fig. 2-The south-eastern sector of the complex as seen from White Ridge. BeddedRarroo sediments on Outjivero (left) and the Otjongundu Plateau (distance), arcuateridges of ridge gabbro in centre, and core gabbros exposed on Martin (middle distance,right).

PLATE XXVI

Fig. I-Rhythmic layering in core gabbro, south summit of Martin.Fig. 2-Inner ridge gabbro (Ok. 7). Note clouded plagioclase, vermicular iron ore

orthopyroxene intergrowth replacing olivine, and biotite rims around primary iron ore.Fig. 3-Middle ferrogabbro (Ok. 106). Large columnar orthopyroxene crystal

associated with and enclosing optically continuous rounded crystals of olivine (higherrelief, dark fractures). Note clouded plagioclase and clear orthoclase.

PLATE XXVII

Fig. I-Incipient granulation of gabbro-picrite (Ok. 99) with zonally arrangrdschiller inclusions in pyroxene and minute granules of iron ore in plagioclase.

Fig. 2-Final stage of granulation of gabbroic rock (Ok. 93) showing granular textureand segregated lenticular patches of plagioclase. (Crossed nicols.)

Fig. 3-Fingerprint intergrowth between felspathoid (stained with methylene blue)and alkali felspar in andesine-essexite (Ok. 195).

Fig. 4-Stellate sodalite-tinguaite (Ok. 189). Note disoriented laths of albite.(Crossed nicols.)

PLATE XXVIII

Fig. l--Camptonite (Ok. 58). Olivine, subidiomorphic amphibole and raggedpyroxene set in a groundmass composed of plagioclase laths and subordinate felspathoid.

Fig. 2-Gauteite (Ok. 57). Ragged phenocrysts of zoned titanaugite and alkaliamphibole set in an altered felspathic groundmass with abundant muscovite and analcite(clear patches).

Fig. 3-Melanephelinite (Ok. 54). Phenocrysts of titanaugite and mica set in agroundmass composed of small nepheline crystals separated by dark minerals.

Fig. 4--Alnoite (Ok. 85). Note phenocrysts and numerous small flakes of biotite,together with melilite tablets, irregular garnet and small black octahedra of iron ore andperovskite.

Composite photograph of the Okonjeje complex from the eastern extremity of the ridge syenite outcrop_

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V:i.a~

V:i.~

d~

N~,...,

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Dkonjej'

Composite photograph of the Okonjeje complex from the eastern extremity of the ridge syenite outcrop.

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TRANS. GEOL. SOC. S.A., VOL. LVII PLATE XXV

FIG. 1Structural relations north of Zbigniew Ridge, facing south.

FIG. 2The south-eastern sector of the complex as seen from White Ridge.

TRANS. GEOL. SOC. B.A., VOL. LVII PLATE XXVI

FIG. 1Rhythmic layering in core gabbro, south summit of Martin.

FIG. 2 FIG. 3

TRANS. GEOL. SOC. 8.A., VOL. LVII PLATE XXVII

TRAlfS. GEOL. SOC. S.A., VOL. LVII PLATE XXVIII

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