Geochemistry and REE minerals of nepheline syenites from theMotzfeldt Centre, South Greenland

American Mineralogist, Volume 70, pages 1087-1 100, 1985

sphene. Electron probe analyses including the REE in sev-eral of these minerals are reported and discussed.

The Motdeldt Centre is especially interesting because alate stage peralkaline magma, lujavrite, was developed.This provides a link between the bulk of silica-undersaturated syenite plutons in the Gardar and the un-usual peralkaline Ilimaussaq Intrusion (Ferguson, 1964,1970; Bailey et al., 1981; Sorenson, 1969). We suggest thatincorporation of earlier feldspar-rich cumulates in additionto extensive crystal fractionation could have played an im-portant role in the overall petrogenesis of the syenites.

Geology

The Motzfeldt Centre was formed by a series of overlap-ping intrusions emplaced by a combination of block stop-ing and 5-10 km radius ring fractures. The country rock ismostly gneiss, known as Julianehib granite (Allart, 1973)and a sequence of supracrustal rocks including metasedi-ments, agglomerates and lavas collectively called theEriksfiord Formation (Emeleus and Upton, 1976). Meta-somatized and altered relics of the supracrustal rocks formlarge foundered raftlike xenoliths within the syenites andmeasure up to several kilometers in length and a fewhundred meters in thickness. Adjacent lavas in the supra-crustal volcanic rocks, including the altered xenoliths,range from alkali basalts to phonolites and are possibly

1087

A. P. JoNrsr

Diuision of Geological and Planetary SciencesCalifurnia Institute of Technology, Pasadena, California 91125

.rNp L. M. LensrN

Grsnlands Geologiske U nder sogelse@ster Voldgade 10, DK-1350, Copenhagen, Denmark

Abctract

Whole-rock analyses including the REE are given for nineteen samples from four of the sixoverlapping nepheline syenite intrusions which constitute the Proterozoic (- 1310 ma) Motz-feldt Centre. Larvikite margins of one unit (SM5) are apparently related to the nephelinesyenites by early fractionation of Ol + Pl followed by fractionation of Ol + Pl + Cpxt Amph at current levels of exposure. Positive Eu-anomalies in the larvikites might beexplained by large-scale incorporation of earlier Pl-rich cumulates. Low initial 875r/865r

ratios imply a mantle source, and the most basic magma was a late alkali gabbro giant dikewith plagioclase megacrysts. REE- and incompatible element-rich peralkaline lujavrites withlarge negative Eu-anomalies are late stage products of extreme fractionation. Electron probeanalyses of REE in eudialyte confirm this mineral as the major host to Zr and REE in thelujavrites. Other REE-bearing minerals in the nepheline syenites are Ca-Na{e-Ti-Zr-Nb-silicates of the rinkite-mssandrite and l6venite series, with up to 22 wt.% RE-oxides; somemay be related to metasomatic fluids which had geochemical signatures similar to the lujav-rites.

Introduction

The Motzfeldt Centre is the older of four major centersof igneous activity which make up the Igaliko Complex ofnepheline syenites (Emeleus and Harry, 1970). It is situatedat the eastern end of the Proterozoic alkaline igneousGardar Province of southern Greenland (Emeleus andUpton, 1976). The Motzfeldt Centre has a determinedRb/Sr (whole-rock) age of 1310+31 ma and an initial8?5r/865r ratio near 0.702 (Blaxland et al., 1978). TheMotzfeldt Centre covers an area of approximately 350 km2and is well exposed as a steeply dissected and glaciated2000 m plateau. The mineralogy of the mafic silicates to-gether with a briefoutline ofthe geology has been given byJones (1984).

In this paper we present new whole-rock geochemicaldata for major and trace elements including the rare earthelements (REE). In particular the REE are contained in avariety of minerals, some of which show evidence of meta-somatism. These minerals are principally eudialyte andalkali-zircono-silicates of the rinkite-mssandrite andlivenite series, and to a lesser extent, apatite, zircon and

I Current address, School of Geological Sciences, KingstonPolytechnic, Penrhyn Road, Kingston Upon Thames, EnglandKTl 2EE.

0003-004x/85/ l l 1 2-1087$02.00

JONES AND LARSEN: NEPHELINE SYENI?ES FROM THE MOTZFELDT CENTRE

Fig. l. Geology of the Motzfeldt Centre and location of samples used for whole-rock analyses in this study. Map after Jones (1984)with revised eastward extension to the major E-W fault after Tukiainen et al (1984). AGGD is alkali gabbro giant dike; EM : EastMotzfeldt syenites; NM : North Motzfeldt syenites. Ornamentation, solid : supracrustal rocks. NQ : North Q6roq Centre(Chambers, 1976), SQ : South Q6roq Centre (Stephenson, 1976).

related to the Motzfeldt Centre itself (Jones, 1980). Thecenter is cut by a major transcurrent E-W fault (TheFlink's Dal Fault, Emeleus and Harry, 1970) which wasshown to have a left lateral displacement of several kilome-ters (Jones, 1980) measured as 6 km to the east of theinland lake, Moztfeldt So (Tukiainen et al., 1984). The Geo-logical Survey of Greenland is currently engaged in de-tailed investigations of several syenites and of radioactivemineralization in and around the center. There are sixmajor syenite units, prefixed "SM" and numbered I to 6 inorder of intrusion as indicated on the summary geologicalmap in Figure 1. Future refinements to the geologicalboundaries of the syenite units are unlikely to affect themajor conclusions of this paper, but might provide answersto some of the outstanding problems. A unit of late-stagelujavrites (SM6) and a sub-unit of cancrinite-rich roof-zonesyenites (HY) were recognized subsequent to the work ofEmeleus and Harry (1970) by Jones (1980). Unequivocaltime relationships between units SM5 and SM6 have notbeen established, since they are separated by the Flink'sDal Fault. The center is cut by a regional swarm of ENE-trending trachytic dikes and a prominent alkali gabbrogiant dike (AGGD) intruded in similar orientation. A fewnear-vertical carbonatitic and ultrabasic breccia pipes upto 200 m across also cut the center, but are not shown onFigure 1. The minor intrusions remain an attractive goalfor future study and it is not known how they relate to thesyenites. Thin discontinuous fluorite- and calcite-rich veinsare found sporadically throughout the center and may pro-vide clues to late stage metasomatic processes seen in some

structurally high levels. Pegmatites are sometimes associ-ated with contacts between xenoliths and host syenites.Several pegmatites, microsyenite sills and altered limonite-rich shear zones in the center are enriched in radioactiveminerals (Tukiainen et al., 1984).

The simplified geological map in Figure 1 shows thelocation of analyzed whole-rock samples (XRF) selected foradditional REE determination by Instrumental NeutronActivation Analysis (INAA). The XRF analyses were ob-tained by Jones (1980) from which the following summaryis largely extracted. The ranges of zoned mafic minerals,especially Fe/(Fe + Mg) and Na/(Na + Ca) in pyroxene,are related to the calculated normative differentiation index(D.L) of the host rock. The D.L was calculated after Thorn-ton and Tuttle (1960) with the addition of normativeacmite (NaFe3Si2O6), which is important in the more frac-tionated syenites. Whole rock P and Ti decrease with in-creasing D.I. and increasing Fe/Mg, consistent with p€tro-graphic textures which indicate early crystallization ofapa.tite and Ti-magnetite. Many of the syenites show cumulustextures in thin section and classic mineral banding, al-though rare, does occur in the center (Emeleus and Harry,1970; Jones, 1980). Mineral compositions show normalzoning and combined trends which are the expectod conse-quences of crystallization from increasingly fractionated oy-enite magmas. In addition to continuous zonation, themafic minerals also show discontinuous zonation of thetype pyroxene-amphibole-pyroxene (Jones, 1984). Bulkfeldspar probe analyses are difficult to obtain because ofperthitic textures and show a wide range in Ca-poor com-

Fig. 2. Feldspar compositions (Ca Na-K). Symbols: solidtdangles : plagioclase cores in larvikites of SM5; open triangles: rims and normal feldspar from larvikitic syenites of SM5; opencircles : feldspar from nepheline syenites of SM5; hexagon(AGGD) : plagioclase megacryst from alkali gabbro giant dike.Ruled areas are loci of feldspar (including plagioclase relics) fromchilled facies of augite syenite at Ilimaussaq (Larsen, 1981). Rangeof feldspar compositions (bulk) of Motdeldt syenites also indicat-ed by dashed lines.

positions, illustrated in Figure 2. Calculated equilibrationtemperatures from nepheline compositions (Hamilton,1961) range from 1050"C in the larvikites of unit SM5 to750'C in the lujavrites of SM6, and some grains are nor-mally zoned. Syenites from near the roof zones often showconsiderable replacement of nepheline by sodalite andlorcancrinite, coincident with peripheral enhancement ofalbite rims on perthite feldspars. Considerable subsolidusalkali exchange must have occurred, and calculations usingthe feldspar-nepheline geothermometer of Powell andPowell (1977) give a range of lower temperatures from 875+90 to 600+60"C for the majority of the unaltered syen-ites from Motzfeldt.

The current samples were chosen from the three largestsyenite units (SM1, SM4, SM5) and the late stage lujavrites(SM6), in order to cover the range of fractionation androck types present. Table 1 lists these samples and summa-rizes their rock types and special features; Table 1 alsoindicates which samples have been used in other pub-lications. One sample from unit SMI (63760) contains Nb-rich astrophyllite and has been hydrothermally altered.Unit SM5 is particularly useful since it grades from larvi-kites at its margin to eudialyte-bearing nepheline syenite atits interior. The mafic larvikites have weakly schillerizedfeldspar similar to Oslo region larvikites and are otherwisesimilar to augite syenite. Augite syenite occurs in associ-ation with many of the Gardar centers and is widely con-sidered to be parental to many of the syenites (e.g., Upton,1964; Emeleus and Upton, 1976). Feldspar in a partial ring

1089

dike related to the margins of unit SM5 (SM5{') containswell defined plagioclase cores with irregular outlines andlamellar twinning abruptly terminated by mantles of crypt-operthitic alkali feldspar. Three reconnaissance microprobeanalyses of the plagioclase show a range in compositionfrom Ab.rAnrrOr, to AbroAnrrOr,n and are plotted inthe feldspar ternary in Figure 2. Similar, though slightlyless calcic plagioclase cores (<An2o) occur in chilled faciesof augite syenite from Ilimaussaq (Larsen, 1981). The bulkcomposition of the larvikite ring dike provides the bestestimate of a basic magma available at Motzfeldt that wasdirectly associated with the syenites (Table 2). However, themost basic rocks in the center, apart from the little studiedcarbonate plugs, are represented by the late AGGD (Fig.l). The AGGD carries abundant plagioclase megacrysts(-Anruor.) and is similar to a Ti-rich alkali basalt incomposition, with about 47 wt.Vo SiO, and l4%o nephel-ine (Ne) in the CIPW norm at Fe3lFe2:0.1 (Jones, 1980).

Analytical procedure

X-ray fluorescence onalyses (XRF)

Original rock samples, typically 0.5 to 2.0 kg in size dependingon grain size, were prepared by hand removal of weathered ma-terial, crushing in a jaw crusher and then milling in a tungstencarbide Tema disc mill. 100-200 g powdered samples were pressedinto pellets and analyzed for selected major and trace elementsusing a Philips PWl2l2 automatic spectrometer with automaticsample loader at Durham University. Detailed procedures andstandards are given in Jones (1980) and are available on request.

I nstrumental neutron actiu ation analy ses ( I N A A )Samples of 300-'500 mg powder were irradiated for 3 hours at

Riss National Laboratory, Denmark, in a reactor with a neutronflux of 4.5 x 1013 nfcm2fsec. The samples were arranged inbatches of four unknowns and one pure FerO. comparator, fol-lowing the procedure of Girardi et al (1965). The counting wasperformed with a multichannel analyzer fitted with a Ge (Li) de-tector in two rounds; first after a cooling tirne of 7-8 days for20-30 minutes, and second after a cooling time of 42-50 days for80-120 minutes. The results were processed by computer using theprogram lncoN written by R. Gwod4 Copenhagen. This pro-cedure identified peaks, performed interference correlations, andcalculated concentrations. The results from one typical standardare given in the footnote to Table 3.

Electron micropobe

Mineral analyses were all made with a Cambridge Geoscan MkII electron microprobe at Durham University. Major and minorelements were analyzed using analytical methods outlined in Jones(1984). Alkali-rich minerals such as feldspars, were analyzed with adefocussed beam of approximately 20 pm diameter. Special con-ditions for the REE, which were analyzed separately, followedprocedures written by A. Peckett (given in Jones, 1980) and arebriefly outlined here. An accelerating voltage of 20 kV, probecurrent of 80 nA as measured with a Faraday Cage in standardblock, and LIF analyzing crystal in conjugation with a gas-glowproportional counter was used. A background was calculatedbased on average atomic number (Z) of the specimen, as scaled toabsolute backgrounds measured on SiOr, NaNbO. and SnO2which have a large range in Z. Collimating slits in front of the

JONES AND LARSEN: NEPHELINE SYENITES FROM THE MOTZFELDT CENTRE

0rAb

1090 JONES AND LARSEN: NEPHELINE SYENITES FROM THE MOTZFELDT CENTRE

Table l. Rock samples analyzed

U n i t Sampl e Rock type S p e c i a l f e a t u r e s M a f i c m i 4 e r a l sana l yzed"

st41 63703AM46376058007.541 63D

58039c58065AM65

5803658037AM54AM39AM38AMl1OAt482

Al"l13946261At4137AI11 40

syen 1 ten e p h e l i n e s y e n i t esyen i ten e p h e l i n e s y e n i t e

n e p h e l i n e s y e n i t en e p h e l i n e s y e n i t en e p h e l i n e s y e n i t e

l a r v i k ' i t e l i n g d y k el a r v i k i t e r i n g d y k e' l a r v i k i t e

r i n g d y k el a r v i k i t i c m a r g i nl a r v i k i t i c m a r g i nn e p h e l i n e s y e n i t en e p h e l i n e s y e n i t e

w h i t e l u j a v r i t ed a r k l u j a v r i t ew h i t e l u j a v r i t ew h i t e I u j a v [ i t e

AMPNZ r - r ' i c h a e g i r i n eN b - r i c h a s t r o p h y l l i t e A M P H

o'l i vi ne-beani ng AMPHAMPH, AENIG

i n te rcumulus amphibo le

p l a g i o c l a s e r e l i c s 0 Lp l a g i o c l a s e r e l i c s M I C Ap l a g i o c l a s e r e l i c so l i v i n e - r ' i c h C P X . 0 Lo l i v i n e - r i c h C P X . 0 L

CPXc u m u l u s e u d i a l y t e ( r a r e ) A E N I G

eud ia ly te phenocrys tseud ia ly te -poor

AMPN

SM4

SM5

SM6

- M a f i c s i l i c a t e a n a l y s e s g i v e n i n J o n e s ( 1 9 8 4 ) f o r A M P H = a m p h i b o l e . 0 L = o l i v i n e , C P X

= c l inopyroxene. AENIG = aen igmat i te and l t4 ICA as ind ' i ca ted ,Dwho le

rock sample no t ana lysed fo r ma jor e lements bu t be l ieved s i rn i la r to 58007.c R b

a n d S r d a t a g i v e n i n B l a x l a n d e t a l ( 1 9 7 8 )

S a m p l e s f r o m e a c h u n i t a r r a n g e d w i t h i n c r e a s i n g D . I . d o w n w a r d s ( s e e T a b l e 2 ) .

5 -d ig i t numbers are GGU (Green land Geo log ica l Survey)s AM-pre f ixes are f rom A,P.Jones .

Table 2. Whole-rock major element analyses (wt.%)

Unit

Sdrple rc: 63703 Al44

SIt[ Sl'14

63760 58007 58039 58066 a,r65

s!15

58o36a58o3fAM54a lr',r39a a,r38 Ar{Lo Ar.482

s},t5

AI'1I39 46251 AI"1I37 AXt40

si-o2

Tio2

N20:F"203

FeO*

IlrO

r@

Na20

K2o

H20

SUM

60.98 56.51 63.69 s8.27 54- '17 54.85 56.2 '7 s1.r0 sr .87 53.44 49.73 55.95 57-24 5 '1.93 54.04 55.97 55.59 58.0?0 .88 0 .36 0 .22 0 .3 I 0 .91 0 .88 0 .09 2 .4 ' 1 2 .30 2 .26 I . 58 1 .58 0 .34 0 .34 0 .37 0 .02 0 .16 0 .07

16.55 20.s l 15.48 20.14 18.99 19.30 21.55 15.82 15.96 t5. '? ' t 16. I4 r '1.91 20.22 20.66 16.68 18.72 19.47 2L.293 . l o o .oo 0 .28 0 .48 Lo .25 7 .42 5 . c3

4 .4 ' 1 5 .06 3 .05 4 .09 ' 1 . 93 6 .54 2 .6 ' 1 10 .87 9 . s4 9 .46 I2 .3 r 6 .70 4 .30 2 .9L 1 .48 1 .39 I . 26 2 .3 r0 .19 0 . ] - 8 0 .2L O .L7 0 .30 0 .28 0 . r 7 0 .29 0 .25 0 .25 0 .51 0 .21 0 .20 0 .14 0 .37 0 .17 0 .27 0 .300 .67 0 .60 0 .08 0 .29 0 .87 0 .93 0 .04 2 . ' t 2 2 .50 L .73 2 .43 1 .21 0 .44 0 .30 0 .48 0 .03 0 .03 0 .184 ,32 I . 45 0 .57 1 .10 2 . t 4 2 .L8 I . L2 5 .65 6 .40 s .26 4 . r 5 3 .43 r .O3 0 .92 0 .99 0 .?5 0 .92 0 .35s . s8 8 .06 6 .41 8 .00 7 .35 ' 7 . ' 72 r 0 .50 4 .38 4 . s3 5 .24 5 .99 6 .00 9 . I 1 8 .74 10 .92 10 .5s 9 .40 r0 .706 .27 5 .94 5 . s0 6 .28 5 .46 5 .51 6 .24 3 .92 3 .10 4 .34 4 . s r 4 .67 5 .80 6 .28 3 .61 3 .96 5 .15 4 .850 .24 0 . l r 0 . 02 0 .16 0 .25 0 .62 0 .o7 1 .85 2 .01 1 .30 r . 43 1 .20 O ,14 0 .13 0 .04 0 .02 0 . t I 0 . 03

0 .36 0 .74 0 .85 0 .47 0 .94 0 .05 2 .51

l 00 . l s 98 .78 99 .69 98 .81 98 .9 '7 98 .87 98 .72 99.08 99 .05 99 .79 99 .92 98 .92 98 .82 99 .30

Na, / (Na+K) 0 .57 0 .67 0 .64 0 .65

(Na+K) /AI 0 .96 0 .96 1 .00 0 .99

I .PRM Ne(Qz) I .3 19 .1 (4 .3 ) 16 .6

D . r . 8 2 . 8 8 8 . I 9 1 . 5 9 1 . 7

100.17 100.06 100.00 98.15

0 .82 0 .80 0 .73 0 .77

1 .30 1 .16 I . 08 I . 07

r6-s 20.2 16.9 24.3

73 .9 82 .2 90 .8 93 .0

0 .6 ' 1 0 .68 0 .72 0 .53 0 .65 0 .65 0 .67 0 .55 0 .70 0 .68o .94 0 .97 r - r 2 0 .72 0 .72 0 .84 0 .91 0 .83 1 .05 1 .0216 .0 I 8 .4 30 .0 1 .5 1 .0 5 .1 l 3 . s 3 .6 22 .8 22 .580.8 83.5 89.0 s9.4 59.8 66.2 66.6 76.0 8 ' t .9 90.7

Note I - *totil ircn wtse Fe2o3 rct detenhed. ArEIyss trrarged in ircrsirlg order of differotiatj-on index (D.I.) for eadl mit.a*nples frm ring dyke ard rerqiDl syenite @ntainirg plagioctase reli6. Al-I arElyses bV XXF of pressed pcdder pellets at Durhm

UniveFiq', q€Pt H2O ard Fe2O3 bV titraLion. Avsage Fe3Ae2 values for iFFM €ldlatioN obtairEd for eadr uit rerer SMt = I.8,

944 = 0.66, SM5 = 0.05 (58036, 58037. AI,154) i ld 0.30 (ttE Gt), St'16 = 5.6.

JONES AND LARSEN: NEPHELINE SYENITES FROM THE MOTZFELDT CENTRE

Table 3. Whole-rock trace element analyses (ppm)

UnitSarple 63703 AI'14 63760 58007 54163 58039 58066 AM65 58035 58037 AI454 A1439 4438 Al4I10 AI\,182 A![39 46261 AI!'1137 AI{140

1091

Ba 2L42 568Rb 107 22'tsr '169 280

ra 53 .3 86 .3Ce I33 199Nd 48.7 69.9Sn 8.35 11.0Eu 5.09 I .60rb o.77 L.23Yb t .6r 3.24Lu 0 .25 0 .48

v 2 4 3 3rh 3 .4 10 .9u 4 . 0 2 . 6

zr 308 870Hf -t.4 20Nb 73 205Ta 4.8 12Sc 11 I .2zn L22 123Pb 23 10

Ey'E\r* 2.33 0.52ce/Yb 82.6 6I.4zr4t t 41.6 43.5Hf/"ra 1.54 I.67NbAa 15.2 I7.1

195 I29 r204I4 274 273r20 80.4 98.s17 .3 12 .8 15 .41 .27 1 .53 1 .58l q q t a ? t a t

o - z J J . z t z . ) l

0 .91 0 .48 C .38

58 3022 .6 L3 .L 9 .7113 8 .9 7 .3

558 6342 8 ^ 1 8

1589' 176^- -a . ̂ZJ ' LZ

1 . 6 1 . 5454 r5858 19

1254 898 57232 185 2804t3 412 55

r19 167 2IL290 397 444t1l 137 16315. I 17-5 26-42 . 1 6 5 . 0 0 2 . 1 5L . 7 7 2 . 0 6 3 . L 24 - 7 3 5 . 0 1 8 . 6 80 . 7 5 0 . 8 6 1 . 1 7

42 52 98L7-2 lr9.6 25.96 . 0 4 . 0 6 . 9

748 5192I T4r'17 L77t 3 6 . 67 . 5 0 . 6176 91 .5

24

4618 514341 37

1219 160r

72-2 60.4151 ]-2282.7 63.715.0 11.56 . 8 4 6 . 6 0r . 7 0 0 . 8 22 . r2 I . 9 lo . 3 2 0 , 2 7

33 292 . 0 2 . 32 . 7 7 . O

2635 170289 t22

872 841

82.7 L96181 43063.0 19sL4.2 28-8

l -68 2 .753 - 4 4 5 . 5 20 . 5 5 0 . 8 0

5 . r 1 0 . 12 . O 8 . 2

1 .49 0 .9152.6 77.934 .5 34 .0L .73 r . 4413 .6 t 4 . 0

104 83 .1 6? .12L5 I73 L429 9 . 1 5 9 . 2 4 8 . 31 5 . 0 9 . 9 5 8 . 3 78-33 2 .45 2 .901 . 2 8 1 . 3 5 0 . 9 6z . L L ) . J ) Z - O L

o . 2 9 0 . 7 8 0 . 4 1

32 37 263 . 6 7 . 9 1 0 . 05 . 7 r 0 . 0 4 . 5

1324 70'l4I 162'tI r5426 l0

107 84.5vl 27

z - L 5 U - 6 L L . Z Z

101 .9 32 .3 54 .43A.6 32.3 44.2f . r z r . t 6 f . o u12 .6 r 0 .4 15 .4

270 352

2t9 2Ea.4 45-rr . 95 2 .523 .19 8 .158 .13 34 .5r . 22 4 -74

86 25r? a ? q q

27 2r

20c5 423053 r47608 4782'1 793 . 6 0 . 3

51 48

o .24 0 .L774.2 L9.337.8 28.81 .96 1 .86z z . > o . L

339 253577 460199 I842'1.2 24.3r . 87 1 .823 .30 4 .78(3 .6 ) 12 .3l . 0 r 1 .68

1040 244926 50688 595L2 340 . 9 0 . 3-189 68257 58

c .24 0 .22

40 .0 40 .82 .L7 L .76

f f ) / u d 5 &

422 25L 20071 284 131

3766 433 878 190 40 250 26384 222 201 313 357 482 542

1992 24r 428 50 26 't3 56

108 L2'l186 9r.4s8 2r

40610

t o

9 4 . 3

0 . 2 8 0 . 4 4 0 . 3 6o b . f d J . 6 t u b . z

23.9 35.2 40.60 .12 1 .50 1 .285 .8 r 4 .7

0 .50 0 .99 0 .2861.3 '19 .2 5r .233 .6 35 .6 37 .1r . 50 L .60 2 . I 216.7 2I-3 17.7

1 . 6 1 2 . 3 87 5 . 9 6 3 . 935.2 34 .61 ? O 2 I t

1 2 . 8 I 7 . 5

L76 180 448 783 2825 .0 5 -2 13 23 7 .346 42 102 224 82

3 .6 2 .4 7 .5 L6 5 .525 35 51 9 . s 6 .629'1 337 287 r47

2 2 3 3 4 1 4

80824267t68 . 0263t3

Note: - Ba Rb Sr Y Pb zr ard Nb ffi1fred by )(RF at Drrtrdn University, tlE rmirder weie mlyzed at Risd (t)eIrwk) by INAt s€tdt for details. IIq staJdard rsults for GPG-BR versus international referc in brad<etst- Sc 22.2 (26.2) r Zn 55.8 (50), Ia8 2 . 9 ( 8 0 ) , c e 1 4 0 ( 1 4 0 ) , N d 5 2 . 5 ( 6 0 ) , s m 1 0 . 9 ( 1 2 ) , E u 3 . s 2 ( 3 . 7 ) 1 b 1 . 2 2 ( 1 . 1 ) , Y b . 8 1 5 ( 2 . 0 ) , I r . I 9 4 ( . t - 3 - . 2 ' 1 \ , 1 1 1 9 . 7 7 ( I 2 ) .asarple

contaire Nb-rich astjcphyUite.

detector gave a resolution of approximately 4'arc, or 5 eV for the

region of interest, but several overlapping lines could not be

avoided, despite selection of La or Lp lines, and an interference

matrix was established by experiment. Thus, interferen@s among

the REE were removed iteratively and finally conventional ZAF

corrections were applied to the complete analysis. Errors becomeparticularly serious at the low levels of heavy REE (especially Ho,

Tm, Lu). Subsequent inspection of the results showed residual

interference from Mn on Dy, which was discarded on Mn-rich

minerals, and Eu represents a maximum since a small contri-

bution from Pr or Sm at the EuLp peak used could not be avoid-

ed.

Whole rock geochemistry

Major elements

Whole rock major element analyses are given in Table 2and include FerO. and HrO for several samples. Calcu-lated differentiation indices (D.L) from the CIPW norm arebased on the D.I. of Thornton and Tuttle (1960) plus nor-mative acmite. D.I. values range from 59 to 93 (Table 2)and on this basis samples can be conveniently referred toas more- or less-fractionated. The syenites all contain nor-mative nepheline (Ne) which, apart from the low values,corresponds more or less with modal proportions esti-mated from hand specimens and thin sections. One obvi-ous exception is sample 63760 of unit SMI which has nor-mative quartz (Qz) at the Fe3*/(Fe2* + Fe3*; value used.

This sample has been hydrothermally altered and containsneither free quartz nor nepheline. The peralkalinity indexincreases with D.I. such that the fractionated samples ofeach unit are peralkaline with (Na + K)/Al > 1, as are allsamples of unit SM6 (lujavrites). The ratio Na/(Na + K)varies little within each unit, and the total range for all ofthe units is from 0.57 to 0.82.

A plot of the major oxides versus SiOr (not illustrated)shows the same features exhibited by the larger data base(90 additional samples) of Jones (1980). Such Harker variation diagrams are of limited use, due to the restrictedrange of SiO, in most units, except for SM5. The latter unitshows simple trends of decreasing MgO, FeO, PrOr andTiO, with increasing SiOr; CaO decreases after a hiatusnear 52 wt.% SiOr, and AlrO. remains nearly constant toapproximately 53 wt.ok SiOr, above which it increases.Apart from the two silica-rich samples of SMl, all themajor oxides of the other syenites cluster loosely aroundthe SM5 data in the range 54-58 wt.% SiO2.

Minor elements

Table 3 gives analyses for 8 REE and 13 other minorelements for the 19 samples. Blank spaces in Table 3 arevalues below detection limits and the precision for eachelement can be approximately inferred from the number ofsignificant figures quoted. Ba, Sr and Sc decrease by ap-proximately two orders of magnitude from their highest

tw2 JONES AND LARSEN: NEPHELINE STENITES FROM THE MOTZFELDT CENTRE

lo

UJFEozOIO

YOOE

[!ulE

Loc. s#u

values in the least-fractionated samples and have theirlowest values in the lujavrites of SM6. Zn decreases withincreasing D.I. in each unit but is highest again in unitSM6 and the hydrothermally altered sample of unit SMI(63760\. Zr, Hf, Nb and Ta are highest in unit SM6, al-though these elements do not increase systematically withD.I. or any other obvious element. The average ZrlHf ratiois 37 (+4.2,2o) lor the nineteen samples, excluding 63760which has ZrlHf :23.9. The ratios Hf/Ta and Nb/Ta arereasonably constant at approximately 1.5 and l5 respec-tively, for most samples except 63760 which contains Nb-rich astrophyllite. The Nb/Ta ratio is also particularly vari-able in SM6 which contains pyrochlore.

REEThe REE whole-rock data are given in Table 3 and were

normalized using the customary chondrite abundances ofNakamura (1974\.The normalized REE (REEN) are plottedfor each syenite unit in Figure 3. Recent revisions ofchondritic abundances, such as those employed by Davis etal. (1982) would increase the REEI levels overall, but causeonly minor differences in their relative distributions. Fordescriptive purposes, a distinction is made between leastfractionated and fractionated samples of each unit, basedon D.I. This does not require that all samples are relatedby crystal fractionation, but is a useful mechanism for dis-cussl0n.

As illustrated in Figure 3, all of the samples are steeplylight REE-enriched, with high positive values of Ce/Yb andmost have a Eu-anomaly. The Eu-anomaly may be nega-tive or positive and varies greatly in relative size. It can be

YbLuLoc. srEu

expressed numerically as Eu/Eu* where Eu* is the expectedEu interpolated from the two adjacent REE (in this caseSm and Tb) assuming no Eu-anomaly. The least frac-tionated sample grouped with unit SMI (63703) has thelowest REE and a prominent positive Eu-anomaly. This isvery different from other samples of SMI and is nowthought to belong to an earlier syenite that predates SMI(Tukiainen, pers. comm. 1984) and is excluded from Figure3. All samples from SMI have a negative Eu-anomalywhich shows some correlation between its size (Eu/

Eu* < l) and the level of total REE. The most fractionatedsample of SMI contains the most REE, except for Eu, andis the silica-rich hydrothermally altered 63760. The threesamples of unit SM4 have similar REE levels (Fig. 3) de-spite their large geographical separation (Fig. l) and mea-sured range in major element chemistry (Table 2). Onesample has a negligible Eu anomaly and two have substan-tial negative Eu-anomalies. By a narrow margin, unit SM5has the overall lowest REE levels and five of the sevensamples have positive Eu-anomalies of varying size. Thelatter include three samples from the earliest marginal lar-vikite (SM5* partial ring dike, Table l) which containplagioclase cores to alkali feldspars. There is a suggestionthat the three samples of SM5 with small Eu-anomalies(AM82, AMll0, AM54) have a kinked REE pattern, whichis steep from La to Sm but flatter in the heavy REE fromTb to Lu; the same is true for two samples of SMl. Thisreally requires a closer spread of analyzed heavy REE be-tween Tb and Lu to analyze further. Unit SM6 lujavriteshave the highest REE levels, and these become less steep(CelYb from 162 to 19, Table 3) with increasing total REE

YbLu

Fig. 3. Whole-rock REE normalized to chondrite abundances (Nakamura, 1974) plotted on log scale.

Ne- syenif e SM4 Lu jovr i te SM6

A

oA

o 8 o63760

200 400 m0 8m 1m0 Dm 1400 1600 18m 2mSr pprn

Fig. 4. Eu/Eu* versus Sr (whole-rock). Eu* is straight line in-terpolated value between Sm and Tb, assuming no Eu-anomaly.The data show good correlation between decreasing Eu/Eu* andSr. Sample 63760 indicated for SMI is hydrothermally alteredsyenite.

abundances. The one dark lujavrite (46261) typified by eu-dialyte phenocrysts (Fig. 5) has the highest REE measured,and the pattern is more obviously kinked. Comparison ofall the patterns in Figure 3 shows that of the three syeniteunits analyzed, SMI and SM4 have the closest similarity tounit SM6; this is important when considering the pet-rogenesis of the lujavrites. Thus, there is a reasonable over-lap between the highest samples of SMI (63760) and SM4(AM65) and the lowest REE sample of the lujavrites(AM140).

There is an interesting relationship between the Eu-anomaly and Sr for all the syenites, shown in Figure 4.Eu/Eu* decreases uniformly with decreasing Sr, withEu/Eu* > I in Sr-rich larvikites to Eu/Eu* < I (i.e., nega-tive Eu-anomaly) in Sr-poorer nepheline syenites of SM5.A similar relationship is shown by the other syenites, withlowest values of Eu/Eu* representing the largest negativeEu-anomalies found in Sr-deficient lujavrites of SM6. Simi-lar correlations are found in analogous plots of Eu/Eu*versus Ba and Ca (not shown). This is important since Ba,Sr, Ca and P all decrease with increasing D.I. ahd SiOr,and both apatite and plagioclase feldspar are common inthe early larvikites of SM5. Their relative roles might beexpected to directly influence Eu-anomalies and are dis-cussed in a later section.

REE-bearing minerals

The REE are present in a number of different minor andaccessory minerals in the Motzfeldt syenites and some ofthese increased or diminished in importance possibly inresponse to changing compositions of host magmas. Somemay have been replaced during circulation of metasomaticfluids. A number of these minerals have been analyzed byelectron microprobe and representative analyses are givenin Table 4. Examples of their textural relationships areillustrated in Figure 5. Apatite, which is characteristicallylight REE-rich (e.g., Henderson, 1980; Larsen, 1979) and

r093

usually has a negative Eu-anomaly (Watson and Green,1981) is common in the early and mafic syenites, such asthe larvikites of SM5, but is largely absent from the morefractionated syenites. This is reflected in whole-rock PrOtvalues in Table 2, which decrease with increasing D.I.Zircon, which has a heavy REE-dominated pattern (e.g.,Exley, 1980) is common in many of the nepheline-poorsyenites. Euhedral zircon forms up to 10% of the mode inrare mafic bands with melanite garnet and euhedral py-rochlore in unit SMI (Jones, 1980). Zirconium also occursin solid solution in late-stage aegirine, with up to 7.0 wt.o/oZrO2 in syenites of unit SM3 (Jones and Peckett, 1980). Inall analyzed aegirine, however, REE levels were below de-tection (50 to 100 pprn etrement) even in the most Zr-richaegirine. More irnportantly" Zr occurs in the mineral eu-dialyte, Naa(Ce,Ce,Fe)tZrSiuOtt(OH,F,Cl)2 in the mostfractionated syenites and this mineral contains up to 7.0wt.% RE-oxides (RE2O3, Table 4). Eudialyte occurs as anessential mineral in the lujavrites (SM6) where its habit ofphenocryst or interstitial texture (Fig. 5) distinguishes darklujavrite from white lujavrite (Jones, 1980). Eudialyte fromunit SM4 (AM49, Fie. 6) is light REE-enriched and eu-dialyte from white lujavrite (Table 4, AMl59) contains thehighest RE2O3. It should be emphasized that the heavyREE measured by electron probe are close to theoreticaldetection limits and have serious limitations in precision(circa * l0oh or worse). Nevertheless, the analyzed eu-dialytes have a kinked REE pattern which is steep from Lato Nd and flatter from Sm to Yb (Fig. 6). Y generallybehaves like Ho, a heavy REE (e.g., Jones and Ekam-baram, 1985) and measured values for the two eudialytegrains in Table 4 have chondrite-normalized abundances inthe range 3-7 x 103, in agreement with the heavy REEshown in Figure 6. This kinked pattern for eudialyte com-pares favorably with the whole-rock REE for dark lujavrite(46261') which is similarly kinked (Fig. 3) and shown forcomparison in Figure 6. REE in the dark lujavrite mightreasonably be expected to reflect the REE-bearing eu-dialyte. In detail, the ratios ZrlCe and ZrlYb for eudialyteand whole rock do not agree very closely. This is probablybecause of additional Zr and REE mineral hosts in thelujavrites including alteration products of eudialyte. Wholerock Zr abundances for the dark lujavrites (Table 3) agreewith modal estimates of circa 5% eudialyte phenocrysts(i.e., 5o/o x L2.2 wt.%o ZrOr:4500 ppm Zr rock). Conse-quently, the REE are expected to be 2-3 times lower inunanalyzed interstitial eudialyte from white lujawite, whichhas similar amounts of eudialyte but lower whole rock Zr.Much of the eudialyte in the lujavrites has been replacedby secondary hydrous minerals, including catapleiiteNarZrSi3Oe'2HzO (Fig. 5). The same lujavrites show re-placement of nepheline by cancrinite, fine-grained whitemica (?Li-rich) and sodalite; coexisting feldspars of albiteand microcline are uneffected. The evidence suggests eomealteration by alkali-rich, Cl-bearing hydrous fluids, whichmight also have modified the REE, ZrlCe and ZrlYb rockratios.

Anhedral sphene (CaTiSiOr) is locally common as an

JONES AND LARSEN: NEPHELINE SYENI?ES FROM THE MOTZFELDT CENTRE

tw4 . IONES AND LARSEN: NEPHELINE SYEN/TES FROM THE MOTZFELDT CENTRE

Fig. 5. Photomicrographs of typical accessory phases in the Motzfeldt syenites: all have width near 2 mm, all plane polarized lightexcept (c) which has partly crossed polars. (a) Clear, well-cleaved (and parallel twinned, not shown) rinkite in center is interstitial toturbid alkali feldspar, clear nepheline and dark amphibole (AM7, unit SM4). (b) High relief and irregularly fractured phenocrysts ofeudialyte (center and above center) with aegirine, nepheline, albite and microcline (various shades) in dark lujavrite (AM164, unit SM6).(c) High relief interstitial and partly altered eudialyte (just right of and below center) with aegirine blades (dark) in white lujavrites(AM138, unit SM6). (d) Moderate relief interstitial Mn-pectolite with good cleavage (near center) with albite (clear), microcline (turbid)

and bladed aegirine (dark) in white lujavrite (AM139, unit SM6).

accessory mineral in the Motzfeldt syenites; althoughbased on typical analytical totals near to IOO%, it is not amajor repository of REE. Sphene can be light REE-enriched in felsic rocks (e.g., Exley, 1980; Fleischer, 1978;Henderson, 1980). The majority of the Motzfeldt syenitescontain no sphene but usually have accessory mineralsof a variety of Ca-Na-Ce-Ti-Zr-Nb-silicates of therinkite-mssandrite and livenite series; these contain up to-22 wt.Yo RE2O3 (Table 4). The same minerals are also

common in the adjacent North Q6roq Centre, whereChambers (1976) established a correlation between theabundance of rinkite and the degree of contamination ofthe syenites by metasomatized country rocks. These min-erals are usually interstitial, or less commonly euhedral,and colorless to pale yellow;they are rarely visible in handspecimen. They often have anomalous birefringence andcan have lamellar twinning superficially resembling plagio-

clase. Rinkite can be represented by the formula

JONES AND LARSEN: NEPHELINE SYENIT'S FROM THE MOTZFELDT CENTRE

Table 4. Electron probe analyses of accessory minerals (wt.%)

1095

2 3 * 4 * 5 c * 5 r * 6 7 * 8 rsphene z i rcon 1av r i n k r i n k r i n k l a v e u d eud Mn-pec t

N b 2 o 5 0 . 9 0

s i o 2 2 9 . 4 2

T i O 2 3 1 . 4 1

z r o 2 0 . 2 0

Hfo 2

A I 2 O 3 0 . 6 4

Y ̂ O .

La2O3

C e 2 0 3

P t 2 0 3

N d 2 0 3

Sm203

Gd203

D y ̂ o .

- " 2 '3Yb203

F e O 1 . 3 5

M n O 0 . 1 3

M g O 0 . 1 9

C a O 2 7 . 8 3

N a 2 O 0 . 8 4

K 2 O 0 . 0 3

3 . 0 r 1 . 6 1

3 1 . 1 9 3 0 . 0 4

I 0 . 3 4 6 . 6 9

6 . 2 6 9 . 1 5

1 . 3 8 0 . 1 9

2 . 6 5 1 . 9 0

1 . 5 3 0 . 4 r

f . f J t - ) z

0 . 5 3 0 . 2 9

2 . 2 0 0 . 7 3

0 . 5 1 0 . 2 4

0 . 4 8 0 . 2 5

0 . 6 3 0 . 3 6

0 . 2 8 0 . 2 4

0 . 2 5 0 . 2 r

0 . 9 7 0 . 8 2

0 . 7 9 0 . 9 1

n . d . n . d .

2 0 . 3 0 2 8 . 2 9

3 . r 2 8 . r 2

0 . 2 L 0 . 0 8

1 3 . 5 0 L . 7 4

3 0 . 2 4 4 7 . 3 3

r . . 0 1 o . 1 5

1 5 . 0 9 7 2 . 5 2

0 . 0 8 0 . r 8

0 . 9 -

0 . 4 9

t . 2 9

o . 2 2

0 . 4 1

0 . 1 5

0 . 1 0

o . 1 2

0 . 0 6

0 . 7 8 6 . 9 ' , 7

0 . 7 1 2 . r 9

0 . 1 I n . d .

3 0 . 5 4 1 0 . 4 0

2 . L 6 1 2 . 1 3

n . d . 0 . I 7

2 . 4 7 n . a .

4 6 . 5 1 5 5 . 9 5

0 . 1 1 0 . o 2

t 2 . 2 7

0 . ' 13 0 . r 1 .

1 . 5 -

1 . 4 0

2 . 8 7

0 . 3 9

0 . 1 6

0 . 2 6

0 . 2 8

b

0 . 2 0

0 . 2 5

6 . 7 3 1 . 0 7

4 . 5 9 1 5 . 5 6

n . d . 0 . 1 6

b . z ) f o . 4 {

r 0 . 7 9 8 . 8 2

0 , t 2 0 . 0 8

3 2 . 2 9

0 . I 5

6 6 . l 3

1 . 0 3

0 . 0 5

0 . 2 7

0 . 2 2

0 . 0 8

0 . 0 4

0 . 0 5

0 . 0 2

2 . 6 3

3 0 . 3 0

o . 7 4

I 7 . 1 3

0 . 0 1

0 . 1 7

0 . 4 9

0 . 0 9

0 . 2 6

0 . 1 8

0 . 1 5

b

0 . 1 3

0 . 0 7

1 . 5 3

I . 3 7

0 . 0 5

3 0 . 7 8

6 . 4 9

0 . 0 2

2 . 6 6

3 0 , 4 5

1 0 , 4 1

I . 4 7

5 . 0 3

2 . 7 ' 1

3 . 4 6

1 0 . 4 9

I . U ]

4 . 3 6

o . 7 7

0 . 6 8

0 . 1 4

0 . 2 7

0 . 1 5

6 . 6 3

0 . 0 9

0 . 0 3

1 0 . 1 8

I . 9 6

u . a 5

9 3 . 2 L9 9 . 5 0 1 0 0 . 3 3 9 4 . 5 6 9 2 . 2 6 9 2 . 0 6 9 4 . 2 2 9 1 . 5 3 9 8 . 4 3 9 8 . 2 L

Note : - *REE pat te rns i l l us t ra ted fo r these minera ls in F ig . 5 . I = A laS l in te rs t i t ia l sphenein coarse-gra ined ne-syen i te (S Ia5) . 2 = 63717 z i rcon core o f euhedra l g ra in (0 .2 cm) j .n maf ic bandeds y e n i t e ( S M 3 ) . 3 = A M 5 l ? l a v e n i t e i n t e r s t i t i a l g r a i n i n f o y a i t e ( S M 4 ) . 4 = A M 8 1 y e l l o w e u h e d r a l? r i n k i t e ( 0 , 3 c m ) i n c o a r s e - q r a i n e d n e - s y e n i t e ( s M 5 ) . 5 = A M 8 4 c o r e ( c ) a n d r i m ( r ) o f y e l l o w z o n e d? r i n k i t e e u h a d r a l g r a i n ( 0 . 5 c m ) i n c o a r s e - g r a i n e d n e - s y e n i t e ( S M 5 ) . 6 = 6 3 7 2 5 N b - r i c h ? l a v e n i t ei n t e r s t i t i a l g r a i n ( s M 4 ) . 7 - A M 4 9 i n t e r s t i t i a f c o l o r l e s s e u d i a l y t e i n f o y a i t e ( s I 4 4 ) . 8 = A M 1 5 9i n t e r s t i t i a l p a l e y e l 1 o w e u d i a l y t e i n w h i t e l u j a v r i t e ( S M 6 ) . 9 = A M 1 3 8 c o l o r l e s s t o p a L e y e l l o w /b r o w n i n t e r s t i t i a l M n - p e c t o l i t e i n w h i t e l u j a v r i t e ( S M 5 ) .aes t imated f rom peak counts on ly , bser ious

in te fe rence f rom Mn. n .d . = rDtde tec ted . .

(Ti,Nb,Zr{Na,Ca)r(Ca,Ce)o- (SirO?)r(O,F)u (Galli and Al-berti, 1971) and a typical example from Motzfeldt is shownin Figure 5. A more general formula for this family wouldbe XYZTOF(SirO?) and a lew representative (partial)analyses are given in Table 4. Recasting the cation propor-tions of seven such analyses assuming Si : 2.00 gave totalsfor (X + Y + Z) from 3.01 to 4.05, with cation totals vary-ing from 7.91 to 9.25, indicating more than one mineraltype. There is a chemical similarity between therinkite/lavenite minerals and sphene (Table 4) consideringthe nearly constant SiO, and the common substitutions ofNa. Nb and Zr for Ca and Ti seen in other minerals.Sphene and rinkite do in fact share some structural ele-ments (P. B. Moore, pers. comm., 1984) and chemicallyrinkite represents an enriched variety of sphene. The not-ably fresh appearance of rinkite in some altered syeniteswith clouded feldspars and general petrographic textureshint at a possible metasomatic origin. Detailed electronprobe analyses of the REE in rinkite are plotted in Figure

6. The larger crystals ( > I mm) are zoned from REE-richcores to relatively REE-poor rims (Table 4).

Interpretation

Petrogenesis i

The systematic variations shown by major elements be-tween early mafic larvikites and evolved nepheline syenitesof unit SM5 can be related either by fractional crys-tallization or partial melting. Using the most basic mineralcompositions analyzed from the larvikites (Jones, 1984) thetrends have been modelled (not shown) by early extractionof olivine (Ol) and plagioclase (Pl) in approximately equalproportions, followed by extraction of Ol + Pl with ad-ditional clinopyroxene (Cpx) with or without amphibole(Amph). Alternatively, successively derived partial meltswould be in equilibrium with residues of Ol + Pl + Cpx

t Amph and Ol + Pl respectively. Olivine and clinopyrox-ene occur as phenocrysts or early formed crystals, and

1096

Fig. 6. Chondrite-normalized REE data for accessory minerals.(a) RinkiteJivenite types including core and rim of zoned grain;(b) eudialyte from unit SM4 (AM49) and unit SM6 (AM159)showing kinked REE patterns (steep La to Sm, flatter Gd to Yb)which compare well with the whole-rock data for a lujavritesample, which has eudialyte phenocrysts (46261, similar to illustra-tion in Fig. 5 (b)).

plagioclase is found in cores of alkali feldspars in the larvi-kites; amphibole only occurs as an interstitial mineral.Field relations show that the larvikitic margins graduallymerge into nepheline syenites and occasional banding withcumulus textures is found in the nepheline syenites of SM5(Emeleus and Harry, 1970). Plagioclase compositions usedwere constrained by relations between CaO, AlrO, andSiO, to be at least as An-rich as the most calcic cores ihthe larvikites (<Anze) for the low silica part of the trends(SiO, 49-54 wt.%o). ln fact, using more anorthitic plagio-clase, such as that analyzed as megacrysts in the AGGDlAnrrO{r) worked better, with lower residuals. It is con-sidered unlikely that the apparent changes in bulk.compo'sitisn and mineral chemistry could be explained by rapidlychanging conditions in a source region duiing partial melt-ing, and crystal frectionatibn can be justified by, for exam-ple, the phenocrysts or early-formed crystals present. ThFchange in mineral assemblage controlling the trends, i.e.,

JONES AND LARSEN: NEPHELINE SYENITES FROM THE MOTZFELDT CENTRE

oL

o-c.(-)ulluGE

subtraction of Cpx + Amph above approximately SiO2 :

54 wtJ/o SiO, coincides with the transition from larvikitesto nepheline syenites. This might be interpreted as a changefrom Ol * Pl controlled magmas in the ring dike toOl + Pl * Cpx * Amph in the main chamber at currentexposure levels. We believe that the magmas are derived bycrystal fractionation processes.

By analogy with unit SM5, the other syenite units of theMotzfeldt Centre are taken to have similar origins. This issupported by overlapping whole-rock and mineralogicaltrends (Jones, 1980, 1984) and field relations, although onlySM5 is clearly associated with larvikitic magmas. Timeintervals between overlapping intrusions were at least longenough to allow solidification. Rather similar fractionationschemes were deduced by Upton and Thomas (1980) for animportant suite of transitional olivine basalts in the LateGardar Younger Giant Dike Complex at Tugtutdq(YGDC). The Tugtutdq YGDC was related by relativelylow pressure (crustal, < 10 kbar) Ol + Pl fractionation, andan even deeper level evolution apparently dominated bypyroxene and garnet. The Tugtutdq YGDC encompasses alarger compositional range than 8 preliminary analyses ofthe Motzfeldt AGGD, but their ultimate petrogenetic his-tories are undeniably similar. Even plagioclase megacrystsin the Motzfeldt AGGD and anorthosite inclusions in theTugtut6q YGDC have similar compositions (Anrr,Anrr_rr). These giant dikes tapped a magma source thatmay have been available in large quantities throughout theGardar Period. Indeed they may have been directly relatedto the intrusive centers by dilation and subsequent ringfracture, as envisaged by Bridgwater and Harry (1968). Theconsistently low initial E7Sr786Sr ratios are taken to implya mantle source (Blaxland et al., 1978; Patchett et al., 1976)and preclude large scale involvement of the JulianehibGranite (upper crust).

Eu-anomalies

The most fractionated syenites have the largest negativeEu-anomalies, there is a simple relationship for Eu/Eu*versus Sr (Fig. a), and Sr decreases with increasing frac-tionation. Thus, large positive anomalies exist in the least-fractionated Sr-, Ba- and P-rich larvikites of SM5, whichare considered to be related to the evolved nepheline syen-ites by crystal fractionation (section above). Similar posi-tive Eu-anomalies exist in the Oslo region alkaline rocks,in most of the lardalites (nepheline-rich larvikites) andabout half of the larvikites (Neumann, 1980), and theircharacter and compositions are comparable to the larvi-kites of SM5. Neumann interpreted the Oslo samples withpositive Eu-anomalies as cumulates, derived by a complexscheme of addition and subtraction of feldspar, apatite andfemic minerals in specific proportions. A model involvingBeparation of clinopyroxene, apatite and kaersutite wasproposed to explain much smaller positive Eu-anomalies inanorthoclase phonolites from Ross Island (Sun andHanson, 1971, p. 149). It is well established that the anor-thite (Ca) component in feldspar exerts a strong preference

Lo Ce ft Nd Sm Gd Tb Dy Er '/b Lu

for Eu2* over the other (trivalen| REE (e.g., Cox et al.,1979). Removal of plagioclase from a silicate melt thereforetends to leave a melt with REE relatively depleted in Eu.Partial melts derived from a source with residual plagio-clase will suffer similar depletion of Eu2+. Experimentshave shown that Sr, and by analogy Eu2+, are less con-centrated by apatite than are the trivalent REE; althoughSr is still greater in apatite than the coexisting silicate liq-uids (Watson and Green, 1981). Only under unusually oxi-dizing conditions would apatite have no Eu-anomaly(Watson and Green, 1981) and the relatively apatite-richlarvikites have low measured Fe3*/Fe2+ whole-rock values(Table 2). Thus, apatite removal from a melt could enrichthe melt in Eu2* and potentially generate a positive Eu-anomaly. Extraction of both plagioclase and apatite, asdeduced from the whole rock chemistry for SM5, wouldhave opposite effects on the Eu-anomaly of the derivativemelts. Extraction of both minerals would successfullyremove Sr, but the transition from a positive Eu-anomalyto a negative Eu-anomaly implies that plagioclase had thedominant role. This leaves the problem of the initial posi-tive Eu-anomaly in the erly larvikites of SM5. An interest-ing, but speculative explanation is provided by the Motz-feldt data and rests on the observation of plagioclase cores,irregularly mantled by alkali feldspar in the larvikites. Wesuggest that the positive Eu-anomalies resulted directlyfrom large scale incorporation of pre-existing feldspar-richcumulates. Such early-formed Ol- and Pl-rich cumulatesfrom deeper levels would have contained relativelyanorthite-rich feldspar which would also have been Eu-richand the observed positive Eu-anomalies could therefore beexplained by mixing.

The amount of heat required for assimilation of existingcrystalline material would have been large, but could havebeen derived partly from contemporaneous heat of crys-tallization and in any case the plagioclase cores indicatethat the material was not completely melted. The largeamount of heat involved would also be intuitively consis-tent with the tendency towards increasing basicity of suc-cessive magmas. Excluding the lujavrites, whose relativeage is not well constrained, the last major syenite unitSM5, crystallized from the most basic syenite magmas atMotzfeldt, namely the larvikites. In a sense, this basic trendwas continued in the form of the later AGGD which cutsSM5 (Fig. l) and carries plagioclase megacrysts (Emeleusand Harry, 1970; Jones, 1980). A similar trend ofincreasingbasicity with time has been noted for other centers in theGardar province (e.g., Stephenson, 1976) and the commonassociation of plagioclase megacrysts with many of thebasic dikes was the subject of a lengthy study by Bridgwa-ter and Hary (1968). In view of the late intrusion of thelardalites in the history of the Oslo alkaline province, it ispossible that their positive Eu-anomalies could also haveresulted from incorporation of earlier plagioclase-richcumulates. A mechanism of partial melting was consideredby Neumann (1980) to be unlikely, but a combination ofassimilation followed by fractional crystallization could ex-plain their basic chemistry combined with relatively high

1097

Table 5. Proposed hierarchy of obvious accessory REE-bearingminerals

apat l te

r i nki te- l lveni te

zircon eud i a lyte

+ increaslng fract ionat ion

JONES AND LARSEN: NEPHELINE SYENITES FROM THE MOTZFELDT CENTRE

N o t e : - s e e a f s o J o n e s ( 1 9 8 4 ) f o r g e n e r a l c r y s t a l l i s a t i o n s c h e m e .

incompatible element and REE contents with positive Eu-anomalies.

REE-minerals

Given the range in the degree of fractionation of thesyenite samples (Table 2; D.I 59 to 93) and two orders ofmagnitude reduction in Sr, Ba and Sc, it is perhaps sur-prising that the variations in REE are so modest (Table 3).This is probably because the REE do not represent trulyincompatible elements and were not simply concentrated inshrinking amounts of residual magmas, Therefore, bulkdistribution coeflicients (Ko's) for the REE were probablynear 1.0 in the Motzfeldt syenites, and were similarly closeto unity in the Oslo larvikites (Neumann, 1980, p. 516).Several different minerals were responsible for extraction ofthe REE from the syenite magmas (Table 5) and their REEpatterns might give information about the geochemistry ofthe evolving magmas; they might also record the effects ofmetasomatism.

The expected light REE-enriched patterns for apatiteand sphene (not shown) in the less-fractionated syenites arepartly counterbalanced by the role of zircon, which isheavy REE-rich. With the exception of the positive Eu-anomalies, the REE in the basic syenites might be rational-ized in terms of different admixtures of apatite, sphene andzircon. In reality, of course, some REE are also distributedas trace elements throughout the major rock-forming min-erals. Calculations based on the distribution of La betweenknown modal minerals and measured whole rock valuesfor a nepheline trachyte dike (Larsen, 1979 and unpub-lished data), slightly more evolved than the larvikites ofSM5, indicate that about half of the trachyte's REE arelocated in fine-grained accessory minerals. Other REE-bearing minerals in Motzfeldt include melanite garnet andpyrochlore in mafic bands from unit SMI and pyrochlorein SM6. The role of sphene is apparently replaced by min-erals of the livenite-rinkite family and it is not surprisingthat these too are light REE-dominated. The Zr-rich vari-eties of livenite occur primarily in syenites devoid ofzircon, which may explain their flatter REE patterns (Fig.6 , AMsl ) .

The principal Zr-mineral changes from zircon to err-dialyte in the peralkaline syenites, possibly reflecting in-creasing availability of alkalies and volatile species. Theeudialyte shows light REE-enrichment but also has a

1098

l l i m a u s s a q

4 6 2 6 1

L a L uFig. 7 Chondrite-normalized REE distributions for the lujav-

rites (unit SM6, 46261) from Motzfeldt compared with the hypo-thetical parental magma for the llimaussaq intrusion (Larsen,r979).

kinked pattern due to some heavy REE. It is possible thatmost of the REE which would have been present in zircon,are instead accomodated in eudialyte in the more frac-tionated syenites. Thus, there is a strong and expected cor-relation between the probe-measured REE in eudialyte andthe INAA-measured whole-rock analyses of the lujavrites(Fig. 6), whose primary REE host is eudialyte. Similar com-parisons between probe analyses of dominant REE-minerals and host rock REE exist between perovskite andkimberlite (Jones and Wyllie, 1984).

D euelopment of luj aurite

Field evidence shows that the lujavrites intruded earliersyenites and behaved as magmas (Jones, 1980). The sim-plest interpretation is that the lujavrites are late-stageproducts derived by extensive fractionation of the syenitemagmas of unit SM 1 or SM4, and they were emplaced at ahigh structural level. Their remarkably high Zr contentsare consistent with experimental results and are taken to bedue to complexing of dissolved Zra* with "free" alkaliesnot associated with Al in the melt; a natural consequenceof peralkalinity (Watson, 1979; Watson and Harrison,1983). In terms of their chemistry, mineralogy and oc-currence, the lujavites (SM6) from Motzfeldt are com-parable with lujavrites and other agpaitic rocks in theIlimaussaq Intrusion, S. Greenland (Ferguson, 1964; Baileyet al., 1978) and in Lovozero, U.S.S.R. (Vlasov et al., 1966;Gerasimovsky and Kusnetsova, 1967). The SM6 lujavritesare somewhat coarser-grained than their counterparts inIlimaussaq, and have geochemical aflinities with kakorto-kites from the latter intrusion (Ferguson, 1964). Likelyranges for the REE content of the initial magma to theIlimaussaq Intrusion (?olivine-phonolite) were estimated

JONES AND LARSEN: NEPHELINE SYENITES FROM THE MOTZFELDT CENTRE

by Larsen (1979) and these are remarkably similar to thedark lujavrite (46261) of SM6 from Motzfeldt, as shown inFigure 7. The euhedral outlines and petrographic texturesof eudialyte in fractionated syenites from the central re-gions of SM5 (Jones, 1980) suggest that eudialyte justachieved the status of a cumulus mineral at Motzfeldt. Itseems reasonable that extrapolation of this process couldhave led to the formation of layered cumulate eudialyte-syenites similar to those so well displayed in the IlimaussaqIntrusion (Ferguson, 1970). The lujavrites at Ilimaussaq arelate-stage products, just as in Motzfeldt, and future com-parative work between the two occurrences might be veryuseful, although it is beyond the scope ofthis paper.

Metasomatism

Evidence for metasomatic fluids is seen in the lujavritesin the form of replacement of eudialyte by catapleiite andother minerals. On a more widespread basis, syenites fromelsewhere in the center show nepheline replaced by sodaliteand cancrinite, with the development of albitic rims to per-thites. By analogy with experimentally-derived stabilitylimits of sodalite in aqueous syenite systems (Wellman,1970) these replacement textures are interpreted to haveresulted from metasomatism by aqueous fluids. These fluidsmay have aflected whole rock ZrlCe and ZrlYb ratios, andmight even have been responsible for some growth of rink-ite minerals. The fairly common replacement of nephelineby natrolite may also be related to the same fluids.

Given that metasomatic fluids were widely available inthe Motzfeldt syenites, especially in the roof zones, then thesample of altered SMI syenite (63760) becomes particularlyinteresting. This sample is enriched in many of the sameelements found in the lujavrites, notably Zn and Pb (Table3), and is even more enriched in Nb and Ta, due to thepresence of Nb-astrophyllite. The REE, including the nega-tive Eu-anomaly, are also quite similar to the lujavrites(Fig. 3). This suggests that the metasomatic fluids, althoughnot necessarily contemporaneous, have apparently en-riched some of the normal syenites with the same elementsthat are characteristic of the lujavrites. The origin of thelujavrites was not metasomatic, as outlined above, but itshould be noted that the processes of late stage igneousenrichment and metasomatic fluids are not mutually exclu-sive, and may of course be related.

Summary

The following petrogenesis of the center has been pro-posed above:

(1) Sr isotopic evidence suggests an ultimate mantlesource and the most basic magma available was alkaligabbro (Ti-rich olivine-basalt) seen in the late giant dike(AGGD). Alkali gabbro, larvikites and nepheline syenitescan be related by progressive removal of Ol + Pl at deeperlevels iollowed by Ol + Pl + Cpx t Amph at current levelsof exposure.

(2) Positive Eu-anomalies in the larvikites have been ex-plained by incorporation of earlier plagioclase-rich cumu-

lates and are seen as plagioclase relics. This could be testedsince plagioclase separates from the larvikites, and prob-ably from the AGGD should have substantial positive Eu-anomalies.

(3) The lujavrites are late-stage products derived by ex-tensive fractionation of nepheline syenites, either of unitSMI or SM4.

(4) Metasomatic fluids apparently affected many of thesyenites and have geochemical signatures similar to thelate-stage lujavrites. A related origin is implied but morework is needed.

This scheme is in general agreement with the widely ac-cepted view of parental augite syenite (similar to SM5 lar-vikites) to many of the Gardar centers, and emphasizes theimportance of giant dikes such as those of transitional oli-vine basalt from Tugtut6q (Upton and Thomas, 1980), inthe Gardar province. The unexpectedly high positive Eu-anomalies and the apparent significance of plagioclase leadback to the ideas of Bridgwater and Harry (1968), whichseem to warrant renewed interest.

AcknowledgmentsThe Danish Natural Science Research Council are thanked for

grant no. 1l-O42'1. J. G. Holland and A. Peckett gave useful assist-ance with XRF and electron probe techniques at the University ofDurham, and P. B. Moore (Chicago) is thanked for help withsome REE-minerals. The Geological Survey of Greenland provid-ed generous support at all times and this paper is published withthe Director's permission.

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Manuscript receioed, December 3, 1984;accepted for publication, J uly 30, 1985.