School of Natural Resource Sciences

Queensland University of Technology

MAGMATIC EVOLUTION OF THE

SHIRA VOLCANICS, MT

KILIMANJARO, TANZANIA

By

Stephen John Hayes

B.App.Sc. (QUT)

2004

Supervisor:

Associate Professor David A. Gust

Queensland University of Technology

A Thesis submitted for the degree of Master of Applied Science

(Queensland University of Technology)

KEYWORDS

Kilimanjaro, East African Rift, alkalic magmatism, petrogenesis, magma

evolution, fractional crystallisation

I

ABSTRACT

Mt Kilimanjaro, Africas highest mountain (5895m), is a large, young (

II

TABLE OF CONTENTS

INTRODUCTION...................................................................................................................................I

GEOGRAPHIC LOCATION ........................................................................................................................... 2 EAST AFRICA ............................................................................................................................................ 5 KILIMANJARO ........................................................................................................................................... 9 SHIRA...................................................................................................................................................... 11

METHODS ........................................................................................................................................... 13

FIELD INVESTIGATIONS........................................................................................................................... 13 SAMPLE PREPARATION ........................................................................................................................... 13 Petrography, Microprobe and Laser Ablation....................................................................................... 13 Geochemistry / Analytical Techniques ................................................................................................... 15

RESULTS ............................................................................................................................................. 18

PETROGRAPHY ........................................................................................................................................ 18 PHASE CHEMISTRY ................................................................................................................................. 19 Olivine.................................................................................................................................................... 21 Clinopyroxene ........................................................................................................................................ 22 Feldspar ................................................................................................................................................. 22 Spinel...................................................................................................................................................... 24 Feldspathoid........................................................................................................................................... 25 LASER ABLATION RESULTS .................................................................................................................... 26 Olivine.................................................................................................................................................... 26 Clinopyroxene ........................................................................................................................................ 27 Feldspar ................................................................................................................................................. 29 Spinel...................................................................................................................................................... 29 GEOCHEMICAL RESULTS......................................................................................................................... 31

DISCUSSION ....................................................................................................................................... 43

FRACTIONAL CRYSTALLISATION MODELS .............................................................................................. 43 Groups 1 and 2....................................................................................................................................... 48 Group 3 (K813-K820-K825) .................................................................................................................. 53 Group 4 (K361-K897-K894) .................................................................................................................. 53 Summary................................................................................................................................................. 56 CRUSTAL CONTAMINATION / MAGMA MIXING MODELS ........................................................................ 56 PRIMITIVE MAGMAS, MELTING AND SOURCES ....................................................................................... 62 Partial Melting Models .......................................................................................................................... 71 Source Characteristics and Formation .................................................................................................. 76

CONCLUSION..................................................................................................................................... 80

REFERENCE LIST............................................................................................................................. 82

III

LIST OF FIGURES

Figure 1. East African Rift and location of Mt Kilimanjaro 3 Figure 2. Kilimanjaro regional geology, lava correlation and Shira cross section 4 Figure 3. Active versus passive rifting models 7 Figure 4. Hypothetical East African Rift model 7 Figure 5. Principle igneous centres of the East Africa Rift 8 Figure 6. Kilimanjaro geology, topography and sample locations 10 Figure 7. Shira geology, topography and sample locations 12 Figure 8. Sr and Ba comparisons for ICP-AES and LA-ICP-MS 16 Figure 9. Olivine microprobe results 20 Figure 10. Clinopyroxene microprobe results 22 Figure 11. Feldspar microprobe results 23 Figure 12. Spinel microprobe results 24 Figure 13. Feldspathoid microprobe results 25 Figure 14. Olivine LA-ICP-MS results 27 Figure 15. Clinopyroxene LA-ICP-MS results 28 Figure 16. Feldspar LA-ICP-MS results 30 Figure 17. Spinel LA-ICP-MS results 31 Figure 18. Major element analysis results 38 Figure 19. Trace element analysis results 39 Figure 20. Chondrite normalised REE and primitive mantle normalised

multi-element spider diagrams 40 Figure 21. Total alkalis silica and silica saturation diagrams 41 Figure 22. Mg number versus CaO/Al2O3 and K2O versus P2O5 42 Figure 23. Normative plots distinguishing groups 42 Figure 24. Fractional crystallisation paths of Shira samples 44 Figure 25. Fractionation vectors produced from the removal of olivine,

clinopyroxene, spinel and plagioclase 45 Figure 26. KSH08-KSH03-K679-KSH02 fractionation model results 50 Figure 27. K2225-K803 fractionation model results 52 Figure 28. KSH01-K802 fractionation model results 52 Figure 29. K813-K820-K825 fractionation model results 54 Figure 30. K361-K897-K894 fractionation model results 55 Figure 31. Zr/Hf and Nb/Ta versus Mg number diagrams 57 Figure 32. Magma mixing model path 58 Figure 33. Backscanned image of KSH05 clinopyroxene 1, with LA-ICP-MS

results showing oscillatory zonation 59 Figure 34. Magma mixing results normalised to KSH11 60 Figure 35. Chondrite normalised REE and primitive mantle normalised

multi-element spider diagrams for magma mixing model 61 Figure 36. Paths produced from addition of equilibrium olivine 65 Figure 37. Chondrite normalised REE and primitive mantle normalised

IV

multi-element spider diagrams for equilibrium olivine addition 65 Figure 38. Compilation of primitive samples plotted on CaO versus Mg number 66 Figure 39. Paths produced from addition of clinopyroxene and olivine 67 Figure 40. Chondrite normalised REE and primitive mantle normalised

multi-element spider diagrams for addition of clinopyroxene and olivine 67 Figure 41. MELTS models of primary fractionation corrected magmas using pressure = 0.5kb, H2O = 0.2% and fO2 = QFM. 69 Figure 42. REE and primitive mantle normalised multi-element spider diagrams

of reverse modal equilibrium batch melting models 73 Figure 43. REE and primitive mantle normalised multi-element spider diagrams

of reverse non-modal equilibrium batch melting models 73 Figure 44. Primitive mantle normalised multi-element spider diagram of forward

modal equilibrium batch melting models 74 Figure 45. OIB normalised multi-element spider diagram of fractionation

corrected samples 76 Figure 46. Model of the genesis and evolution of Mt Kilimanjaro and the Shira region 77

LIST OF TABLES

Table 1. Analytical precision of EDS microprobe results 15 Table 2. Analytical precision of ICP-AES major element results 16 Table 3. Shira volcanic rock group classification 18 Table 4. Samples analysed by EDS microprobe and LA-ICP-MS 18 Table 5. Representative microprobe analyses 19 Table 6. Group 1 geochemical results 34 Table 7. Group 2 geochemical results 35 Table 8. Group 3 geochemical results 36 Table 9. Group 4 geochemical results 37 Table 10. Partition coefficients used in modelling 46 Table 11. Microprobe results used in modelling 47 Table 12. Fractional crystallisation models 49 Table 13. Magma mixing model 60 Table 14. Compositions of primary fractionation corrected magmas 68

LIST OF APPENDICES

Appendix A. Fractional crystallisation models 92 Appendix B. Magma mixing model 97 Appendix C. Primary magma compositions 99 Appendix D. Reverse partial melting models 101 Appendix E. Forward partial melting models 103

V

STATEMENT OF ORIGINAL AUTHORSHIP

The work contained in this thesis has not been previously submitted for a degree or

diploma at any other higher education institution. To the best of my knowledge, this

contains no material previously published or written by another person except where

due reference is made.

Signed:..

Date:..

VI

ACKNOWLEDGMENTS

Several people have provided invaluable assistance through the duration of my

project which I would sincerely like to thank. Firstly, I would like to gratefully

acknowledge the time, work, fieldwork assistance and financial assistance (through

numerous jobs) of my supervisor Associate Professor David Gust.

I would also like to acknowledge Dr Michael Carpenter and Dr Sally Gibson

(Cambridge University) for providing access to samples from the Sheffield University

Kilimanjaro rock collection. Furthermore, thankyou to Professor Richard Arculus

(Australian National University) for performing trace element analyses of all

Kilimanjaro samples and for his assistance when I went to Canberra for LA-ICP-MS

analysis.

Thankyou to the QUT technical staff, in particular Bill Kwiecien and Loc Duong, and

to Dave Purdy for showing me the ropes on all the machines at QUT. Thanks must

also go to Franco (Kilimanjaro guide) and our porters for not leaving us stranded on

the mountain or telling the national parks about our souvenir rocks, and also to

Luke for his endless supply of music and entertainment.

Finally, special thanks must go to my family, and Therese for their support and

encouragement and for tolerating me over the last two years.

1

INTRODUCTION

The petrogenetic modelling of primitive mafic, alkalic rocks provides valuable

information on large parts of the earths interior which are otherwise

inaccessible. When combined with geophysical studies, the geochemical

studies of alkalic rocks may hold the key to understanding the composition and

evolution of the Earths mantle (Spath et al., 2001). Geochemical and

mineralogical studies yield valuable information concerning the magmatic

evolution and magma chamber dynamics of melts once segregated from their

source. Extensive geochemical and mineralogical studies have been

performed on numerous tectonic settings, including arc volcanics, ocean island

volcanics and continental flood basalts (e.g. MacDonald et al., 2001). However,

the source, production and evolution of large mixed-association, off rift axis

stratovolcanoes remains enigmatic within the studies of continental rifting and

has only recently been addressed (e.g. Spath et al., 2001). This thesis

investigates the geochemistry and mineralogy of the Pliocene to Pleistocene

Shira Volcanics, Mt Kilimanjaro to determine the processes responsible for their

evolution as well as speculate on their source and conditions of partial melting.

Continental rifting, in which voluminous alkalic magmatism is commonly

associated, has been the subject of geochemical investigations for decades

(e.g. Williams, 1970; 1971; Bailey, 1974; Baker, 1987; MacDonald et al., 2001).

Problems addressed by these studies include source region characteristics,

partial melting process, and magmatic evolution with respect to time and space.

Rifting processes are divided into active (plume driven) or passive

(lithospheric extension driven) (Keen, 1985; Wilson, 1989). In both models,

rising asthenosphere results in decompression melting of the asthenosphere,

metasomatism, and partial melting of the lithosphere due to conductive heating

(Turner et al., 1996). Proposed sources for rift magmas include the

subcontinental lithospheric mantle (McKenzie & Bickel, 1988; White &

McKenzie, 1989; Arndt & Christensen, 1992) and the asthenosphere / upwelling

mantle. The enriched incompatible element signatures observed in alkalic

rocks (e.g. Kay & Gast, 1973; Irving, 1980; Frey & Prinz, 1978; Frey et al.,

1978; Wass, 1980; Kempton et al., 1987) is attributed to either extremely low

degrees of melting (Green & Ringwood, 1967a; Green, 1969; 1973) or slightly

higher degrees of melting of a metasomatised/enriched source (e.g. Frey et al.,

1978; Bailey, 1987; Morris et al., 1987; Spath et al., 2001).

2

The East African Rift (EAR) as a classic example of a continental rift, has been

studied for decades (e.g. Gregory, 1921; Willis, 1936; Williams, 1970; 1971;

Girdler, 1972; Rosendahl, 1987; Kampunzu & Mohr, 1991; Morley, 1999;

Morley et al., 1999; Rogers et al., 2000; MacDonald et al., 2001). In recent

times, plume-related models of rifting and rift magmatism dominate EAR

literature with one or more mantle plumes being postulated beneath the Kenya

Rift Valley or nearby Tanzanian Craton (e.g. Karson & Curtis, 1989; Ebinger et

al., 1997; Mechie et al., 1997; Simiyu & Keller, 1997; Rogers et al., 1999; 2000).

Upwelling mantle is believed to be responsible for lithospheric extension,

metasomatism of the lithosphere and partial melts of both the asthenosphere

and subcontinental lithospheric mantle within the EAR.

Upon segregation from their source, melts are subject to numerous magma

chamber processes including fractional crystallisation, assimilation of the

country rock, magma recharge and magma mixing. The role that these

processes play in the evolution of rift stratovolcanoes remains unaddressed,

and when better understood, will contribute to understanding the development

of continental rifts.

Mt Kilimanjaro is a large, young (

3

Mt Kilimanjaro

DODOMA

300km

N

Kivu Lake

Mobutu Lake

Tanganyika Lake

Victoria Lake

Rukwa Lake

Malawi Lake

WESTE

RN

BRA

NC

H

EA

STE

RN

BRA

NC

H

ADEN GULF

INDIAN OCEAN

Volcanic Provinces

Lake

Fault

Mt Kilimanjaro - 5895m

NAIROBI

Major City

ADDIS ABABA

ASMARA

DJIBOUTI TOWN

LILONGWE

LUSAKA

Unguja/Zanzibar Island

KIGALI

BUJUMBURA

MOGADISHU

LEGEND

EAST AFRICAN RIFT

See Figures 2 and 6

Tanzanian craton

Figure 1. Location of Mt Kilimanjaro on the East African Rift. Also shown are the

eastern and western branches, major faults, extent of volcanic activity and location

of Tanzanian craton (after Kampunzu & Mohr, 1991).

4

5

GEOLOGY

East Africa

The East African Rift (EAR) originates at the Afar triple junction and forms a

3500km SSW-trending branch from the Red Sea and Gulf of Aden spreading

zones (Figure 1). Rifting began about 45Ma in Ethiopia and has propagated in

a southerly direction at a rate of between 2.5cm/year (Oxburgh & Turcotte,

1974) to 5cm/year (Kampunzu & Mohr, 1991). Rifting occurs in three distinct

pulses (Baker, 1987) with an initial early Eocene (44-38Ma) period followed by

a middle Miocene (16-11Ma) episode, and a final and current rifting in the

Pliocene-Pleistocene (5-0Ma).

This style of rifting reflects an active rifting process (Kampunzu & Mohr, 1991;

Spath et al., 2001), driven by upwelling mantle (Figure 3), rather than a passive

rifting process where differential stresses in the lithosphere result in extension

and mantle plumes. Evidence for active rifting includes synchronous

magmatism with rift initiation, enriched magma sources in the early stages of

rifting, and a decrease in the lithospheric mantle component of the mafic lava

geochemistry (Kampunzu & Mohr, 1991).

The EAR is located above at least two mantle plumes (Rogers et al., 2000).

These plumes include the Afar plume (Kampunzu & Mohr, 1991), and the

southern Kenya Plume (e.g. Mechie et al., 1997; Simiyu & Keller, 1997; Rogers

et al., 1999; 2000). The Kenya Plume has different isotopic and trace element

characteristics to that of Afar (Rogers et al., 2000).

The EAR divides into an eastern and western branch in southern Ethiopia

(Figure 1), with the eastern branch having more profuse volcanic activity

(Kampunzu & Mohr, 1991). The western branch is highly potassic and extends

into Malawi and Mozambique, and the eastern branch is highly sodic. It extends

into northern Tanzania where it terminates into a diffuse (approximately 300km

wide) zone of normal faults. The potassic magmas of the western branch are

believed to be generated at greater depths than those of the eastern branch

(Girdler, 1983; Wilson, 1989) and are thought to be the result of a smaller,

related shoulder plume. The most popular hypothesis for the formation of this

shoulder plume is that the Archaen Tanzanian Craton (located between the

6

eastern and western rift branches) has deflected a portion of the rising

asthenosphere (Kampunzu & Mohr, 1991; Zeyen et al., 1997; Winter, 2001).

EAR magmatism is diverse with ultra-alkalic / carbonatitic, alkalic, transitional

and tholeiitic suites all identified (Kampunzu & Mohr, 1991). Two contrasting

views of the evolution of magmatism in rift zones are proposed (Kampunzu &

Mohr, 1991). The first suggests that the diverse range of rock suites show a

progressive decrease in alkalinity with rift development, from ultra-alkalic

magmas associated with pre-rift regional uplift, to alkalic magmatism associated

with graben development, followed by tholeiitic magmatism upon initiation of

seafloor spreading (Gass, 1970; 1972; Baker et al., 1978, Lippard & Truckle,

1978; Baker, 1987). The second view is that magma alkalinity does not

significantly evolve, and continental and oceanic rifting magmatism can not be

correlated (Le Bas, 1971; Bailey, 1974). The complexity of correlating lava

flows and the close temporal and spatial association of alkalic, transitional and

tholeiitic rocks makes it extremely difficult to distinguish between these

hypotheses.

Transitional and tholeiitic rocks have occurred before, during and after rifting

within the EAR, and can predate, postdate or occur concurrently with alkalic

rocks of the same region (Kampunzu & Mohr, 1991). This indicates that

structural setting exerts an important, though not always predictable control on

magma composition within the EAR (Gass, 1970; Mohr, 1970). Volcanic

products have been shown to vary transversely across rifts, with the magmas

erupted on the flanks tending to be more alkalic and less voluminous than those

lavas erupted in the axial graben (Wilson, 1989). This variation probably results

from differing degrees of melting due to depth of magma production increasing

with distance from the axial graben.

Individual eruptive centres, occurring predominatly on the flanks and

propagating end of the EAR produce either mixtures of rock suites, or only one

rock suite (Figure 5). This off-rift volcanism can be explained as the result of

individual mantle plumes (e.g. Burke, 1996) or diapirs of plume material

deflected from the main plume along pre-existing structures (e.g. Bosworth,

1987, 1989; Mechie et al., 1997; Ritter & Kaspar, 1997; Spath et al., 2000;).

7

Figure 3.

A)

B)

Active vs. Passive rifting models for continental rifting (Wilson, 1989 after Keen, 1985).

Active rifting, whereby mantle upwelling has caused lithospheric extension and regional uplift,

compared with Passive rifting where differential stresses in the lithosphere have resulted in

lithospheric extension, causing the mantle to plume in the area of thinned crust.

Figure 4. Hypothetical cross section (no vertical exaggeration) showing a proposed model for the current

stage of development of the East African rift system. This is the intermediate stage between initial

asthensopheric diapir rising and sea floor spreading (asthenospheic material reaching crustal levels).

Decompression melting results from the ascent of an asthenospheric diapir, which in turn can cause

metasomatism of the sub-continental lithospheric mantle (SCLM), and partial melting resulting in variably

alkaline melts. The reversed decollement (D1) provides room for the rising asthenosphere which can in

turn result in crustal anatexis. Eruption of alkaline lavas, mostly from a deep asthenospheric source fills

the rift valley with volcanic and volcaniclastic material. (Winter, 2001 after Kampunzu & Mohr, 1991)

grassoThis figure is not available online. Please consult the hardcopy thesis available from the QUT library

grassoThis image is not available online. Please consult the hardcopy thesis available from the QUT library

8

0km 100km

Mixed association central volcanoes

Nephelinite-phonolite central volcanoes

Basalt-trachyte, trachyte-rhyolite and trachyphonolite shields

Quaternary basalts

Volcanic province

Ngorongoro Crater

Lake

Mt Kenya

Mt KilimanjaroNgorognoro

Oldoinyo Lengai

Mt Meru

Chyulu Hills

Napak

Kadam

Elgon

Moroto

Yelele

LakeVictoria

Emuruangogolak

Silali

Paka

Menengai

LongonotSuswa

OlorgesailieOlesakut

LakeTurkana

34 35 37 3836

4

3

2

1

0

-1

-2

-3Shira

KiboMawenzi

Figure 5. A map showing the distribution, alignment and eruption types of the principle

igneous centres of the East African Rift (after Kampunzu & Mohr, 1991 and Baker, 1987).

Note that there are a variety of rocks erupted, with some centers producing only one rock

suite, whilst others produce a mix.

9

Kilimanjaro

Mt Kilimanjaro forms a shield approximately 96km in length by 64km width, with

the long axis trending WNW. The summit Kibo is the only volcanic centre

currently regarded as active, and reaches an elevation of 5895m (Uhuru Peak)

at the coordinates 305S, 3720E (Figure 6).

Magmatic activity of Mt Kilimanjaro began in the lower Pleistocene, with several

eruptive centres creating a mix of alkalic, transitional, tholeiitic and pyroclastic

rock suites (Figures 2 & 6). Initial volcanic activity produced olivine basalts of

the Ol Molog, Kibongoto and Kilema regions (Figure 2a) approximately 1 million

years ago (Downie & Wilkinson, 1972). Faulting controlled the location of

magmatic activity, building a low complex shield. In the lower Pleistocene,

activity became focused at three main volcanic centres (Kibo, Shira and

Mawenzi). Initially, all three centres operated simultaneously producing basalts

of similar composition. Towards the end of the lower Pleistocene these centres

developed their individual characteristics. Shira produced silica-undersaturated

lavas, ankaramites and nephelinites followed by strongly silica under-saturated

lavas, ijolites and associated lavas from a smaller unknown centre to the east of

Shira. Mawenzi lavas changed from basalts to trachybasalts to trachyandesite,

with activity moving from the Neumann Tower to the main Mawenzi centre

before becoming extinct. The activity of Kibo is similar to that of Mawenzi, with

the production of trachyandesites long after the cessation of the Mawenzi

volcanic centre (Downie & Wilkinson, 1972). The final stage of Mt Kilimanjaros

evolution involved the production of aegerine phonolite flows, and creation of

the present caldera and ash pit. Kilimanjaro has remained dormant through the

Holocene, with only fumarolic activity taking place (Downie & Wilkinson, 1972).

Petrographic studies on Kilimanjaro by Abdullah (1963), Saggerson (1964),

Wilkinson and Downie (1965), Wilkinson (1967), Sahu (1969), Williams (1969),

and Downie and Wilkinson (1972) result in a correlation of the many lavas of

Kilimanjaro (Figure 2c).

Glaciation occurred episodically throughout the late Pleistocene and Holocene,

between periods of volcanic activity. The current glaciers of Kilimanjaro are

rapidily disappearing, exposing many previously unseen rock surfaces

(Hastenrath & Greischar, 1997; Irion, 2001).

10

11

Shira

Shira consists of many distinct landmarks including the Shira Ridge, Platzkegel

(German for cone place), East Shira Hill, Shira Cathedral and the Shira

Plateau (Figure 7). Flows from parasitic cones obscure the Shira lavas to the

north and south, whilst vegetation covers a great deal of the flanks below the

ridge and plateau (Downie & Wilkinson, 1972). The distinct Shira Ridge

(Figures 2b & 7 resulted from a caldera collapse (Wilcockson, 1956; Downie &

Wilkinson, 1972). The lavas on the western and southern slopes dip radially

outwards, from about 20 on the upper slopes to 2 to 3 on the lower slopes

and cover the lava units of Ol Molog in the north and Kibongoto in the south.

Reconstruction of the Shira volcano suggests it may have once reached a

height of 5400m (Downie & Wilkinson, 1972).

The geology of Shira (Figure 7) is described by Downie and Wilkinson (1972);

they conclude that the petrogenesis of its magmas reflect significant fractional

crystallisation of ferromagnesian minerals. Shira volcanic units are not dated,

however they are older than the Upper Rectangle Porphyry group of Kibo (Nvq2

Figure 7), a unit that partially covers the degraded Shira crater.

Shira contains the most primitive and alkalic rocks of the Mt Kilimanjaro region.

The rocks are mainly mafic, silica-undersaturated lavas with considerable

amounts of pyroclastic material. Shira rocks include olivine basalt, trachybasalt,

trachyandesite, ankaramite, basanite, nephelinite, agglomerate and augite-

bearing tuff (Downie & Wilkinson, 1972). A 480m thick section, measured from

just below the Shira Ridge upwards identified 3 distinct groups. These are the

upper trachybasalt group (Nvd2 on Figures 6 & 7) ultramafite and

melanephelinite group (Nvu on Figures 6 & 7;inner face of the Shira Ridge) and

lower trachybasalt group (Nvd1 on Figures 6 & 7). The upper trachybasalt

groups (upper ridges and western escarpment) consists of trachybasalt with

large platy feldspar phenocrysts, the ultramafite and melanephelinite group

consists of large augite crystal tuffs, ankaramite, basanite and melanephelinite,

and the lower trachybasalt group (southern ridge and the upper slopes of

Shira) is comprised of trachybasalt with small platy feldspars.

12

13

With a relief of approximately 240m, Platzkegel rises from the centre of the

Shira Plateau. Eruptive products from the Platzkegel vent consist of

agglomerates with analcime-basalt fragments in a tuffaceous matrix, and thin

basalt flows (Downie & Wilkinson, 1972). Platzkegel has been intruded by

various dolerite, analcime, syenite and essexite intrusions penetrating along

NNE-SSW fissures. Many other Shira dykes form a radial swarm focusing on

Platzkegel. These dykes are more or less vertical, ranging in thickness from

0.5 to 1m, and are generally of similar composition to the trachyandesite and

trachybasalt lavas with a few dykes representative of the basalts, ankaramites,

atlantites and melatrachybasalts. The density of dykes appears to decrease

with increasing distance from the crater. A number of inclined dykes dipping

outwards at about 45 intrude into the flanks. They are approximately 100m

apart, 1 to 1.5m thick, and are composed of the equivalents of the trachybasalt

and melatrachybasalt lavas (Downie & Wilkinson, 1972).

A nephelinite centre (Nvn Figure 7) occurs approximately 3.5km northwest of

Platzkegel, and post dates the agglomerate and dykes.

METHODS

Field Investigations

Eleven samples were collected from Mt Kilimanjaro, near the East Shira Hill and

Shira Cathedral. Five samples were collected from a 20m vertical exposure

capped by pyroclastics. Thirty seven samples were obtained from Cambridge

University, and came from the 1953 and 1957 joint surveys of the Geological

Survey of Tanganyika (Tanzania) and Sheffield University. This sample set

includes 26 samples large enough for geochemical analysis, and 11 samples of

sufficient size for the creation of polished sections. Sample locations are shown

in Figures 6 and 7.

Sample Preparation

Petrography, Microprobe and Laser Ablation

Polished sections were made of all collected samples for petrographic,

microprobe and laser ablation ICP-MS analysis. Microprobe analyses were

undertaken at the QUT Analytical Electron Microscopy Facility using a JEOL

JXA-840A Scanning Electron Microprobe with an Energy-Dispersive

Spectrometry (EDS) detector. Operating conditions for the quantitative

grasso

grasso

14

determination of mineral chemistry were: 15kV accelerating voltage, beam

current of ~3nA, count time of 100 seconds, 38mm working distance, 40 take

off angle for the EDS detector and a focused 5-10 m diameter beam.

Calibration was performed using pure copper from the Astimex Scientific

MINM25-53 standard mineral mount, with a standards file based on albite for

Na, olivine for Mg and Si, plagioclase for Al, apatite for P, sanidine for K,

diopside for Ca, rutile for Ti, chromium for Cr, rhodonite for Mn, aluminium

garnet for Fe, nickel silicide for Ni and cobalt for Co. EDS spectra were

collected and interpreted through Moran Scientific quantitative EDS software.

13 samples were analysed, with phenocrysts probed between 2 and 12 times

from core to rim, dependant upon size and whether zonation was apparent in

backscattered images. Three groundmass analyses of each phase were also

determined. Table 1 shows comparisons between analysed and accepted

values for several relevant mineral standards. The maximum deviation from

accepted values is approximately five percent (relative) with most elements

being determined within two percent (relative).

Microprobe analyses are recalculated as a proportion of end member

compositions for olivine, clinopyroxene, feldspar, nepheline, and spinel. Olivine

microprobe analyses are calculated as a percentage of forsterite whereas

clinopyroxenes are calculated as a percentage of both enstatite (En) - ferrosilite

(Fe) - wollastonite (Wo) and Ti-Aliv-NaM2 (e.g. Kempton et al., 1987). Pyroxene

En-Fe-Wo calculations used PX-NOM, a pyroxene spreadsheet calculator

(Sturm, 2002), based on the classification schemes of the International

Mineralogical Association. Fe3+ values were determined using the methods of

Droop (1987). Plagioclase compositions are presented as percentage

anorthite-albite-orthoclase (An-Ab-Or), with nepheline cast as a percentage of

nepheline-kalsilite-silica (Ne-Ks-Q) (Deer, Howie & Zussman, 1992).

Mg/Mg+Fe2+ and Cr/Cr+Al for spinel analyses followed methods of Kempton et

al. (1987).

Laser Ablation Inductively Coupled Mass Spectrometry (LA-ICP-MS) analysis of

trace elements (Sc, V, Cr, Ga, Rb, Sr, Y, Zr, Nb, Cs, Ba, La, Ce, Nd, Sm, Eu,

Gd, Dy, Er, Yb, Lu, Hf, Ta, Pb, Th and U), Ca, Si and Al were performed at The

Australian National University (ANU) on an ArF (193nm) EXCIMER laser and a

Fisons PQ2 STE ICPMS. Full instrument details are outlined in Eggins and

Shelley (2003). A laser diameter of 71m was used, with repetition rate of 5Hz.

15

Counting time was set at 60 seconds, including 20 seconds of background data

collection. Instrument calibrations were performed after approximately 12

analyses on internal glass standard NIST 612 to account for drift. Analyses

were performed on 5 samples (KSH05, K2225, KSH01, KSH03 and K811), on

phenocryst points previously analysed through EDS in order to reduce the data

and gain quantitative results. Ca was used to reduce analyses of

clinopyroxene, Si for plagioclase and olivine, and Al for spinel.

Table 1. Comparison between EDS analysed and accepted compositions of the Astimex Scientific MINM 25-53 mineral mount for a range of minerals

comparable to Shira samples.

Albite Albite Plagioclase Plagioclase Diopside Diopside Cr Diopside Cr Diopside Olivine Olivine Chromite Chromite

(analysed) (accepted) (analysed) (accepted) (analysed) (accepted) (analysed) (accepted) (analysed) (accepted) (analysed) (accepted)SiO2 67.5 68.52 52.25 54.21 55.41 55.36 54.19 55.13 41.73 41.84 0.25Al2O3 19.46 19.54 28.64 28.53 0.32 0.09 0.27 0.08 12.61 13.79FeO 0.01 0.3 0.37 0.13 0.05 1.25 1.21 7.34 6.51 18.4 17.5MnO 0.03 0.05 0.03 0.12 0.34MgO 0.01 0.13 19.17 18.62 17.7 17.46 51.64 51.57 12.23 13.6CaO 0.06 0.13 12.37 11.8 26.93 25.73 25.48 25.55 0.01Na2O 11.02 11.59 4.43 4.35K2O 0.21 0.22 0.4 0.41 0.1 0.07 0.06TiO2 0.05 0.03 0.07 0.08 0.06 0.05 0.31 0.15P2O5 0.14 0.19Cr2O3 0.05 0.07 0.11 0.58 0.58 54.28 54.4NiO 0.09 0.03 0.11 0.35 0.14CoO 0.03 0.19 0 0.03 0.05 0.2 0.02Total 98.49 100 98.71 99.87 102.45 99.98 99.82 99.98 101.28 100.24 98.59 99.44

Geochemistry / Analytical Techniques

Samples were prepared for chemical analysis at the University of Queensland

(UQ) sample preparation laboratory. Samples were washed, crushed using a

hardened steel jaw crusher and dried overnight on a hot plate. Rock chips were

crushed for 15 minutes in an agate mill with approximately 70-120g of powder

produced.

Major elements (Si, Ti, Al, Fe, Mn, Ca, Na, K and P) and two trace elements

(Ba and Sr) were determined by Inductively Coupled Atomic Emission

Spectrometry (ICP-AES) at the QUT School of Natural Resource Science

Geochemical Analytical Facility using a Varian Liberty 200 ICP-AES.

Solutions were prepared using the methods of Kwiecien (1993), involving

hydrofluoric acid digestion and dilution to 200ml. H2O+, CO2 and S were

determined as loss on ignition (LOI) for each sample by heating samples to

950C over a period of 5 hours, then maintaining this temperature for a period

of 15 minutes.

16

Samples were run manually in batches of 10, including at least 1 blank and 4

calibration standards, along with USGS standards Nim-L and Nim-S for

comparison (Table 2). Calibration standards used were internal standards QUT

353, 446, 1552, 2769 and 161 which are referenced to USGS standards W1,

GSP1, AGV1, BCR1 and G2. Correlation coefficients were all extremely close

to 1, and the USGS standard values correlated extremely well with accepted

values for all elements.

Table 2. Comparison of ICP-AES analysed and accepted USGS standards Nim-L and Nim-S.

Sample Nim-L (analysed) Nim-L (accepted) Nim-S (analysed) Nim-S (accepted)SiO2 52.18 52.40 64.77 63.63Al2O3 12.98 13.64 16.78 17.34Fe2O3 (total) 9.93 9.96 1.46 1.40Fe2O3 8.74 1.07FeO 1.13 0.30MnO 0.76 0.77 0.01 0.01MgO 0.25 0.28 0.45 0.46CaO 2.57 3.22 0.72 0.68Na2O 7.97 8.37 0.43 0.43K2O 5.38 5.51 15.47 15.35TiO2 0.49 0.48 0.04 0.04P2O5 0.04 0.06 0.12 0.12LOI 3.72 2.48 0.31 0.31Total 96.26 97.17 100.56 99.77

Sr(ppm) 4396.00 4600.00 65.74 62.00Ba(ppm) 307.40 450.00 2550.00 2400.00

Trace element analyses were performed by Professor Richard Arculus (ANU).

Glass discs were prepared by fusing 0.5 grams of powdered sample with 1.5

grams of Li-borate flux for 15 minutes at 1190C. All trace element

concentrations (Sc, V, Cr, Ga, Rb, Sr, Y, Zr, Nb, Cs, Ba, La, Ce, Nd, Sm, Eu,

Gd, Dy, Er, Yb, Lu, Hf, Ta, Pb, Th and U) were determined on the glass discs

by Laser Ablation, Inductively-Coupled Plasma Mass Spectrometry (LA-ICP-

MS) at the Research School of Earth Sciences, ANU. The LA-ICP-MS employs

an ArF (193nm) EXCIMER laser and a Fisons PQ2 STE ICPMS. Full instrument

details are outlined in Eggins and Shelley (2003). Analyses were performed on

Li-borate fusion discs using a spot size of 100m and a repetition rate of 5Hz.

Counting time was set at 70 seconds. Instrument calibration was against NIST

612 glass and background analysis time was 30 seconds. 43Ca was employed

17

as the internal standard isotope, based on CaO concentrations previously

measured by ICP-AES.

Sr and Ba results from LA-ICP-MS correlate well with ICP-AES results

indicating consistency between both techniques (Figure 8).

0 1000 20000

1000

2000

Ba

(ppm

)-

LA

-IC

P-M

S

Ba (ppm) - ICP-AES

0 1000 2000 3000 40000

1000

2000

3000

Sr

(pp

m)

-L

A-I

CP

-MS

Sr (ppm) - ICP-AES

Figure 8. Sr and Ba comparisons for ICP-AES and LA-ICP-MS analyses showing a close

correlation.

grad

ient

=1

grad

ient =

1

18

RESULTS

Petrography

Petrographic examination of Kilimanjaro rocks show that they are relatively

fresh to slightly altered, microporphyritic to porphyritic rocks with varying

vesicularity. Phenocrysts of predominantly clinopyroxene, with lesser amounts

of olivine, plagioclase, nepheline and spinel, constitute up to 55 modal percent

in some samples. Shira samples are split into four groups based on

phenocryst assemblages (Table 3), with the presence of feldspar and/or

feldspathoid phenocrysts used to distinguish between Groups 1, 2 and 3;

Platzkegel samples (Group 4) are distinguished by their distinctive intergranular

textures. Results are discussed with respect to the following groups:

Group 1: Olivine and clinopyroxene phenocrysts microphenocrysts of

feldspar.

Group 2: Clinopyroxene and feldspar olivine phenocrysts.

Group 3: Feldspathoid, clinopyroxene and feldspar phenocrysts

microphenocrysts of olivine.

Group 4: Platzkegel samples.

Group 1 samples are similar to Group 2, which is distinguished by larger

feldspar phenocrysts and fewer olivine phenocrysts. Olivine phenocrysts in

Group 1 samples often contain magnesiochromite inclusions (absent in all other

groups), and lack ulvspinel phenocrysts common to Group 2 samples.

Complex zonation patterns are apparent in many large clinopyroxene

phenocrysts in Groups 1, 2 and 3 with abundant melt, apatite, spinel and olivine

inclusions found in these phenocrysts. Smaller clinopyroxene phenocrysts and

olivines are generally normally zoned or unzoned and contain far fewer

inclusions (occasional magnetite speckles or feldspar) than the larger samples.

Some Group 2 clinopyroxenes are sector zoned and often occur in

glomerocrysts. Plagioclase phenocrysts occur predominantly in Group 2

samples and are extremely variable. In general the larger phenocrysts are

complexly zoned, whilst the smaller phenocrysts are normally zoned or

unzoned and occur as glomerocrysts or as individual crystals aligned in a

trachytic texture. Large nepheline and clinopyroxene phenocrysts are

prominent in Group 3 samples, with olivine appearing solely as

microphenocrysts. Group 4 samples are much more equigranular and contain

19

distinctive intergranular textures. Groundmasses in Groups 1 to 3 are

predominantly cryptocrystalline to microcrystaline and are speckled with

titaniferous magnetite. Group 4 groundmasses are fine to medium grained and

composed of clinopyroxene, plagioclase, nepheline, titaniferous magnetite,

magnetite and in some samples, olivine, interstitial biotite and minor apatite.

Phase Chemistry

Samples for EDS microanalysis were taken from each group (Table 4).

Representative clinopyroxene and spinel results for EDS microanalysis are

presented in Table 5.

Table 3. Shira volcanic rocks and classification into four groups.

Group 1 Group 2 Group 3 Group 4KSH03 KSH01 K686 K361KSH05 KSH02 K689 K832aKSH07 KSH04 K811 K832bKSH08 KSH06 K813 K894KSH09 KSH10 K820 K897K693 KSH11 K825 K1043K2225 K679 K829

K695 K1039K696K802K803K804K821K822K895K891K1038

Table 4. Samples analysed by EDS microprobe and LA-ICP-MS

Samples for EDS Analysis Samples for LA-ICP-MS AnalysisSample Group Sample GroupKSH03 1 KSH05 1KSH05 1 K2225 1KSH08 1 KSH03 1KSH09 1 KSH01 2K2225 1 K811 3KSH01 2KSH04 2KSH06 2K802 2K811 3K820 3K361 4K894 4

20

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mass

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mp

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(co

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px

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px

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94

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SiO

252.3

747.3

344.7

04

7.8

644.0

046.3

048.0

345.2

746.1

944.4

641.0

040.7

3T

iO2

0.2

81.7

72.9

91.8

43.4

72.4

81.7

22.3

91.7

42.8

03.7

94.6

2A

l 2O

32.2

86.8

68.8

64.8

68.0

25.2

46.4

08.3

96.3

68.6

010.9

211

.14

Cr 2

O3

0.9

30.2

50.1

70.0

10.0

00.0

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50.0

00.0

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4F

e2O

3*

1.6

33

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4.4

62.5

84.1

93.8

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*1.8

43.9

25.0

06.2

15.9

87.4

76.0

16.4

013.4

75.7

16.2

46.5

5M

nO

0.1

00.1

90.1

80.1

30.0

70.2

10.2

40.1

10.5

00.1

90.2

20.1

1M

gO

17.5

313.3

911.6

312.5

610.4

411.5

111.9

210.7

86.2

111.0

09.0

79.3

2C

aO

22.9

523.2

62

2.7

222.7

122.1

622.3

423.0

823.0

920.5

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2.8

122.9

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2O

0.0

60.1

00

.09

0.0

80.0

80.0

40.1

10.0

70.1

70.0

00.0

90.0

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a2O

0.0

00.0

00.1

60.1

50.4

90.1

40.3

40.1

21.0

60.0

70.0

40.0

0P

2O

50.0

00.0

00.3

70.5

70.2

30.1

70.0

00

.00

0.9

50.0

00.1

20.7

4N

iO0.1

00.1

00.1

00.1

20.1

70.0

00.0

00.2

10.0

00.2

10.1

20.0

0C

oO

0.0

00.0

00.0

00.0

00.0

00.0

00.0

00.0

00.0

00.0

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10.2

9To

tal

10

0.0

7100.3

1101.4

499.6

899.3

099.7

2101.0

699.6

699.3

198.4

497.3

399.1

5

Su

mC

ati

on

s(6

O)

4.0

04.0

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Al

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-su

bS

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su

bS

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su

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ip

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48.7

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9.3

835.1

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332.7

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120.8

034.3

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9.6

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211.4

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616.1

3w

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nit

e45.9

04

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647.5

149.3

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l(iv

)0.9

20.8

30.7

80.7

50.7

00.7

40.7

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90.5

80.7

80.7

80.7

6T

i0.0

80.1

70.1

90.2

10.2

20.2

20.1

70.1

90.1

60.2

10.2

20.2

4N

a(m

2)

0.0

00.0

00.0

30.0

40.0

80.0

30.0

80.0

20.2

50.0

10.0

10.0

0M

g/(

Mg

+F

e2+)

94.4

485.8

880.5

678.2

875.6

773.3

177.9

675.0

245.1

177.4

672.1

471.7

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Sp

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Gro

up

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sK

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Sp

1K

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03

GM

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2K

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04

Sp

9K

SH

04

GM

Sp

1K

82

0S

p2

K8

20

GM

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2K

894

Sp

1a

K894

GM

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1S

iO2

0.2

80.9

80.3

22.8

00.4

54.9

50.1

81.9

7T

iO2

1.6

720.8

720.8

924.7

01

5.2

52

0.7

420.8

725.3

1A

l 2O

313.0

72.4

85.3

83.8

47.9

72.8

07.4

31.8

5C

r 2O

344.4

90.1

20.2

40.0

50.0

40.0

70.0

80.0

0F

e2O

3**

8.7

921.2

521.7

45.1

028.4

99.4

21

7.3

85.5

1F

eO

**16.9

149.0

644.0

154.2

839.0

452.5

643.9

352.5

6M

nO

0.4

50.7

30.5

10.5

80.5

10.8

31.9

10.3

8M

gO

11.5

00.5

44.1

50.3

14.1

40.4

22.8

90.0

0C

aO

0.0

00.3

60.0

60.5

60.0

00.9

60.0

20.8

3K

2O

0.0

00.0

20.0

00.2

50.0

10.0

30.0

00.0

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a2O

0.0

00.0

10.0

00.0

90.0

00.6

70.0

00.0

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2O

50.0

00.0

40.0

00.2

20.0

10.0

40

.01

0.0

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iO0.1

50.0

70.0

20.1

30.0

20.0

00.0

10.0

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0.0

00.0

00.0

60.0

00.0

00.0

00.0

00

.00

To

tal

97

.31

96

.53

97

.38

92.9

195.9

493.4

894.7

188.5

8

Su

mC

ati

on

s(4

O)

3.0

03.0

03.0

03.0

03.0

03.0

33.0

03.0

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g/(

Mg

+F

e2

+)

54.8

01.9

214.3

91.0

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01.4

010.5

00.0

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r/(C

r+A

l)69.5

43.1

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10.8

70.3

41.6

50.7

20.0

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lvo

Sp

inelC

om

po

nen

t1.8

40.6

60.6

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10.4

50

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0.6

60

.91

*=

Fe

and

Fe

adju

ste

dsto

ichio

metr

ically

thro

ugh

the

meth

ods

of

Dro

op

(1987)

2+

3+

**=

Fe

an

dF

eadju

ste

dsto

ichio

metr

ically

2+

3+

21

Olivine

Olivine phenocrysts occur in all groups and are compositionally homogenous or

normally zoned. Olivine phenocrysts from Group 1 samples have higher

forsterite contents than those from other groups; phenocryst cores (Fo85-92) are

compositionally homogenous, with thin rims that have dramatically lower

forsterite content (Fo77-80). Group 2 samples vary from Fo75 cores to ~ Fo40 rims.

Olivine phenocrysts from Group 3 and 4 samples are more homogenous (Fo65 to Fo60 and Fo55 to Fo48, respectively). Compositions of groundmass olivine

varies, but is always less than the least forsteritic phenocryst rim composition;

groundmass olivine in Group 3 is considerably lower than the rim compositions

of the phenocrysts. Olivine phenocryst compositions are less than

compositions calculated to be in equilibrium with the bulk rock (Figure 9), with

groundmass analyses considerably lower.

30 40 50 60 70 80

10

30

50

70

90

Fo

%

Mg number of rock

Figure 9. Comparison of forsterite values of phenocryst and groundmass olivines withMg number calculated from bulk rock analysis. The dashed line represents the olivinecomposition in equilibrium with the bulk rock using K = 0.3. (arrows show typical

Fo change from core to rim where applicable).D(Ol/Liq)

Fe/Mg

Group 1 Phenocryst

Group 1 Groundmass

Group 2 Phenocryst

Group 2 Groundmass Group 3 Groundmass

Group 3 Phenocryst Group 4 Phenocryst

Group 4 Groundmass

rim

-co

re

22

Clinopyroxene

Clinopyroxene phenocrysts commonly occur in all four petrographic groups.

Phenocrysts range from large subhedral crystals with resorption rims,

numerous melt, apatite, olivine and spinel inclusions and complex zonation

patterns to small, unzoned or normally zoned euhedral crystals with few

inclusions. Most clinopyroxenes are aluminium diopsides (Figure 10a).

Phenocryst cores contain significant amounts of Cr2O3 (eg. Table 5, KSH05

Cpx 4a (0.93 weight percent)); TiO2 contents reach 4.6 weight percent in some

rim / groundmass analyses. The majority of samples that plot in the others

quad (Figure 10b) are rim or groundmass analyses. According to the

boundaries defined by Aoki and Kushiro (1968) on an octahedral aluminium

(AlM1) versus tetrahedral aluminium (AlT) plot (Figure 10c), all clinopyroxene are

of low pressure origin.

Group 1 samples vary from chromian aluminium augite to ferrian sub-silic

aluminium wollastonite. Groundmass analyses dominantly plot towards the

diopside/wollastonite end of this band. Group 3 and 4 samples show similar

trends however span much smaller compositional bands, whilst Group 2

phenocrysts and groundmass compositions overlap.

Feldspar

Feldspar phenocrysts are well developed only in Group 2 samples. These

phenocrysts are sub- to euhedral, coarse to very fine-grained with a variety of

zonation patterns. Most phenocrysts are unzoned or normally zoned; An

content varies from An70 to An45. Some larger Group 2 phenocrysts show

oscillatory or reverse zonation. Group 1 and 3 samples contain sub- to

euhedral micro phenocrysts that are normally zoned from An80 to An60 and An70

to An50, respectively. Groundmass plagioclase generally overlaps phenocryst

rim compositions and extends to lower An contents. Group 2 samples contain

sanidine in the groundmass.

23

NaM2

AlT

CaTSJD

NaTi TiAlNaTiAl

Ti

enstatite ferrosilite

Hedenbergitediopside hedenbergite

Group 1 Phenocryst

Group 1 Groundmass

Group 2 Phenocryst

Group 2 Groundmass Group 3 Groundmass

Group 3 Phenocryst Group 4 Phenocryst

Group 4 Groundmass

Figure 10. A) Microprobe analyses of phenocryst and groundmass clinopyroxenesfrom Shira samples presented in the Mg-Ca-Fe (enstatite-wollastonite-ferrosilite) triangle(arrows indicate general trend from core to rim, circles indicate groundmass compositionregions) and

.

Samples have been split into the four petrographic groups based on phenocryst assemblagesas discussed in the text. C) Plot of octahedral aluminium (Al ) versus tetrahedral aluminium(Al ) in clinopyroxenes, and the pressure fields of Aoki and Kushiro (1968).

M1

T

B) others quadrilateral (JD = jadeite, CaTS = Ca-Tschermaks, TiAl = Ti-Alaugite, NaTiAl = Na-Ti-Al augite, NaTi = Na-Ti augite (Ti end member is fictive CaTiAl O )2 6

0.0 0.1 0.2 0.3 0.4 0.50.0

0.1

0.2

0.3

0.4

AlM

1

AlT

Intermediate Pressure

High Pressure

Low Pressure

A)

B)

wollastonite

C)

core- rim

trend

groundmasssamples

24

Ab An

Or

Albite Oligoclase Andesine Labradorite Bytownite Anorthite

Anorthocl

ase

Sanid

ine

Group 1 Phenocryst

Group 1 Groundmass

Group 2 Phenocryst

Group 2 Groundmass

Group 4 Phenocryst

Group 4 GroundmassGroup 3 Groundmass

Group 3 Phenocryst

Figure 11. Microprobe analyses of phenocryst and groundmass feldspars from Shira samplesplotted as proportion anorthite-albite-orthoclase (An-Ab-Or), with arrow showing the general trendfrom core to rim, and circles showing the groundmass composition regions.

core

Spinel

Analyses of spinels are separated into inclusion, phenocryst and groundmass

phases. Inclusions occur dominantly in Group 1 olivine (one inclusion was

found in a Group 4 olivine). These spinel inclusions are dominantly

magnesiochromite spinels (Figure 12). Spinel phenocrysts and groundmass

phases in all groups are similar in composition, being dominantly titaniferous

magnetites.

25

Figure 12. Microprobe analyses of phenocrysts (including inclusions) and groundmassspinels of Shira samples. Samples have been plotted using Mg numbers (Mg/(Mg + Fe ))and Cr numbers (Cr/(Cr+Al)). Groundmass samples plot very close to the lower left cornerof each diagram or not at all due to 0% Mg or Cr (magnetite / titaniferous magnetite).

2+

Cr/

Cr+

Al

Mg/Mg+Fe2+

00 10 20 30 40 50 60 700

20

40

60

80

10 20 30 40 50 60 700

20

40

60

80

Cr/

Cr+

Al

Mg/Mg+Fe2+

0 10 20 30 40 50 60 700

20

40

60

80

Cr/

Cr+

Al

Mg/Mg+Fe2+

0 10 20 30 40 50 60 700

20

40

60

80

Cr/

Cr+

Al

Mg/Mg+Fe2+

inclusions

inclusion

Group 1 Inclusion

Group 1 Groundmass

Group 2 Phenocryst

Group 2 Groundmass

Group 4 Phenocryst

Group 4 GroundmassGroup 3 Groundmass

Group 3 Phenocryst

Feldspathoid

Nepheline phenocrysts occur in Group 3 and 4 samples; groundmass nepheline

is also present in these as well as in one Group 2 sample (K802). Nepheline

compositions of Group 3 samples have higher nepheline components (Ne70-80)

than Group 4 samples (Ne60-65) (Figure 13). Groundmass and phenocryst

nepheline compositions overlap.

26

50 60 70 800

10

20

Ks

Ne

Group 2 Groundmass

Group 4 Phenocryst

Group 4 GroundmassGroup 3 Groundmass

Group 3 Phenocryst

Figure 13. Nepheline phenocryst and groundmass analyses from Shira samples plotted as

percent nepheline (Ne) versus kalsilite (Ks).

Laser Ablation Results

Samples for LA-ICP-MS (Table 4) were all analysed by EDS prior to LA-ICP-MS

analysis in order to reduce data and gain quantitative results. Results are

presented with respect to phenocryst type, and their respective sample names.

Olivine

Olivine shows consistent trace element concentrations for almost all core to rim

traverses (Figure 14), despite a significant decrease in forsterite content at the

phenocryst rim. KSH05 and K2225 (Group 1) olivine phenocrysts have similar

forsterite contents (Fo80-90) and very similar trace element concentrations (Ni ~

2000ppm, Cr ~ 250-400ppm, V ~ 5ppm and Mn ~ 1500ppm). KSH03 (Group 1)

olivine has slightly lower forsterite content (Fo75-82), and significantly lower Ni (~

1000ppm) and Cr (~ 50ppm), but higher V (~ 7-9ppm) and Mn (~ 2500ppm).

KSH01 (Group 2) olivine has the lowest forsterite contents (Fo72-55) and much

27

lower Ni (178-379ppm) and Cr (1.8-20ppm), yet the highest V (10-33.16ppm)

and Mn (4500-7000ppm) concentrations of all groups.

Clinopyroxene

Traverses of clinopyroxene phenocrysts yield a range of zonation patterns from

relatively unzoned to complex and oscillatory zoned. Results are presented as

a series of chondrite-normalised (Sun & McDonough, 1989) REE diagrams,

with an inset diagram of Mg number (Mg #) and Sc variation from core to rim for

comparison (Figure 15). REE diagrams show smooth enriched curves,

increasing in degree of enrichment from La to Nd, then decreasing from Nd to

Lu. Many samples also show slight positive Gd anomalies. In general, Mg #

and Sc concentrations show opposing trends (increasing Mg # versus

decreasing Sc); low Mg #s and high Sc concentrations correlate with greater

REE enrichment. Many samples show distinct steps in REE enrichment (i.e.

Figure 15, KSH05 cpx 2, 4 and 5) in which there is a drastic increase in REE

enrichment over a small increase in distance from the phenocryst core.

REE contents in individual clinopyroxene phenocrysts can vary by up to a factor

of 10, however in most cases, variation is restricted to a factor of ~3. The

overall degree of REE enrichment in clinopyroxene increases from Group 1

(samples KSH05, KSH03 and K2225) to Group 2 (KSH01) and Group 3 (K811).

28

Sc V Cr Mn Ni Fo%

Figure 14. Results of LA-ICP-MS core to rim analyses of 3 olivine phenocrysts from

each sample KSH05, KSH03, K2225 and KSH01 showing the variation of Sc, V, Cr,

Mn and Ni, along with the EDS determined forsterite content.

1.00

10.00

100.00

1000.00

10000.00

KSH05 Olivine 1

Ol1a Ol1b Ol1c Ol1d Ol1e Ol2a Ol2b Ol2c Ol2d Ol2e Ol2f Ol3a Ol3b Ol3c Ol3d Ol3e

KSH03 Olivine 1

Ol1a Ol1b Ol1c Ol2a Ol2b Ol2c Ol4a Ol4b Ol4c

K2225 Olivine 1

Ol1a Ol1b Ol1c Ol2a Ol2b Ol2c Ol4a Ol4b Ol4c

KSH01 Olivine 1

Ol1a Ol1b Ol1c Ol4a Ol4b Ol4c Ol5a Ol5b

20.00

40.00

60.00

80.00

100.00

Fo

rste

rite

%(E

DS

)

1.00

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100.00

1000.00

10000.00

20.00

40.00

60.00

80.00

100.00

Fo

rste

rite

%(E

DS

)

1.00

10.00

100.00

1000.00

10000.00

20.00

40.00

60.00

80.00

100.00

Fo

rste

rite

%(E

DS

)

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100.00

1000.00

10000.00

20.00

40.00

60.00

80.00

100.00

Fo

rste

rite

%(E

DS

)

KSH05 Olivine 2 KSH05 Olivine 3

KSH03 Olivine 2 KSH03 Olivine 4

K2225 Olivine 2 K2225 Olivine 4

KSH01 Olivine 4 KSH01 Olivine 5

Ab

un

dan

ce

of

ele

men

t(p

pm

)A

bu

nd

an

ce

of

ele

men

t(p

pm

)A

bu

nd

an

ce

of

ele

men

t(p

pm

)A

bu

nd

an

ce

of

ele

men

t(p

pm

)

28

29

Feldspar

Three plagioclase phenocrysts from sample KSH01 were analysed, and are

presented in order of decreasing anorthite content (determined by EDS

microprobe analyses) (Figure 16a). Only minor changes in trace element

concentrations were noted between each analysis. Chondrite normalised (Sun

& McDonough, 1989) REE diagrams (Figure 16b) show decreases in degree of

enrichment from La (42 x chondrite) to Yb (0.14 x chondrite), with distinct

positive Eu anomalies (up to 22 x chondrite). Degree of enrichment increases

with decreasing An %.

Spinel

Due to the small size of spinel phenocrysts (samples KSH01 (Group 2) and

K811 (Group 3)) and inclusions (sample KSH05 (Group1)), traverses were

unable to be conducted. Analyses have instead been plotted against

decreasing Mg #s as determined through EDS microprobe analyses in order to

show trace element variations (Figure 17). Distinct changes are observed with

the most notable being decreases in Cr and Ni, yet increases in Ti, Mn, V, Zr

and Nb. Zr/Nb is lowest in K811 analyses (0.56-0.61), and increases in KSH05

(0.93-1.33) and KSH01 (1.71).

30

KSH01 Plagioclase LA-ICP-MS Results

1.00

10.00

100.00

1000.00

10000.00

Pl7aPl7bPl1a

Analyses

Lo

g(p

pm

)

40.00

60.00

80.00

An

ort

hit

e%

(ED

S)

Ti Mn Ga Sr Ce An%

Figure 16. A) LA-ICP-MS analyses of Ti, Mn, Ga, Sr, Ce and An% in plagioclase phenocrystsfrom sample KSH01. Samples have been plotted in order of decreasing anorthite content asdetermined through EDS microprobe analysis. B) Chondrite normalised (Sun & McDonough,1989) REE diagram of analysed plagioclase phenocrysts.

KSH01 Plagioclase LA-ICP-MS Results

0.10

1.00

10.00

100.00

1000.00

La Ce Nd Sm Eu Gd Dy Er Yb Lu

REE

Ch

on

dri

teN

orm

ali

sed

RE

E

(Su

n&

McD

on

ou

gh

,1989)

Plag 1a Plag 7b Plag 7a

A)

B)

31

Figure 17. LA-ICP-MS analyses of spinel inclusions (KSH05 samples) and phenocrysts (K811and KSH01 samples). Samples have been plotted in order of decreasing Mg number asdetermined through EDS microprobe analysis.

Spinel LA-ICP-MS Results

1.00

10.00

100.00

1000.00

10000.00

100000.00

1000000.00

KS

H01-S

p3

K811-S

p2a

K811-S

p3a

K811-S

p4a

KS

H05-S

p3a

KS

H05-S

p4a

Analyses

Lo

g(p

pm

)

0.00

20.00

40.00

60.00

Mg

nu

mb

er

(ED

S)

Ti Mn V Cr Ni Zr Nb Mg#

Geochemical Results

All Shira samples were analysed for both major and trace elements. Results

are presented with respect to petrographic groups (Tables 6, 7, 8 and 9) and

graphically in Figures 18, 19 and 20. Mg #s (Mg/Mg+Fe2+) were adjusted to a

FeO ratio of 0.85 (FeO/Fe2O3+FeO). CIPW normative mineralogy was

calculated using IGPET (Igpet32) petrologic software (Terra Softa Inc.).

Samples are classified using the total alkalis-silica (TAS) diagram (Le Bas et al.,

1986).

The Shira volcanic rocks are all strongly alkalic, ranging from nephelinite to

picro-basalt, basanite and trachybasalt (Figure 21) and are all nepheline

normative; Mg #s vary from 77 to 36. The Shira samples have a limited range

in SiO2 content (40.46 wt % to 49.31wt %), a broad range in MgO content

(16.51wt % to 3.11wt %) and Al2O3 content (8.35wt % to 17.72wt %). CaO

abundances (15.76wt % to 7.09wt %) and CaO/Al2O3 (molecular proportions)

(0.73 to 3.71) have positive correlations with Mg # (Figure 22). Abundances of

Fe2O3, TiO2, K2O, P2O5, Na2O, Sr and Ba all show negative correlations with

32

Mg# (Figure 18), however both Fe2O3 and TiO2 show inflections at

approximately Mg# 45.

Groups identified on the basis of petrographic character are easily discernible

on most major element and trace element graphs (Figures 18, 19 and 21) and

normative mineralogy (Figure 23). Group 1 samples (picrites, basanites and

alkali-olivine basalts) are easily separated due to their much higher Mg #s and

CaO contents, and much lower Al2O3, Na2O, P2O5 and K2O abundances (Figure

18). Group 1 samples generally show low incompatible element concentrations

(Figures 19 & 20), relatively high normative plagioclase compositions (Figure

23b) and low normative albite contents (Figure 23c).

Group 3 (nephelinites and basanites) samples, although having similar Mg #s

to Group 2 and 4 samples, are distinguished by their high P2O5 and MnO, and

low SiO2 content (Figure 18). Group 3 samples also have higher CaO contents

and CaO/Al2O3 ratios (Figure 22a) at comparable Mg #s to Group 2 and 4

samples, as well as higher Sr, Ce, Yb, Zr, Nb and Ta abundances (Figure 19).

Group 3 samples have the highest normative nepheline contents (Figure 23a),

high normative plagioclase compositions (Figure 23b), and low albite contents

(Figure 23c) at comparable Mg #s to Groups 2 and 4.

Group 2 and 4 samples (trachy-basalts and basanites) cover broad, but similar

chemical composition ranges (Figures 18, 19, 22 and 23). Group 2 samples

have lower CaO/Al2O3 (Figure 22a), than Group 3 and 4 samples of similar Mg

#. Group 2 samples generally contain slightly higher Sr, Ba, Rb, Ce, Yb, Zr, Hf,

Nb and Ta contents at comparable Mg #s (Figure 19) than Group 4 samples,

whereas the majority of Group 4 samples contain higher normative plagioclase

compositions (Figure 23b) and lower normative albite content (Figure 23c) than

Group 2 samples of comparable Mg #s.

Although broad geochemical trends are apparent over the entire range of Shira

samples (negative trends for incompatible elements (i.e. Sr, Ba, REE, Zr, Hf, Nb

& Ta) and positive trends for Cr, Sc and V), smaller intra-group trends are also

apparent, with some intra-group trends opposing the broader Shira trend.

Group 2 samples show positive trends for Nb and Ta, whilst Groups 1, 3 and 4

show negative trends. Similarly, Groups 1 and 3 show positive Rb trends,

whilst Groups 2 and 4 show negative trends. Hf shows a negative correlation

33

for Groups 1 and 4, yet a positive correlation for Group 3 samples, and broad

scatter of Group 2 samples.

Chondrite-normalised REE patterns of Shira samples are light-REE enriched.

La concentrations range between 100 and 400 times chondritic levels, with Lu

concentrations approximately 10 to 20 times chondritic levels. Ce/Yb ratios

vary from 36 to 70. REE patterns shallow towards the heavy REE, with Ce/Sm

values between 9.9 and 16.3 and Sm/Yb values of between 3.17 and 4.41.

Chondrite-normalised REE patterns are smooth and near parallel (Figure 20),

with very minor Eu anomalies observed in only five Group 2 samples, three

Group 3 samples, and one Group 4 sample. The degree of REE enrichment

increases from Groups 1 to 3, with Group 4 covering a broader range. Groups

have distinct multi-element spider diagram trends when normalised against

primitive mantle values (Figure 20) (Sun & McDonough, 1989). All groups show

distinct K depletions, but uncharacteristically, Pb enrichments (not as

pronounced in Group 3 samples) (Figure 18). Group 1 and 2 samples have

similar characteristics, with Group 1 tending to be less enriched than Group 2.

Positive anomalies are shown for Pb, Nb, Nd, and Ti relative to neighbouring

elements, and negative anomalies are shown for P, K and Zr in Groups 1 and 2,

with larger anomalies in Group 1 than Group 2. Group 3 multi-element spider

diagrams are similar to Group 2, with larger negative K anomalies, but smaller

Pb anomalies. The multi-element diagram for Group 4 is very similar to that of

Group 1.

34

Table 6. Geochemical results of Group 1 samples (BSN=basanite, PBAS=picrobasalt, AOB=alkali olivine basalt).

ICP-AES Major Element Results (values in weight percent except where stated)Sample KSH03 KSH05 KSH07 KSH08 KSH09 K2225Rock Type BSN AOB PBAS AOB AOB BSNSiO2 44.46 46.37 44.37 45.18 45.15 43.33Al2O3 13.28 7.96 9.33 7.52 8.35 12.91Fe2O3 (total) 12.93 10.62 11.49 10.23 11.10 13.33MnO 0.21 0.16 0.18 0.16 0.17 0.21MgO 10.13 16.17 13.06 16.51 14.53 8.46CaO 11.55 14.30 15.68 15.34 15.76 13.13Na2O 2.02 1.16 1.43 1.04 1.14 3.33K2O 0.96 0.45 0.36 0.37 0.34 0.56TiO2 1.97 1.50 1.75 1.49 1.65 2.21P2O5 0.44 0.23 0.29 0.24 0.24 0.49LOI 1.65 1.09 1.06 0.98 1.14 2.47Total 99.59 100.01 99.00 99.07 99.58 100.42

Sr (ppm) 622.10 388.10 355.50 291.90 338.10 590.60Ba (ppm) 371.80 123.80 152.70 82.31 109.10 271.70

LA-ICP-MS Trace Element Results (values in ppm)Sc 34.23 47.94 59.23 52.29 59.51 35.03V 276.81 237.98 305.80 254.00 289.19 325.96Cr 316.44 1417.44 764.80 1571.65 886.98 193.40Ga 18.48 11.07 13.59 11.83 12.44 18.79Rb 22.35 85.98 44.29 11.09 46.04 9.57Sr 671.84 395.26 378.01 329.97 369.07 595.74Y 22.64 14.04 17.55 14.82 16.38 24.42Zr 194.15 100.10 120.73 105.55 107.93 193.67Nb 63.50 32.25 37.88 33.64 33.30 71.26Cs 0.24 0.98 0.55 0.11 0.44 0.28Ba 455.77 197.76 208.96 188.02 207.47 366.92La 46.78 24.50 26.09 24.83 24.71 48.82Ce 85.65 43.67 49.92 45.89 45.82 88.91Nd 39.39 21.35 25.09 22.11 22.76 39.90Sm 7.15 4.30 5.03 4.32 4.62 7.31Eu 2.13 1.23 1.53 1.29 1.38 2.16Gd 5.99 3.61 4.46 3.85 4.29 6.53Dy 4.47 2.70 3.28 2.97 3.29 4.79Er 2.10 1.28 1.69 1.40 1.57 2.39Yb 1.68 1.05 1.38 1.14 1.27 1.91Lu 0.26 0.16 0.21 0.16 0.19 0.28Hf 4.48 2.57 3.36 2.58 2.99 4.61Ta 3.69 1.89 2.28 2.03 2.13 4.25Pb 6.50 4.34 4.68 4.67 4.61 6.00Th 4.89 2.68 2.88 2.80 2.57 5.96U 0.88 0.57 0.63 0.58 0.49 1.34

FeO/(Fe 2 O 3 + FeO) = 0.85

Fe2O3 2.16 1.77 1.92 1.71 1.85 2.22FeO 10.99 9.03 9.77 8.70 9.44 11.33

Mg/(Mg+Fe 2+ ) and CaO/Al 2 O 3 calculated using molecular proportions (analysed weights)

Mg/(Mg+Fe2+) 62.2 76.2 70.5 77.2 73.3 57.1CaO/Al2O3 1.6 3.3 3.1 3.7 3.4 1.8Fo% (0.3) 84.6 91.4 88.8 91.9 90.1 81.6%AN 66.9 68.1 91.0 88.5 87.0 75.5CIPW Normative Results - Weight norm calculated using dry weights recalculated to 100%, and 0.85 Fe 2+ values

or 5.78 2.68 2.17 2.23 2.04 3.37ab 12.28 7.19 1.81 1.95 2.52 6.16an 24.79 15.32 18.32 15.02 16.90 18.97ne 2.78 1.47 5.70 3.79 3.93 12.21di 24.97 43.84 47.57 48.64 48.84 36.15ol 21.37 23.49 17.52 22.40 19.31 14.43mt 3.19 2.59 2.84 2.52 2.72 3.28il 3.81 2.87 3.39 2.88 3.18 4.28ap 1.04 0.54 0.68 0.57 0.56 1.16Total 100.00 100.00 100.00 100.00 100.00 100.00

35

Table 7. Geochemical results of Group 2 samples (BSN=basanite, TBAS=trachybasalt). ICP-AES Major Element Results (values in weight percent except where stated)Sample KSH01 KSH02 KSH04 KSH06 KSH10 KSH11 K679 K802 K803 K804 K895Rock Type TBAS TBAS BSN TBAS BSN TBAS TBAS TBAS BSN TBAS TBASSiO2 45.73 49.31 42.79 49.01 43.38 46.61 46.62 47.93 44.52 48.57 47.25Al2O3 16.08 17.03 16.27 17.72 15.74 16.77 15.71 17.47 16.44 16.91 15.81Fe2O3 (total) 13.66 12.05 14.20 11.62 15.17 12.42 12.42 12.57 13.02 11.56 12.57MnO 0.22 0.20 0.21 0.19 0.22 0.19 0.21 0.23 0.24 0.18 0.20MgO 5.57 4.41 4.49 3.94 4.74 4.85 6.31 3.70 5.66 4.26 6.20CaO 9.15 7.36 9.74 7.09 11.95 8.47 9.50 7.93 10.22 7.88 9.06Na2O 4.12 4.42 3.37 3.74 2.12 3.30 3.62 4.64 4.34 3.37 3.86K2O 1.27 2.25 1.64 2.48 1.13 1.95 2.05 1.89 1.68 2.40 2.15TiO2 2.40 2.06 2.60 2.08 2.74 2.36 2.26 2.56 2.49 2.17 2.24P2O5 0.58 0.73 0.61 0.68 0.58 0.60 0.73 0.70 0.75 0.59 0.71LOI 0.51 0.65 4.82 1.87 2.88 2.20 1.26 0.83 0.79 1.17 0.69Total 99.28 100.46 100.73 100.41 100.66 99.71 100.68 100.43 100.14 99.05 100.73

Sr (ppm) 848.30 959.20 741.60 869.80 728.40 827.10 848.60 877.80 941.10 1289.00 875.00Ba (ppm) 645.90 787.00 424.40 758.10 526.90 631.80 617.70 680.80 635.60 589.50 586.20

LA-ICP-MS Trace Element Results (values in ppm)Sc 19.50 13.97 15.55 11.72 20.79 14.43 19.77 9.38 17.32 15.04 18.49V 263.11 180.06 241.78 170.59 338.87 246.89 235.06 145.12 235.50 210.71 224.18Cr 36.65 35.39 29.08 30.48 18.63 - 99.07 - - 69.24 151.10Ga 23.62 23.20 23.52 23.10 23.72 24.99 22.32 22.95 22.91 27.93 22.40Rb 66.38 62.75 53.66 92.84 125.47 89.15 54.02 53.94 59.30 116.09 58.71Sr 822.05 945.01 771.66 872.76 778.23 881.56 877.47 1000.05 954.54 1316.21 892.83Y 27.84 29.74 27.85 29.67 28.69 30.29 28.34 29.68 29.17 39.01 27.47Zr 258.20 279.91 226.23 268.65 221.08 270.48 296.65 262.01 328.48 325.54 287.77Nb 90.25 97.42 82.56 99.83 75.50 92.91 107.72 88.29 107.97 94.29 107.37Cs 0.36 0.56 0.37 0.57 1.31 0.71 0.54 0.76 0.49 0.38 0.85Ba 634.96 781.96 487.50 769.35 587.53 695.26 690.43 710.36 703.35 704.94 672.67La 67.88 75.85 53.14 75.95 51.50 72.09 71.85 69.41 74.34 78.98 72.38Ce 121.85 137.00 97.43 137.06 95.52 129.50 131.61 124.89 136.24 143.20 129.80Nd 52.98 58.10 43.03 57.58 44.38 53.39 55.55 53.15 58.48 60.55 53.98Sm 9.44 9.68 7.88 9.85 8.31 9.75 9.85 9.46 10.11 10.80 9.00Eu 2.84 2.64 2.46 2.57 2.51 2.71 2.72 2.63 2.98 2.91 2.68Gd 7.53 7.60 6.72 7.56 7.24 7.66 7.53 7.41 8.02 8.78 7.32Dy 5.37 5.65 5.35 5.72 5.68 5.88 5.52 5.64 5.80 7.24 5.37Er 2.53 2.79 2.63 2.85 2.73 2.89 2.67 2.80 2.71 3.83 2.60Yb 2.16 2.51 2.18 2.55 2.23 2.48 2.23 2.48 2.37 3.40 2.28Lu 0.31 0.37 0.34 0.41 0.32 0.37 0.34 0.38 0.32 0.51 0.34Hf 5.61 5.97 4.84 6.21 4.93 6.04 6.42 5.66 6.82 7.24 6.16Ta 5.14 6.14 4.92 6.09 4.52 5.08 6.41 4.99 6.48 5.18 6.49Pb 5.45 9.57 7.24 10.04 6.08 9.56 8.66 8.48 7.49 8.78 8.49Th 7.50 8.83 6.84 9.59 5.90 9.13 9.38 8.99 7.68 10.52 9.42U 1.40 1.65 1.43 2.99 1.39 1.02 1.12 1.89 1.55 2.60 1.15

FeO/(Fe 2 O 3 + FeO) = 0.85

Fe2O3 2.28 2.01 2.37 1.94 2.53 2.07 2.07 2.10 2.17 1.93 2.10FeO 11.61 10.24 12.07 9.88 12.89 10.56 10.56 10.68 11.07 9.83 10.68

Mg/(Mg+Fe 2+ ) and CaO/Al 2 O 3 calculated using molecular proportions (analysed weights)Mg/(Mg+Fe2+) 46.1 43.4 39.9 41.6 39.6 45.0 51.6 38.2 47.7 43.6 50.9CaO/Al2O3 1.0 0.8 1.1 0.7 1.4 0.9 1.1 0.8 1.1 0.8 1.0Fo% (0.3) 74.0 71.9 68.9 70.3 68.6 73.2 78.0 67.3 75.2 72.0 77.5%AN 51.6 41.0 65.4 44.8 69.0 51.5 54.6 44.4 65.0 46.4 51.4CIPW Normative Results - Weight norm calculated using dry weights recalculated to 100%, and 0.85 Fe 2+ values

or 7.58 13.29 10.08 14.84 6.81 11.79 12.16 11.19 9.97 14.46 12.67ab 20.49 28.77 13.43 30.25 13.82 24.23 17.18 26.62 11.02 28.18 18.37an 21.85 19.98 25.40 24.55 30.70 25.77 20.64 21.30 20.50 24.39 19.41ne 7.97 4.67 8.79 0.97 2.43 2.35 7.35 6.89 14.01 0.48 7.69di 16.77 9.81 17.35 5.50 21.73 10.99 18.05 11.36 21.13 9.61 17.17ol 16.04 14.96 14.77 15.45 14.09 15.79 15.62 13.11 13.72 14.43 15.76mt 3.34 2.91 3.57 2.85 3.74 3.07 3.01 3.05 3.16 2.85 3.04il 4.60 3.91 5.14 4.00 5.31 4.59 4.31 4.87 4.75 4.20 4.24ap 1.36 1.69 1.47 1.60 1.37 1.42 1.70 1.62 1.74 1.39 1.64Total 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00

36

Table 8. Geochemical results of Group 3 samples (BSN=basanite, NEPH=nephelinite/foidite).

ICP-AES Major Element Results (values in weight percent except where stated)Sample K686 K811 K813 K820 K825Rock Type BSN BSN NEPH NEPH NEPHSiO2 41.32 42.72 40.67 40.46 41.48Al2O3 14.49 14.89 14.12 15.29 16.89Fe2O3 (total) 16.07 14.27 14.64 15.86 14.71MnO 0.27 0.24 0.29 0.31 0.28MgO 5.50 5.52 6.22 5.20 4.22CaO 11.94 11.67 13.99 11.37 10.03Na2O 3.71 3.89 3.03 4.46 5.76K2O 0.78 1.83 0.76 1.10 1.47TiO2 2.68 2.41 2.63 2.39 2.27P2O5 0.85 0.75 0.75 0.88 0.92LOI 3.38 2.00 3.14 1.86 1.82Total 100.99 100.19 100.25 99.17 99.84

Sr (ppm) 935.10 977.20 807.50 1138.00 1156.00Ba (ppm) 545.30 669.40 393.70 545.70 719.30

LA-ICP-MS Trace Element Results (values in ppm)Sc 9.30 16.88 13.25 9.37 6.67V 300.48 270.26 335.77 278.34 253.13Cr - 41.65 19.27 13.06 4.12Ga 23.91 21.80 24.44 23.56 23.38Rb 74.02 59.61 126.66 63.60 55.26Sr 980.81 916.55 912.24 1182.32 1151.69Y 34.62 30.99 36.28 39.22 37.90Zr 351.34 297.07 385.83 360.25 353.74Nb 134.44 108.10 121.95 138.41 148.53Cs 0.58 0.44 0.66 0.57 0.61Ba 520.52 574.16 532.86 616.73 762.33La 85.81 70.69 81.92 90.80 92.75Ce 154.99 128.87 150.89 162.13 165.72Nd 65.64 56.95 67.70 68.37 67.35Sm 11.71 10.16 12.09 11.87 11.46Eu 3.36 2.98 3.62 3.32 3.32Gd 9.31 8.17 9.98 9.61 9.38Dy 6.76 6.02 7.26 7.42 7.17Er 3.30 2.95 3.43 3.68 3.63Yb 2.73 2.40 2.90 3.15 3.16Lu 0.40 0.34 0.39 0.48 0.43Hf 6.48 5.75 7.57 6.52 6.02Ta 9.30 7.22 8.51 10.03 10.62Pb 5.92 5.24 5.75 5.65 5.44Th 9.18 7.38 8.50 9.98 10.62U 1.65 1.15 1.85 2.60 2.78

FeO/(Fe 2 O 3 + FeO) = 0.85

Fe2O3 2.68 2.38 2.44 2.64 2.45FeO 13.66 12.13 12.44 13.48 12.50

Mg/(Mg+Fe 2+ ) and CaO/Al 2 O 3 calculated using molecular proportions (analysed weights)

Mg/(Mg+Fe2+) 41.8 44.8 47.1 40.7 37.6CaO/Al2O3 1.5 1.4 1.8 1.4 1.1Fo% (0.3) 70.5 73.0 74.8 69.6 66.7%AN 75.2 87.6 99.7 89.9 81.8CIPW Normative Results - Weight norm calculated using dry weights recalculated to 100%, and 0.85 Fe 2+ values

or 4.71 10.99 4.61 6.66 8.84ab 6.94 2.55 0.06 2.12 3.60an 21.03 18.05 23.30 18.91 16.17ne 13.61 16.73 14.24 19.80 24.92di 28.29 29.82 35.66 27.67 23.87ol 14.24 11.94 11.57 14.17 12.43mt 3.97 3.51 3.63 3.92 3.61il 5.20 4.65 5.13 4.65 4.39ap 2.01 1.77 1.79 2.09 2.17Total 100.00 100.00 100.00 100.00 100.00

37

Table 9. Geochemical results of Group 4 samples (BSN=basanite, TBAS=trachybasalt).

ICP-AES Major Element Results (values in weight percent except where stated)Sample K361 K832A K897 K832B K894Rock Type BSN BSN BSN TBAS BSNSiO2 44.27 45.07 43.92 45.58 43.09Al2O3 16.22 14.20 15.78 15.37 17.51Fe2O3 (total) 14.00 14.47 15.18 14.50 11.83MnO 0.20 0.22 0.23 0.21 0.23MgO 6.10 6.37 5.63 4.31 3.11CaO 11.41 12.51 10.29 9.12 8.07Na2O 3.55 3.31 4.17 2.96 4.83K2O 0.64 0.41 1.44 2.14 2.42TiO2 2.19 2.28 2.46 2.63 2.00P2O5 0.39 0.54 0.63 0.46 0.91LOI 1.36 0.92 0.91 2.13 6.05Total 100.32 100.30 100.63 99.39 100.04

Sr (ppm) 769.80 558.60 702.00 617.10 1198.00Ba (ppm) 384.10 366.50 441.60 597.00 811.80

LA-ICP-MS Trace Element Results (values in ppm)Sc 20.56 28.79 13.98 20.51 4.25V 317.62 290.64 268.64 301.86 133.06Cr - 60.88 24.13 76.76 -Ga 21.58 20.32 24.12 22.97 23.52Rb 13.81 20.97 27.59 48.70 78.61Sr 813.04 574.22 734.79 625.53 1232.35Y 20.56 26.56 27.23 28.47 29.89Zr 165.97 192.93 228.59 260.35 274.90Nb 52.07 62.27 83.86 83.27 152.97Cs 0.29 0.44 0.44 0.18 0.75Ba 439.70 443.48 546.68 687.21 904.48La 38.08 46.88 60.24 52.96 95.73Ce 70.29 86.61 108.49 94.56 161.86Nd 32.13 41.26 46.76 42.85 61.69Sm 6.17 7.61 8.36 7.96 9.95Eu 1.99 2.29 2.49 2.47 2.82Gd 5.18 6.72 6.98 6.86 7.77Dy 4.07 5.17 5.24 5.49 5.68Er 1.93 2.51 2.58 2.73 2.90Yb 1.73 2.14 2.09 2.33 2.31Lu 0.24 0.30 0.32 0.34 0.33Hf 3.94 4.46 4.98 5.66 4.65Ta 3.11 3.73 5.00 5.06 9.17Pb 6.11 7.58 7.53 12.46 10.39Th 4.49 5.12 7.53 5.95 12.71U 1.07 1.17 1.74 1.23 2.89

FeO/(Fe 2 O 3 + FeO) = 0.85

Fe2O3 2.33 2.41 2.53 2.42 1.97FeO 11.90 12.30 12.90 12.33 10.06

Mg/(Mg+Fe 2+ ) and CaO/Al 2 O 3 calculated using molecular proportions (analysed weights)Mg/(Mg+Fe2+) 47.7 48.0 43.8 38.4 35.6CaO/Al2O3 1.3 1.6 1.2 1.1 0.8Fo% (0.3) 75.3 75.5 72.2 67.5 64.8%AN 66.1 59.8 66.4 54.5 59.7CIPW Normative Results - Weight norm calculated using dry weights recalculated to 100%, and 0.85 Fe 2+ values

or 3.81 2.43 8.51 12.97 15.18ab 13.69 15.33 10.18 19.11 13.60an 26.65 22.77 20.09 22.90 20.12ne 8.99 6.93 13.61 3.56 16.13di 23.11 29.87 22.40 17.14 13.56ol 15.24 13.57 15.41 14.51 12.10mt 3.41 3.51 3.67 3.60 3.03il 4.19 4.35 4.67 5.12 4.03ap 0.91 1.26 1.46 1.09 2.24Total 100.00 100.00 100.00 100.00 100.00

38

40 50 60 7040

42

44

46

48

SiO2

Mg/(Mg+Fe2+)

40 50 60 705

10

15

Al2O3

Mg/(Mg+Fe2+)

40 50 60 705

10

15

Fe O

(total)2 3

Mg/(Mg+Fe2+)

40 50 60 701

2TiO2

Mg/(Mg+Fe2+)

40 50 60 70

1

2

3

4

5

Na2O

Mg/(Mg+Fe2+)

40 50 60 700.1

0.2

0.3

MnO

Mg/(Mg+Fe2+)

40 50 60 700

1

2

K2O

Mg/(Mg+Fe2+)

0

40 50 60 70 805

10

15

CaO

Mg/(Mg+Fe )2+

40 50 60 70 800

P2O5

Mg/(Mg+Fe )2+

0.8

0.6

0.4

0.2

40 50 60 70 80

Mg/(Mg+Fe )2+

40 50 60 70 80

Mg/(Mg+Fe )2+

Figure 18. ICP-AES major element resultsfor Shira samples in weight percent plottedagainst Mg numbers.

Group 1

Group 2

Group 3

Group 4

Shira Samples

39

40 50 60 70

200

600

1000

1400

Sr

Mg/(Mg+Fe2+)

40 50 60 70

200

400

600

800

1000

Ba

Mg/(Mg+Fe2+)

40 50 60 700.0

0.5

1.0

1.5

Cs

Mg/(Mg+Fe2+)

40 50 60 700

50

100

Rb

Mg/(Mg+Fe2+)

40 50 60 700

40

80

120

160

Ce

Mg/(Mg+Fe2+)

40 50 60 701

2

3

Yb

Mg/(Mg+Fe2+)

40 50 60 700

100

200

300

400

Zr

Mg/(Mg+Fe2+)

40 50 60 700

2

4

6

8

Hf

Mg/(Mg+Fe2+)

40 50 60 70 800

50

100

150

Nb

Mg/(Mg+Fe )2+

40 50 60 70 80

2

6

10

Ta

Mg/(Mg+Fe )2+

40 50 60 70 80

Mg/(Mg+Fe )2+

40 50 60 70 80

Mg/(Mg+Fe )2+

Figure 19. LA-ICP-MS trace element results (Sr, Ba, Cs, Rb, Ce, Yb, Zr, Hf, Nb & Ta) for Shirasamples in parts per million (ppm) plotted against Mg numbers.

Group 1

Group 2

Group 3

Group 4

Shira Samples

40

1

10

100

La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Rock/Chondrites

1

10

100

La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Rock/Chondrites

1

10

100

La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Rock/Chondrites

1

10

100

La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Rock/Chondrites

1

10

100

1000

CsRbBaTh U Nb K LaCePb Pr Sr P Nd ZrSmEu Ti Dy Y YbLu

Rock/Primitive Mantle

1

10