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
ABSTRACT Mt Kilimanjaro, Africa’s highest mountain (5895m), is a large, young (<1.6Ma) stratovolcano at the southern end of the East African Rift, in northern Tanzania. Consisting of three distinct volcanic centres, Shira, Mawenzi and Kibo, Shira contains the highest proportion of mafic rocks. Shira samples are strongly silica under-saturated rocks, ranging from picro-basalt, to nephelinite and hawaiite (Mg numbers (Mg #) ranging from 77.2–35.5). Phenocrysts constitute up to 55% of some samples, and include aluminous augite (often containing abundant fluid and/or melt inclusions), olivine (Fo92-Fo49), plagioclase (An75-
An42), nepheline (Ne77-Ne68), magnesiochromite and ulvöspinel. Groups identified on the basis of phenocryst assemblages and textures correlate with location. East Shira Hill samples contain olivine and clinopyroxene phenocrysts + microphenocrysts of plagioclase (Group 1), or plagioclase and clinopyroxene phenocrysts + microphenocrysts of olivine (Group 2). Samples with high Mg #’s contain abundant cumulate clinopyroxene and olivine (Fo92-Fo85). Group 3 samples (Shira Ridge) contain nepheline phenocrysts and Group 4 samples (Platzkegel) have distinct intergranular textures. Chondrite normalised REE patterns are steep, with light REE-enrichment up to 400x chondrite. Spider diagrams, normalised to OIB for primitive Shira samples have strong K depletions and Pb enrichments.
The source of the Shira volcanic rocks is most likely an amphibole-bearing spinel lherzolite, in which amphibole remains residual. Similarities in spider diagram patterns and trace element ratios suggest a source similar to average OIB. The Shira volcanic centre is a polygenetic volcano, in which multiple small volume, low degree (4-10%) partial melts from a metasomatised subcontinental lithospheric mantle follow pre-existing structural weaknesses, before ponding in the lithosphere. Evolution of these small volume melts is dominated by shallow fractional crystallisation of clinopyroxene, olivine ± spinel, with plagioclase also fractionating from Group 4 (Platzkegel) samples. A magma mixing origin is suggested for some samples and supported by complex zonation patterns in major and trace element chemistry of clinopyroxene phenocrysts as well as linear mixing arrays. The Shira volcanic centre has since ceased activity, and collapsed to form the present day Shira Ridge and caldera before being overlain by various Kibo and parasitic lavas to the east and northwest of the Shira region.
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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
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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
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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
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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:…………………………………..
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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.
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INTRODUCTION The petrogenetic modelling of primitive mafic, alkalic rocks provides valuable information on large parts of the earth’s 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 Earth’s 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).
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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 (<1.6Ma) stratovolcano located near the flank of the propagating end of the EAR. The last significant geological studies on Mt Kilimanjaro undertaken in 1953 and 1957 by Sheffield University and Geological Survey of Tanganyika (Tanzania) recognised a wide variety of rocks that range from strongly alkalic to tholeiitic compositions (Downie & Wilkinson, 1972). Mt Kilimanjaro is composed of three distinct volcanic centres, Shira, Mawenzi and Kibo (currently active); Shira contains the highest proportion of alkalic rocks.
Geographic Location
Mt Kilimanjaro is an active stratovolcano located near the eastern flank at the southern end of the eastern branch of the EAR, in northern Tanzania (Figure 1). Mt Kilimanjaro forms a shield shape, approximately 95km in length by 65km width, trending WNW, with the summit, Uhuru peak, in the Kibo region reaching an elevation of 5895m. The Shira volcanic centre and rocks are situated between 8km and 40km west and south west of Kibo (Figure 2), and are geographically made up of several distinct landmarks, including the Shira Plateau, Shira Ridge, Shira Cathedral, East Shira Hill and Platzkegel (cone place) lying dominantly between 3000m and 4200m elevation.
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EAST AFRICAN RIFT
ASMARA Major City
Volcanic Provinces
Lake ADEN GULF Fault DJIBOUTI TOWN
Mt Kilimanjaro
N Tanzanian craton ADDIS ABABA
300km
LEGEND H
MOGADISHU Mobutu Lake
EASTERN BRANC NAIROBI Kivu Lake See Figures 2 and 6
KIGALI Mt Kilimanjaro - 5895m Victoria Lake BUJUMBURA INDIAN OCEAN
Tanganyika Lake WESTERN BRANCH DODOMA Unguja/Zanzibar Island
Rukwa Lake
Malawi Lake
LILONGWE
LUSAKA
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).
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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
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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;).
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Figure 3. Active vs. Passive rifting models for continental rifting (Wilson, 1989 after Keen, 1985). A) Active rifting, whereby mantle upwelling has caused lithospheric extension and regional uplift, compared withB) 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)
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34° 35°36° 37° 38°
Lake 4° Turkana
Yelele 3°
Moroto Napak 2° Kadam Emuruangogolak Elgon Silali 1° Paka
0° Lake Menengai Mt Kenya Victoria
Longonot Suswa -1°
Olesakut Olorgesailie -2° Chyulu Hills
Oldoinyo Lengai Shira Kibo -3° Ngorognoro Mawenzi Mt Meru Mt Kilimanjaro
Mixed association central volcanoes
Nephelinite-phonolite central volcanoes
Quaternary basalts
Basalt-trachyte, trachyte-rhyolite and trachyphonolite shields
Ngorongoro Crater
Volcanic province
Lake
0km 100km
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.
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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 3°05’S, 37°20’E (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 Kilimanjaro’s 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).
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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.
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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
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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 µm 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 71µm was used, with repetition rate of 5Hz.
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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 0.25
Al2O3 19.46 19.54 28.64 28.53 0.32 0.09 0.27 0.08 12.61 13.79 FeO 0.01 0.3 0.37 0.13 0.05 1.25 1.21 7.34 6.51 18.4 17.5 MnO 0.03 0.05 0.03 0.12 0.34 MgO 0.01 0.13 19.17 18.62 17.7 17.46 51.64 51.57 12.23 13.6 CaO 0.06 0.13 12.37 11.8 26.93 25.73 25.48 25.55 0.01
Na2O 11.02 11.59 4.43 4.35
K2O 0.21 0.22 0.4 0.41 0.1 0.07 0.06
TiO2 0.05 0.03 0.07 0.08 0.06 0.05 0.31 0.15
P2O5 0.14 0.19
Cr2O3 0.05 0.07 0.11 0.58 0.58 54.28 54.4 NiO 0.09 0.03 0.11 0.35 0.14 CoO 0.03 0.19 0 0.03 0.05 0.2 0.02 Total 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 950ºC over a period of 5 hours, then maintaining this temperature for a period of 15 minutes.
15
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 63.63
Al2O3 12.98 13.64 16.78 17.34
Fe2O3 (total) 9.93 9.96 1.46 1.40
Fe2O3 8.74 1.07 FeO 1.13 0.30 MnO 0.76 0.77 0.01 0.01 MgO 0.25 0.28 0.45 0.46 CaO 2.57 3.22 0.72 0.68
Na2O 7.97 8.37 0.43 0.43
K2O 5.38 5.51 15.47 15.35
TiO2 0.49 0.48 0.04 0.04
P2O5 0.04 0.06 0.12 0.12 LOI 3.72 2.48 0.31 0.31 Total 96.26 97.17 100.56 99.77
Sr(ppm) 4396.00 4600.00 65.74 62.00 Ba(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 1190˚C. 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 100µm 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
16
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).
3000 2000
2000 gradient = 1 gradient = 1 1000
Sr (ppm) -1000 LA-ICP-MS
Ba (ppm) - LA-ICP-MS
0 0 0 1000 2000 3000 4000 0 1000 2000 Sr (ppm) - ICP-AES Ba (ppm) - ICP-AES
Figure 8. Sr and Ba comparisons for ICP-AES and LA-ICP-MS analyses showing a close correlation.
17
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 ulvöspinel 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
18
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 4 KSH03 KSH01 K686 K361 KSH05 KSH02 K689 K832a KSH07 KSH04 K811 K832b KSH08 KSH06 K813 K894 KSH09 KSH10 K820 K897 K693 KSH11 K825 K1043 K2225 K679 K829 K695 K1039 K696 K802 K803 K804 K821 K822 K895 K891 K1038
Table 4. Samples analysed by EDS microprobe and LA-ICP-MS
Samples for EDS Analysis Samples for LA-ICP-MS Analysis Sample Group Sample Group KSH03 1 KSH05 1 KSH05 1 K2225 1 KSH08 1 KSH03 1 KSH09 1 KSH01 2 K2225 1 K811 3 KSH01 2 KSH04 2 KSH06 2 K802 2 K811 3 K820 3 K361 4 K894 4
19
i
40.73
11.14
22.92
99.15
0.24
30.31 16.13 53.57
71.71
wollastonite
K894GMCpx2
i Al-sub Si-T
p1
5.51
41.00
10.92
22.81
97.33
25.31
52.56
88.58
Rim Groundmass
4.00 4.00
0.01 0.00
0.00 0.00 0.91
Group 4 29.69 16.66 53.66
72.14
K894 Cpx1c
Al-sub Si-T
wollastonite
Groundmass
Group 4
2.80 3.79 4.62
0.15 0.00 0.24
5.71 6.24 6.55 0.19 0.22 0.11
0.00 0.09 0.07
0.21 0.12 0.00
p1a K894 GM S
44.46
11.00 9.07 9.32 23.14
98.44
20.87
43.93
94.71
0.21 0.22
Core
34.37 13.67 51.96
Phenocryst K894 Cpx1a
wollastonite
6.21
46.19
20.54
99.31
1 77.46
Al Al-sub Si
4.00 4.00
20.80 29.76 49.44
45.1
hedenbergite
K820 GM Cpx1
p2 K894 S
8.39 6.36 8.60
9.42 17.38
0.96 0.02 0.83
0.00 0.01 0.08
45.27
23.09
99.66
20.74
52.56
93.48
Rim Groundmass
1 Cpx2c
0.19 0.16
3.03 3.00 3.00 1.40 10.50
0.85 0.66
Group 3
33.21 15.66 51.13
75.02
Group 3
K81
wollastonite
Groundmass
3.16 2.83 2.07 2.11 2.90 2.42 6.01 6.40 13.47 0.24 0.11 0.50
p2 K820 GM S
48.03
23.08
15.25
39.04
95.94
side
101.06
Al Al-sub Si
1 Cpx2a
0.75 0.79 0.58 0.78 0.78 0.76
0.08 0.02 0.25 0.01
Core
35.47 15.18 49.36
77.96
diop
Phenocryst K81
adjusted stoichiometrically
46.30
11.51 11.92 10.78 22.34
99.72
side
and Fe
Al-Fe
2+ 3+
34.06 18.43 47.51
73.31
diop
=Fe
KSH04 GM Cpx3
**
p1 K820 S
5.10 28.49
44.00
10.44 22.16
99.30
24.70
54.28
92.91
side
Rim Groundmass
1.01 15.90 0.87 0.34 1.65 0.72 0.91 0.45
Group 2
32.75 17.28 49.96
75.67
Group 2
diop
Groundmass
KSH04 Cpx3c
4.86 8.02 5.24 6.40
0.15 0.49 0.14 0.34 0.12 1.06 0.07 0.04 0.00
p9 KSH04 GM S
47.86
22.71
99.68
20.89
21.74 44.01
97.38
side
Al Al-Fe-sub Si
Core
14.40 48.37
78.28
diop
Phenocryst
KSH04 Cpx3a
44.70
11.63 12.56 22.72
101.44
1 37.23
side
35.1
49.30
80.56
KSH08 GM Cpx2
p2 KSH04 S
2.48 5.38 3.84 7.97 2.80 7.43 1.85 0.12 0.24 0.05 0.04 0.07 0.08 0.00
0.02 0.00 0.25 0.01 0.03 0.00 0.09
S
47.33
13.39 23.26
49.06
96.53
100.31
side diop
Al Al-Fe-sub Si
Rim Groundmass
0.83 0.78 0.75 0.70 0.74
1.92 14.39 3.14 2.91
39.38
49.17
85.88
diop
Group 1
Group 1
Groundmass
KSH05 cpx 4g
0.28 1.77 2.99 1.84 3.47 2.48 1.72 2.39 1.74 2.28 6.86 8.86 0.93 0.25 0.17 0.01 0.00 0.02 0.05 0.00 0.05 1.63 3.14 4.46 2.58 4.19 3.80 1.84 3.92 5.00 6.21 5.98 7.47 0.10 0.19 0.18 0.13 0.07 0.21
0.06 0.10 0.09 0.08 0.08 0.04 0.11 0.07 0.17 0.00 0.00 0.16 0.00 0.00 0.37 0.57 0.23 0.17 0.00 0.00 0.95 0.00 0.12 0.74 0.10 0.10 0.10 0.12 0.17 0.00 0.00 0.21 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.29
0.28 0.98 0.32 2.80 0.45 4.95 0.18 1.97 1.67 20.87
8.79 21.25
0.45 0.73 0.51 0.58 0.51 0.83 1.91 0.38
0.00 0.36 0.06 0.56 0.00 0.00 0.00 0.01 0.00 0.09 0.00 0.67 0.00 0.00 0.00 0.04 0.00 0.22 0.01 0.04 0.01 0.00 0.15 0.07 0.02 0.13 0.02 0.00 0.00 0.06 0.00 0.00 0.00 0.00 0.00
52.37
17.53 22.95
13.07 44.49
16.91
11.50 0.54 4.15 0.31 4.14 0.42 2.89 0.00
97.31
adjusted stoichiometrically through the methods of Droop (1987)
100.07
p 1 KSH03 GM
side
Cr
4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00
5.32 11.45 15.59
0.92 0.08 0.17 0.19 0.21 0.22 0.22 0.17 0.00 0.00 0.03 0.04 0.08 0.03
Core
3.00 3.00 3.00 3.00 3.00
1.84 0.66 0.61
48.78
45.90
94.44
54.80 69.54
and Fe
diop
Inclusion
2+ 3+
KSH05 S
=Fe
KSH05 cpx 4a (core)
*
Representative core, rim and groundmass clinopyroxene and spinel phenocryst / inclusion and groundmass microprobe
)
)
2+
2+
*
pinel Component
**
Cations (6O)
3
Cations (4O) 3
3
3
3
3
atite
5
2
2
5
O
2
2 O
O
O
O
O
O
2
O
(m2)
2
2
2
(iv)
O
2
O
2
2
O
O
2
2
2
Table 5. 2 analyses for each group (number of cations calculated on basis of 6 oxygens for clinopyroxene and 4 oxygens for spinels).
2
Clinopyroxene
Sample SiO TiO Al Cr Fe FeO* MnO MgO CaO K Na P NiO CoO Total
Sum adjective pyroxene enst ferrosilite wollastonite Al Ti Na Mg/(Mg+Fe
Spinel
SiO TiO Al Cr Fe FeO** MnO MgO CaO K Na P NiO CoO Total
Sum Mg/(Mg+Fe Cr/(Cr+Al) UlvoS
20
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.
90
70
50 rim-core Fo %
30
10
30 40 50 60 70 80 Mg number of rock
Group 1 Phenocryst Group 2 Phenocryst Group 3 Phenocryst Group 4 Phenocryst
Group 1 Groundmass Group 2 Groundmass Group 3 Groundmass Group 4 Groundmass
Figure 9. Comparison of forsterite values of phenocryst and groundmass olivines with Mg number calculated from bulk rock analysis. The dashed line represents the olivine Fe/Mg composition in equilibrium with the bulk rock using KD(Ol/Liq) = 0.3. (arrows show typical Fo change from core to rim where applicable).
21
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.
22
A) wollastonite
groundmass diopside samples hedenbergiteHedenbergite
core - rim trend
Group 1 Phenocryst Group 2 Phenocryst Group 3 Phenocryst Group 4 Phenocryst
Group 1 Groundmass Group 2 Groundmass Group 3 Groundmass Group 4 Groundmass enstatite ferrosilite B) Ti
NaTi NaTiAl TiAl
JD CaTS NaM2 AlT
C)
High Pressure 0.4
0.3
Intermediate Pressure M1
Al 0.2
0.1
Low Pressure 0.0 0.0 0.1 0.2 0.3 0.4 0.5 AlT Figure 10. A) Microprobe analyses of phenocryst and groundmass clinopyroxenes from Shira samples presented in the Mg-Ca-Fe (enstatite-wollastonite-ferrosilite) triangle (arrows indicate general trend from core to rim, circles indicate groundmass composition regions) and B) “others” quadrilateral (JD = jadeite, CaTS = Ca-Tschermaks, TiAl = Ti-Al
augite, NaTiAl = Na-Ti-Al augite, NaTi = Na-Ti augite (Ti end member is “fictive” CaTiAl26 O ). Samples have been split into the four petrographic groups based on phenocryst assemblages as discussed in the text. C) Plot of octahedral aluminium (AlM1 ) versus tetrahedral aluminium T (Al ) in clinopyroxenes, and the pressure fields of Aoki and Kushiro (1968).
23
Or
Sanidine
Anorthoclase
core
Albite Oligoclase Andesine Labradorite Bytownite Anorthite Ab An Group 1 Phenocryst Group 2 Phenocryst Group 3 Phenocryst Group 4 Phenocryst
Group 1 Groundmass Group 2 Groundmass Group 3 Groundmass Group 4 Groundmass
Figure 11. Microprobe analyses of phenocryst and groundmass feldspars from Shira samples plotted as proportion anorthite-albite-orthoclase (An-Ab-Or), with arrow showing the general trend from core to rim, and circles showing the groundmass composition regions.
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.
24
80 80
60 60 inclusions
40 40
Cr/Cr+Al Cr/Cr+Al
20 20
0 0 0 10203040506070 0 10203040506070 Mg/Mg+Fe2+ Mg/Mg+Fe2+ 80 80
60 60
40 40 inclusion
Cr/Cr+Al Cr/Cr+Al
20 20
0 0 0 10 20 30 40 50 60 70 0 10 20 30 40 50 60 70 Mg/Mg+Fe2+ Mg/Mg+Fe2+ Group 1 Inclusion Group 2 Phenocryst Group 3 Phenocryst Group 4 Phenocryst
Group 1 Groundmass Group 2 Groundmass Group 3 Groundmass Group 4 Groundmass
Figure 12. Microprobe analyses of phenocrysts (including inclusions) and groundmass spinels of Shira samples. Samples have been plotted using Mg numbers (Mg/(Mg + Fe2+ )) and Cr numbers (Cr/(Cr+Al)). Groundmass samples plot very close to the lower left corner of each diagram or not at all due to 0% Mg or Cr (magnetite / titaniferous magnetite).
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.
25
20
Ks 10
0 50 60 70 80 Ne Group 2 Groundmass
Group 3 Phenocryst Group 4 Phenocryst
Group 3 Groundmass Group 4 Groundmass 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
26
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).
27
KSH05 Olivine 1 KSH05 Olivine 2 KSH05 Olivine 3 10000.00 100.00
1000.00 80.00
100.00 60.00
Forsterite % (EDS) 10.00 40.00
Abundance of element (ppm)
1.00 20.00 Ol1a Ol1b Ol1c Ol1d Ol1e Ol2a Ol2b Ol2c Ol2d Ol2e Ol2f Ol3a Ol3b Ol3c Ol3d Ol3e KSH03 Olivine 1 KSH03 Olivine 2 KSH03 Olivine 4 10000.00 100.00
1000.00 80.00
100.00 60.00
Forsterite % (EDS) 10.00 40.00
Abundance of element (ppm)
1.00 20.00 Ol1a Ol1b Ol1c Ol2a Ol2b Ol2c Ol4a Ol4b Ol4c K2225 Olivine 1 K2225 Olivine 2 K2225 Olivine 4 10000.00 100.00
1000.00 80.00
100.00 60.00
Forsterite % (EDS) 10.00 40.00
Abundance of element (ppm)
1.00 20.00 Ol1a Ol1b Ol1c Ol2a Ol2b Ol2c Ol4a Ol4b Ol4c KSH01 Olivine 1 KSH01 Olivine 4 KSH01 Olivine 5 10000.00 100.00
1000.00 80.00
100.00 60.00
Forsterite % (EDS) 10.00 40.00
Abundance of element (ppm)
1.00 20.00 Ol1a Ol1b Ol1c Ol4a Ol4b Ol4c Ol5a Ol5b
Sc V Cr Mn Ni Fo%
Figure 14. Results of LA-ICP-MS core to rim analyses of 3 olivine phenocryst s 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.
28
28
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).
29
A) KSH01 Plagioclase LA-ICP-MS Results
80.00 10000.00
1000.00
60.00 100.00 Log (ppm) Anorthite % (EDS) 10.00
40.00 1.00 Pl1a Pl7b Pl7a Analyses
Ti Mn Ga Sr Ce An%
B) KSH01 Plagioclase LA-ICP-MS Results 1000.00
100.00
10.00
(Sun & McDonough, 1989) 1.00 Chondrite Normalised REE La Ce Nd Sm Eu Gd Dy Er Yb Lu
0.10 REE Plag 1a Plag 7b Plag 7a
Figure 16. A) LA-ICP-MS analyses of Ti, Mn, Ga, Sr, Ce and An% in plagioclase phenocrysts from sample KSH01. Samples have been plotted in order of decreasing anorthite content as determined through EDS microprobe analysis. B) Chondrite normalised (Sun & McDonough, 1989) REE diagram of analysed plagioclase phenocrysts.
30
Spinel LA-ICP-MS Results
60.00 1000000.00
100000.00
40.00 10000.00
1000.00
20.00 100.00 Log (ppm) Mg number (EDS)
10.00
0.00 1.00 K811-Sp4a K811-Sp3a K811-Sp2a KSH01-Sp3 KSH05-Sp4a KSH05-Sp3a Analyses
Ti Mn V Cr Ni Zr Nb Mg#
Figure 17. LA-ICP-MS analyses of spinel inclusions (KSH05 samples) and phenocrysts (K811 and KSH01 samples). Samples have been plotted in order of decreasing Mg number as determined through EDS microprobe analysis.
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
31
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
32
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.
33
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 K2225 Rock Type BSN AOB PBAS AOB AOB BSN
SiO2 44.46 46.37 44.37 45.18 45.15 43.33
Al2O3 13.28 7.96 9.33 7.52 8.35 12.91
Fe2O3 (total) 12.93 10.62 11.49 10.23 11.10 13.33 MnO 0.21 0.16 0.18 0.16 0.17 0.21 MgO 10.13 16.17 13.06 16.51 14.53 8.46 CaO 11.55 14.30 15.68 15.34 15.76 13.13
Na2O 2.02 1.16 1.43 1.04 1.14 3.33
K2O 0.96 0.45 0.36 0.37 0.34 0.56
TiO2 1.97 1.50 1.75 1.49 1.65 2.21
P2O5 0.44 0.23 0.29 0.24 0.24 0.49 LOI 1.65 1.09 1.06 0.98 1.14 2.47 Total 99.59 100.01 99.00 99.07 99.58 100.42 Sr (ppm) 622.10 388.10 355.50 291.90 338.10 590.60 Ba (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 35.03 V 276.81 237.98 305.80 254.00 289.19 325.96 Cr 316.44 1417.44 764.80 1571.65 886.98 193.40 Ga 18.48 11.07 13.59 11.83 12.44 18.79 Rb 22.35 85.98 44.29 11.09 46.04 9.57 Sr 671.84 395.26 378.01 329.97 369.07 595.74 Y 22.64 14.04 17.55 14.82 16.38 24.42 Zr 194.15 100.10 120.73 105.55 107.93 193.67 Nb 63.50 32.25 37.88 33.64 33.30 71.26 Cs 0.24 0.98 0.55 0.11 0.44 0.28 Ba 455.77 197.76 208.96 188.02 207.47 366.92 La 46.78 24.50 26.09 24.83 24.71 48.82 Ce 85.65 43.67 49.92 45.89 45.82 88.91 Nd 39.39 21.35 25.09 22.11 22.76 39.90 Sm 7.15 4.30 5.03 4.32 4.62 7.31 Eu 2.13 1.23 1.53 1.29 1.38 2.16 Gd 5.99 3.61 4.46 3.85 4.29 6.53 Dy 4.47 2.70 3.28 2.97 3.29 4.79 Er 2.10 1.28 1.69 1.40 1.57 2.39 Yb 1.68 1.05 1.38 1.14 1.27 1.91 Lu 0.26 0.16 0.21 0.16 0.19 0.28 Hf 4.48 2.57 3.36 2.58 2.99 4.61 Ta 3.69 1.89 2.28 2.03 2.13 4.25 Pb 6.50 4.34 4.68 4.67 4.61 6.00 Th 4.89 2.68 2.88 2.80 2.57 5.96 U 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 2.22 FeO 10.99 9.03 9.77 8.70 9.44 11.33
2+ Mg/(Mg+Fe ) 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 57.1
CaO/Al2O3 1.6 3.3 3.1 3.7 3.4 1.8 Fo% (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 75.5 CIPW 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 3.37 ab 12.28 7.19 1.81 1.95 2.52 6.16 an 24.79 15.32 18.32 15.02 16.90 18.97 ne 2.78 1.47 5.70 3.79 3.93 12.21 di 24.97 43.84 47.57 48.64 48.84 36.15 ol 21.37 23.49 17.52 22.40 19.31 14.43 mt 3.19 2.59 2.84 2.52 2.72 3.28 il 3.81 2.87 3.39 2.88 3.18 4.28 ap 1.04 0.54 0.68 0.57 0.56 1.16 Total 100.00 100.00 100.00 100.00 100.00 100.00
34
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 K895 Rock Type TBAS TBAS BSN TBAS BSN TBAS TBAS TBAS BSN TBAS TBAS
SiO2 45.73 49.31 42.79 49.01 43.38 46.61 46.62 47.93 44.52 48.57 47.25
Al2O3 16.08 17.03 16.27 17.72 15.74 16.77 15.71 17.47 16.44 16.91 15.81
Fe2O3 (total) 13.66 12.05 14.20 11.62 15.17 12.42 12.42 12.57 13.02 11.56 12.57 MnO 0.22 0.20 0.21 0.19 0.22 0.19 0.21 0.23 0.24 0.18 0.20 MgO 5.57 4.41 4.49 3.94 4.74 4.85 6.31 3.70 5.66 4.26 6.20 CaO 9.15 7.36 9.74 7.09 11.95 8.47 9.50 7.93 10.22 7.88 9.06
Na2O 4.12 4.42 3.37 3.74 2.12 3.30 3.62 4.64 4.34 3.37 3.86
K2O 1.27 2.25 1.64 2.48 1.13 1.95 2.05 1.89 1.68 2.40 2.15
TiO2 2.40 2.06 2.60 2.08 2.74 2.36 2.26 2.56 2.49 2.17 2.24
P2O5 0.58 0.73 0.61 0.68 0.58 0.60 0.73 0.70 0.75 0.59 0.71 LOI 0.51 0.65 4.82 1.87 2.88 2.20 1.26 0.83 0.79 1.17 0.69 Total 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 1289.00 875.00 Ba (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 18.49 V 263.11 180.06 241.78 170.59 338.87 246.89 235.06 145.12 235.50 210.71 224.18 Cr 36.65 35.39 29.08 30.48 18.63 - 99.07 - - 69.24 151.10 Ga 23.62 23.20 23.52 23.10 23.72 24.99 22.32 22.95 22.91 27.93 22.40 Rb 66.38 62.75 53.66 92.84 125.47 89.15 54.02 53.94 59.30 116.09 58.71 Sr 822.05 945.01 771.66 872.76 778.23 881.56 877.47 1000.05 954.54 1316.21 892.83 Y 27.84 29.74 27.85 29.67 28.69 30.29 28.34 29.68 29.17 39.01 27.47 Zr 258.20 279.91 226.23 268.65 221.08 270.48 296.65 262.01 328.48 325.54 287.77 Nb 90.25 97.42 82.56 99.83 75.50 92.91 107.72 88.29 107.97 94.29 107.37 Cs 0.36 0.56 0.37 0.57 1.31 0.71 0.54 0.76 0.49 0.38 0.85 Ba 634.96 781.96 487.50 769.35 587.53 695.26 690.43 710.36 703.35 704.94 672.67 La 67.88 75.85 53.14 75.95 51.50 72.09 71.85 69.41 74.34 78.98 72.38 Ce 121.85 137.00 97.43 137.06 95.52 129.50 131.61 124.89 136.24 143.20 129.80 Nd 52.98 58.10 43.03 57.58 44.38 53.39 55.55 53.15 58.48 60.55 53.98 Sm 9.44 9.68 7.88 9.85 8.31 9.75 9.85 9.46 10.11 10.80 9.00 Eu 2.84 2.64 2.46 2.57 2.51 2.71 2.72 2.63 2.98 2.91 2.68 Gd 7.53 7.60 6.72 7.56 7.24 7.66 7.53 7.41 8.02 8.78 7.32 Dy 5.37 5.65 5.35 5.72 5.68 5.88 5.52 5.64 5.80 7.24 5.37 Er 2.53 2.79 2.63 2.85 2.73 2.89 2.67 2.80 2.71 3.83 2.60 Yb 2.16 2.51 2.18 2.55 2.23 2.48 2.23 2.48 2.37 3.40 2.28 Lu 0.31 0.37 0.34 0.41 0.32 0.37 0.34 0.38 0.32 0.51 0.34 Hf 5.61 5.97 4.84 6.21 4.93 6.04 6.42 5.66 6.82 7.24 6.16 Ta 5.14 6.14 4.92 6.09 4.52 5.08 6.41 4.99 6.48 5.18 6.49 Pb 5.45 9.57 7.24 10.04 6.08 9.56 8.66 8.48 7.49 8.78 8.49 Th 7.50 8.83 6.84 9.59 5.90 9.13 9.38 8.99 7.68 10.52 9.42 U 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 2.10 FeO 11.61 10.24 12.07 9.88 12.89 10.56 10.56 10.68 11.07 9.83 10.68
2+ Mg/(Mg+Fe ) 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 50.9
CaO/Al2O3 1.0 0.8 1.1 0.7 1.4 0.9 1.1 0.8 1.1 0.8 1.0 Fo% (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 51.4 CIPW 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 12.67 ab 20.49 28.77 13.43 30.25 13.82 24.23 17.18 26.62 11.02 28.18 18.37 an 21.85 19.98 25.40 24.55 30.70 25.77 20.64 21.30 20.50 24.39 19.41 ne 7.97 4.67 8.79 0.97 2.43 2.35 7.35 6.89 14.01 0.48 7.69 di 16.77 9.81 17.35 5.50 21.73 10.99 18.05 11.36 21.13 9.61 17.17 ol 16.04 14.96 14.77 15.45 14.09 15.79 15.62 13.11 13.72 14.43 15.76 mt 3.34 2.91 3.57 2.85 3.74 3.07 3.01 3.05 3.16 2.85 3.04 il 4.60 3.91 5.14 4.00 5.31 4.59 4.31 4.87 4.75 4.20 4.24 ap 1.36 1.69 1.47 1.60 1.37 1.42 1.70 1.62 1.74 1.39 1.64 Total 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00
35
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 K825 Rock Type BSN BSN NEPH NEPH NEPH
SiO2 41.32 42.72 40.67 40.46 41.48
Al2O3 14.49 14.89 14.12 15.29 16.89
Fe2O3 (total) 16.07 14.27 14.64 15.86 14.71 MnO 0.27 0.24 0.29 0.31 0.28 MgO 5.50 5.52 6.22 5.20 4.22 CaO 11.94 11.67 13.99 11.37 10.03
Na2O 3.71 3.89 3.03 4.46 5.76
K2O 0.78 1.83 0.76 1.10 1.47
TiO2 2.68 2.41 2.63 2.39 2.27
P2O5 0.85 0.75 0.75 0.88 0.92 LOI 3.38 2.00 3.14 1.86 1.82 Total 100.99 100.19 100.25 99.17 99.84 Sr (ppm) 935.10 977.20 807.50 1138.00 1156.00 Ba (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 6.67 V 300.48 270.26 335.77 278.34 253.13 Cr - 41.65 19.27 13.06 4.12 Ga 23.91 21.80 24.44 23.56 23.38 Rb 74.02 59.61 126.66 63.60 55.26 Sr 980.81 916.55 912.24 1182.32 1151.69 Y 34.62 30.99 36.28 39.22 37.90 Zr 351.34 297.07 385.83 360.25 353.74 Nb 134.44 108.10 121.95 138.41 148.53 Cs 0.58 0.44 0.66 0.57 0.61 Ba 520.52 574.16 532.86 616.73 762.33 La 85.81 70.69 81.92 90.80 92.75 Ce 154.99 128.87 150.89 162.13 165.72 Nd 65.64 56.95 67.70 68.37 67.35 Sm 11.71 10.16 12.09 11.87 11.46 Eu 3.36 2.98 3.62 3.32 3.32 Gd 9.31 8.17 9.98 9.61 9.38 Dy 6.76 6.02 7.26 7.42 7.17 Er 3.30 2.95 3.43 3.68 3.63 Yb 2.73 2.40 2.90 3.15 3.16 Lu 0.40 0.34 0.39 0.48 0.43 Hf 6.48 5.75 7.57 6.52 6.02 Ta 9.30 7.22 8.51 10.03 10.62 Pb 5.92 5.24 5.75 5.65 5.44 Th 9.18 7.38 8.50 9.98 10.62 U 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 2.45 FeO 13.66 12.13 12.44 13.48 12.50
2+ Mg/(Mg+Fe ) and CaO/Al 2 O 3 calculated using molecular proportions (analysed weights) Mg/(Mg+Fe2+) 41.8 44.8 47.1 40.7 37.6
CaO/Al2O3 1.5 1.4 1.8 1.4 1.1 Fo% (0.3) 70.5 73.0 74.8 69.6 66.7 %AN 75.2 87.6 99.7 89.9 81.8 CIPW 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 8.84 ab 6.94 2.55 0.06 2.12 3.60 an 21.03 18.05 23.30 18.91 16.17 ne 13.61 16.73 14.24 19.80 24.92 di 28.29 29.82 35.66 27.67 23.87 ol 14.24 11.94 11.57 14.17 12.43 mt 3.97 3.51 3.63 3.92 3.61 il 5.20 4.65 5.13 4.65 4.39 ap 2.01 1.77 1.79 2.09 2.17 Total 100.00 100.00 100.00 100.00 100.00
36
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 K894 Rock Type BSN BSN BSN TBAS BSN
SiO2 44.27 45.07 43.92 45.58 43.09
Al2O3 16.22 14.20 15.78 15.37 17.51
Fe2O3 (total) 14.00 14.47 15.18 14.50 11.83 MnO 0.20 0.22 0.23 0.21 0.23 MgO 6.10 6.37 5.63 4.31 3.11 CaO 11.41 12.51 10.29 9.12 8.07
Na2O 3.55 3.31 4.17 2.96 4.83
K2O 0.64 0.41 1.44 2.14 2.42
TiO2 2.19 2.28 2.46 2.63 2.00
P2O5 0.39 0.54 0.63 0.46 0.91 LOI 1.36 0.92 0.91 2.13 6.05 Total 100.32 100.30 100.63 99.39 100.04 Sr (ppm) 769.80 558.60 702.00 617.10 1198.00 Ba (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 4.25 V 317.62 290.64 268.64 301.86 133.06 Cr - 60.88 24.13 76.76 - Ga 21.58 20.32 24.12 22.97 23.52 Rb 13.81 20.97 27.59 48.70 78.61 Sr 813.04 574.22 734.79 625.53 1232.35 Y 20.56 26.56 27.23 28.47 29.89 Zr 165.97 192.93 228.59 260.35 274.90 Nb 52.07 62.27 83.86 83.27 152.97 Cs 0.29 0.44 0.44 0.18 0.75 Ba 439.70 443.48 546.68 687.21 904.48 La 38.08 46.88 60.24 52.96 95.73 Ce 70.29 86.61 108.49 94.56 161.86 Nd 32.13 41.26 46.76 42.85 61.69 Sm 6.17 7.61 8.36 7.96 9.95 Eu 1.99 2.29 2.49 2.47 2.82 Gd 5.18 6.72 6.98 6.86 7.77 Dy 4.07 5.17 5.24 5.49 5.68 Er 1.93 2.51 2.58 2.73 2.90 Yb 1.73 2.14 2.09 2.33 2.31 Lu 0.24 0.30 0.32 0.34 0.33 Hf 3.94 4.46 4.98 5.66 4.65 Ta 3.11 3.73 5.00 5.06 9.17 Pb 6.11 7.58 7.53 12.46 10.39 Th 4.49 5.12 7.53 5.95 12.71 U 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 1.97 FeO 11.90 12.30 12.90 12.33 10.06
2+ Mg/(Mg+Fe ) and CaO/Al 2 O 3 calculated using molecular proportions (analysed weights) Mg/(Mg+Fe2+) 47.7 48.0 43.8 38.4 35.6
CaO/Al2O3 1.3 1.6 1.2 1.1 0.8 Fo% (0.3) 75.3 75.5 72.2 67.5 64.8 %AN 66.1 59.8 66.4 54.5 59.7 CIPW 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 15.18 ab 13.69 15.33 10.18 19.11 13.60 an 26.65 22.77 20.09 22.90 20.12 ne 8.99 6.93 13.61 3.56 16.13 di 23.11 29.87 22.40 17.14 13.56 ol 15.24 13.57 15.41 14.51 12.10 mt 3.41 3.51 3.67 3.60 3.03 il 4.19 4.35 4.67 5.12 4.03 ap 0.91 1.26 1.46 1.09 2.24 Total 100.00 100.00 100.00 100.00 100.00
37
Mg/(Mg+Fe2+ ) Mg/(Mg+Fe2+ ) 40 50 60 70 80 40 50 60 70 80
48
15 46
SiO2 Al2O3
44 10
42
40 5 40 50 60 70 40 50 60 70 Mg/(Mg+Fe2+) Mg/(Mg+Fe2+)
15
Fe23 O TiO2 2 (total)
10
5 1 40 50 60 70 40 50 60 70 Mg/(Mg+Fe2+) Mg/(Mg+Fe2+) 5
0.3 4
MnO Na2O 3
0.2 2
1
0.1 0 40 50 60 70 40 50 60 70 Mg/(Mg+Fe2+) Mg/(Mg+Fe2+)
0.8
2 0.6
K2O P2O5
0.4 1
0.2
0 0 40 50 60 70 40 50 60 70 80 Mg/(Mg+Fe2+) Mg/(Mg+Fe2+ ) Shira Samples
15 Group 1 Group 2 Group 3 CaO Group 4
10 Figure 18. ICP-AES major element results for Shira samples in weight percent plotted against Mg numbers.
5 40 50 60 70 80 Mg/(Mg+Fe2+ )
38
Mg/(Mg+Fe2+ ) Mg/(Mg+Fe2+ ) 40 50 60 70 80 40 50 60 70 80 1400 1000
800
1000
600
Sr Ba 600 400
200 200
40 50 60 70 40 50 60 70 Mg/(Mg+Fe2+) Mg/(Mg+Fe2+)
1.5
100
Cs 1.0 Rb
50
0.5
0.0 0 40 50 60 70 40 50 60 70 Mg/(Mg+Fe2+) Mg/(Mg+Fe2+)
160
3 120
Ce Yb
80 2
40
0 1 40 50 60 70 40 50 60 70 Mg/(Mg+Fe2+) Mg/(Mg+Fe2+)
400 8
300 6
Zr Hf
200 4
100 2
0 0 40 50 60 70 40 50 60 70 Mg/(Mg+Fe2+) Mg/(Mg+Fe2+) Shira Samples
150 Group 1 10 Group 2 Group 3 Group 4 Nb 100 Ta 6
50
2
0 40 50 60 70 80 40 50 60 70 80 Mg/(Mg+Fe2+ ) Mg/(Mg+Fe2+ ) Figure 19. LA-ICP-MS trace element results (Sr, Ba, Cs, Rb, Ce, Yb, Zr, Hf, Nb & Ta) for Shira samples in parts per million (ppm) plotted against Mg numbers.
39
Rock/Chondrites Rock/Primitive Mantle 1000
100 100
10 10
Group 1 1 1 La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu CsRbBaTh U Nb K LaCePb Pr Sr P Nd ZrSmEu Ti Dy Y Yb Lu Rock/Chondrites Rock/Primitive Mantle 1000
100 100
10 10
Group 2 1 1 La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu CsRbBaTh U Nb K LaCePb Pr Sr P Nd ZrSmEu Ti Dy Y Yb Lu Rock/Chondrites Rock/Primitive Mantle 1000
100 100
10 10
Group 3 1 1 La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu CsRbBaTh U Nb K LaCePb Pr Sr P Nd ZrSmEu Ti Dy Y Yb Lu Rock/Chondrites Rock/Primitive Mantle 1000
100 100
10 10
Group 4 1 1 La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu CsRbBaTh U Nb K LaCePb Pr Sr P Nd ZrSmEu Ti Dy Y Yb Lu Figure 20. REE and multi element spider diagrams for all Shira samples (separated into groups and normalised to Sun & McDonough, 1989 chondrite and primitive mantle respectively)
40
A) 16 Shira Samples Phonolite K825 K894 Tephrite 14 Group 1 Basanite KSH02 Group 2 K802 KSH06 Group 3 6 K803 K895 K804 Group 4 K811 K679 Tephri- K820 K897 12 KSH01 Trachy-basalt Other Localities phonolite KSH11 TrachyteKSH04 K832B Kibo samples Parasitic vent samples K686 10 4 K361 Phono- K2225 K813 K832A
2 Tephrite Trachy- Foidite andesite TrachydaciteKSH10 KSH03
8 RhyoliteBasalt 2 Picro- KSH07
Basaltic 2 KSH05 Foidite Na O+ K O % Tephrite trachy- basalt andesite KSH09 KSH08 6 Basanite Trachy- basalt 40 45 50 4 Basalt Basaltic andesite Andesite Dacite 2 Picro- basalt
0 35 40 45 50 55 60 65 70 75
SiO2 % B) 30
ne
20
10 0
50 70 90 %AN
10 Shira Samples
Group 1 approximate plane of silic a saturation 20 Group 2 hy Group 3 Group 4 30
Figure 21. A)Total alkalis silica (TAS) diagram of all Kilimanjaro samples showing the classification scheme of Le Baset al. , (1986) with inset showing sample numbers. B) Plot of normative plagioclase composition (AN=(anx100)/(ab+an)) versus normative nepheline and hypersthene for all Shira samples. The centre line represents the plane of silica undersaturation, whereas the shaded band represents the approximate trace of the plane of silica saturation (e.g. Best & Brimhall, 1974), showing that all Shia samples are alkalic.
41
4 1.0
A) B)
3
23
25
PO
CaO/Al O 2 0.5 Shira SamplesSample
Group 1 1 Group 2 Group 3 Group 4
0 0.0 40 50 60 70 80 0123
Mg/(Mg+Fe2+) KO2
Figure 22. A) Plot of Mg number versus CaO/Al23 O for Shira samples showing the broad
negative trend (decrasingCaO/Al23 O with decreasing Mg number) and differing CaO/Al23 O
values for groups 1 to 4. B) Plot of K225 O versus P O for Shira samples showing the regions in which groups 1 to 4 plot.
30 A) B) 90
20
70 ne %AN
10 50
0 30 40 50 60 70 40 50 60 70 Mg/(Mg+Fe2+) Mg/(Mg+Fe2+)
C) Shira Samples
Group 1 30 Group 2 Group 3 Group 4
ab 20
10
0 40 50 60 70 80 Mg/(Mg+Fe2+) Figure 23. Plots of normative nepheline (A), anorthite percent (B) (%AN = an/ (ab+an)), and albite (C) versus Mg number in order to show normative differences between groups.
42
Comment [ADG1]: You will find that I have not changed DISCUSSION anything in the discussion yet. This is not to say that it is perfect. It does need some Four distinct groups based upon petrographic characteristics are identified as changes some in presentation and some in science), but you composing the Shira Volcanics of Mt Kilimanjaro. In general, these groups also certainly have the bulk of it there. Why I haven’t written have distinctive petrographic, geochemical and geographic characteristics. anything is that after 2 readings, I am still mulling it all Group 1 (East Shira Hill and Shira Cathedral areas) are high-Mg strongly phyric over in my mind. I am not quite sure what the issues are, but I basanites, picrobasalts and alkali olivine basalts. Group 2 samples (from the have this slightly unsettled feeling that there is another same area) are trachybasalts and basanites, Group 3 samples (nephelinites) angle that needs to be thought about. are from the main Shira Ridge near Klute and Kente peaks, and Group 4 I will do this thinking over the weekend and get some samples (basanites and trachybasalts) are from Platzkegel. comment back to you on the discssion by Monday.
Understanding the relationship between these different groups may help to understand the petrogenesis of the Shira volcanic rocks and magma chamber dynamics beneath Mt Kilimanjaro. The petrogenetic relationship of each group is developed through modelling of magmatic processes such as fractional crystallisation, mixing and assimilation, and ultimately, speculating on their source composition and mineralogy, and melting processes.
Fractional Crystallisation Models
Major and trace element trends indicate that the diversity of the Shira suite can be explained by the process of fractional crystallisation (Figure 24) commencing from a suite of slightly different primary magmas.
Quantitative models of fractional crystallisation for the Shira lavas are based on major element mass balance equations using inputs of major element geochemistry and microprobe data. These models provide a basis for trace element calculations using the Rayleigh fractional crystallisation formula (D-1) CL/CO=F (Allegre et al., 1977; Allegre & Minister, 1978) and compilation of relevant partition coefficients (Table 10). Modelling has been performed through both IGPET petrologic software and the creation of Microsoft Excel spreadsheets for comparing calculated daughter trace element concentrations with observed parent and daughter concentrations.
Several criteria are used for selecting suitable solutions to major element mass balance calculations. The first criterion is that the sum of the residuals squared be less than 0.6 (using a weighting of 0.4 for Si, 0.5 for Al and Mn and 1 for Ti,
43
Mg, Ca, Fe, Na, K and P). The second criterion is the plausibility of the fractionating assemblage (whether the relative abundances of fractionated phases from the model is suitable for the observed mineralogy of the samples) whilst the third criterion is the match between calculated daughter trace element concentrations and observed daughter concentrations. When these criteria fail to discriminate between models, the model that uses the least number of fractionating phases is preferred. Comprehensive major and trace element results are presented in Appendix A, and simplified model results are shown in Table 12.
Potential fractionating phases are limited to observed phenocrysts (clinopyroxene, olivine, plagioclase, nepheline and spinel). In some cases, the compositions of the selected phase vary significantly between core and rim. The phase chemistries used in the fractional crystallisation models for each group are presented in Table 11. The vectors produced by fractionation of the wide range of phenocryst compositions are shown in Figure 25 on a CaO versus Mg # diagram. The major observations that can be made from this graphical presentation are that fractionation of olivine drives liquid compositions to lower Mg #’s and higher CaO, whereas fractionation of clinopyroxene drives liquid compositions to lower Mg #’s and lower CaO. In general it takes approximately twice the amount of clinopyroxene fractionation to decrease the Mg # the same amount that olivine fractionation would decrease it. Variation in olivine composition and clinopyroxene compositions create a range of liquid compositions. Plagioclase fractionation produces minimal change, whereas spinel fractionation drives liquids to higher Mg ‘s. The essential conclusion is that the Shira trends can be explained by dominantly clinopyroxene fractionation, or a combination of clinopyroxene and olivine fractionation.
44
16 KSH07KSH09 KSH08
KSH05 14 K813
K2225
K832A 12 KSH10K686 CaO K811 Path B KSH03 K820 K361
K897 K803 Path A 10 K825 KSH04 K679 K832B KSH01 K895 Group 1 and 2 fractionation paths ( , ) Path C KSH11 Group 3 fractionation path ( ) 8 K894 K802 K804 Group 4 fractionation path ( ) KSH02 KSH06 Samples with phase chemistry determined
40 50 60 70 80 Mg/(Mg+Fe2+ ) Figure 24. Mg number versus CaO plot showing the fractional crystallisation paths modelled and microprobed samples used in modeling.
16 Fractionation of olivine phenocrysts Fo89 Fo67 15% Fractionation direction of the range of spinels found in Shira rocks 14 10% 5% Fractionation of plagioclase (An59) (arrow tip = 30% fractionation) 5% Fractionation of plagioclase (An76) 10% 12 (arrow tip = 30% fractionation) 15% CaO 20% 25% 30% 10 Fractionation of the range of cpx phenocrysts found in Shira rocks
8
40 50 60 70 80 Mg/(Mg+Fe2+ ) Figure 25. Mg number versus CaO plot showing the bulk rock chemistry change (starting from sample K2225 black star) produced by the fractionation of a wide range of Shira olivine, spinel, plagioclase and clinopyroxene compositions as determined by microprobe analyses.
45
Table 10. Partition coefficients used for petrogenetic modelling (CPX = clinopyroxene, PLAG = plagioclase, SP = spinel, OL = olivine, NEPH = nepheline, OPX = orthopyroxene, GT = garnet, AMPH = amphibole and PHLOG = phlogopite)
Mineral CPX PLAG SP OL NEPH OPX GT AMPH PHLOG Mn 0.75 2 0 2 0 2 1.45 2 0.044 6 1.4 2 Sc 2.8 4 0.02 4 1.59 4 0.15 4 0.056 6 1.2 2 8.5 9 2.9 1 2 Ti 0.4 2 0.04 2 7.5 2 0.02 2 0.08 6 0.1 2 0.3 2 1.4 1 0.9 V 1.35 2 0 2 26 2 0.06 2 0.6 2 Cr 5.3 7 0.08 7 4.2 7 2.8 7 10 2 1.345 9 1 Rb 0.03 4 0.39 4 0.32 4 0.02 4 1.5 6 0.022 2 0.042 2 0.45 1 5.8 2 Sr 0.16 7 2.7 7 0.68 7 0.02 7 0.04 2 0.012 2 0.376 1 0.081 2 Y 0.9 2 0.03 2 0.2 2 0.01 2 0.18 2 9 2 0.333 1 0.03 2 Zr 0.3 4 0.09 4 3.6 4 0.07 4 0.18 2 0.65 2 0.8 1 0.6 1 Nb 0.005 2 0.01 2 0.4 2 0.01 2 0.15 2 0.02 2 0.2 1 0.14 Cs 0.04 7 0.13 7 0.08 7 0.05 7 0.8 6 1 Ba 0.04 7 0.56 7 0.4 7 0.03 7 0.42 6 0.013 2 0.023 9 0.5 1 2.9 La 0.13 4 0.12 4 0.19 4 0.01 4 0.041 6 0.003 3 0.001 9 0.039 1 2 Ce 0.092 5 0.111 5 2.15 8 0.006 5 0.047 6 0.02 10 0.007 9 0.067 1 0.034 2 Nd 0.23 5 0.09 5 2 8 0.0059 5 0.03 10 0.026 9 0.142 1 0.032 2 Sm 0.445 5 0.072 5 1.65 8 0.007 5 0.059 6 0.05 10 0.102 9 0.188 1 0.031 2 Eu 0.59 4 0.21 4 0.22 4 0.02 4 0.061 6 0.05 10 0.243 9 0.27 1 0.03 2 Gd 0.556 5 0.071 5 0 8 0.01 5 0.09 10 0.68 9 0.03 2 Dy 0.582 5 0.063 5 0 8 0.013 5 0.15 10 1.94 9 0.36 1 0.03 2 Er 0.583 5 0.057 5 0 8 0.0256 5 0.23 10 4.7 9 0.034 2 Yb 0.542 5 0.056 5 1.35 8 0.0491 5 0.086 6 0.34 10 6.167 9 0.61 1 0.042 2 Lu 0.506 5 0.053 5 0.0454 5 0.066 6 0.42 10 6.95 9 0.046 Hf 0.5 4 0.05 7 0.37 4 0.03 4 0.11 6 0.02 3 0.14 9 0.5 4 Ta 0.08 4 0.03 4 0.56 4 0.02 4 0.06 2 0.19 4 Th 0.06 4 0.05 4 0.19 4 0.02 4 0.04 6 0.06 3 0.001 3 0.05 1 U 0.07 4 0.05 4 0.26 4 0.03 4 0.043 6 0.08 4
1 Spath et al. , 2001 (comp of McKenzie & O'Nions, 1991; Frey et al. , 1978; 4 Lemarchand et al ., 1987 5 Dalpe' & Baker, 1994; Chazot et al., 1996; Adam et al., 1993; Fujimaku, 1984 Le Roex et al ., 1990 and references therein; Lemarchand et al ., 1987 6 Onuma et al ., 1981 2 Rollinson, 1993 (comp of Arth 1976; Pearce & Norry, 1979; Green et al., 1989; 7 Villemant et al ., 1981 8 Schock, 1979; Fujimaku, 1984; Dostal et al ., 1983; Henderson, 1982; Schock, 1979 9 Leeman & Lindstrom, 1978; Lindstrom & Weill, 1978; Green & Pearson, 1987) Irving & Frey, 1978 3 10 Kempton et al ., 1987 Arth, 1976
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i
t
1
t
a
a
L
L
3
3
3
3
e
C
C
A
A
5
5
2
O
2
2 O
2
l
O
O O O
T
f
O O
T f
O 2 O
O 2 O
O
O
2
O O
2
O O
2
O
n g
o
2
O
n g
o o
O
O
i
b l a
r a
2 2
i
l
a a
r n
2 2
i
e
O
o
i e
O
e
T S
A F M M
K P C C N
T # F
T A F M S
K M C N
P C T # E
W F
a
T
47
Groups 1 and 2
Although different with respect to phenocryst mineralogy, samples from Group 1 and 2 define a broadly coherent trend on variation diagrams that suggest they are probably genetically related. Three plausible fractionation paths that include samples from both groups are considered. Path A (samples KSH08- KSH03-K679-KSH02), path B (samples K2225-K803) and path C (samples KSH01-K802) (Figure 24) model variation from basanite (Mg # 78) to trachybasalt (Mg # 43). Several other samples from these two groups are difficult to reconcile with a fractional crystallisation origin and suggest magma mixing.
Path A (Samples KSH08-KSH03-K679-KSH02)
Examination of variation diagrams for a number of trace elements indicate that Sr, Ba, Rb, LREE, Y, and Ga are incompatible throughout the entire fractionation path (decreasing Mg #), and Cr and Sc behave compatibly. This pattern of trace element variation suggests fractionation of clinopyroxene ± olivine. Inflections in V, Ti and Fe at KSH03 may indicate the onset of spinel fractionation. Zr, Hf and Th behave incompatibly from KSH08 to K679, yet compatibly from K679 to KSH02, possibly due to minor accessory phase fractionation. Nb and Ta also show inflections at this point attributable to a change in spinel fractionation. Quantitative fractional crystallisation models that incorporate these suggestions are presented as Table 12.
The most primitive sample, a basanite (Mg# 78 (KSH08) probably contains cumulate olivine and clinopyroxene, as evidenced by the high modal abundance of these phases and high forsterite content in some of the olivines. A fractional crystallisation model that uses this sample as the parental magma requires significant fractionation of clinopyroxene and olivine. Fractional crystallisation is dominated by clinopyroxene and olivine with the addition of minor Cr spinel for the parent / daughter pairs KSH03 / K679 and for K679 / KSH02.
Trace element calculations for Rayleigh fractional crystallisation based on each model of Path A show a high degree of consistency between calculated and observed values (Figure 26). Slight discrepancies between observed versus calculated REE for the model that links the most evolved magmas
48
(trachybasalts; K679-KSH02) may reflect minor accessory phase fractionation that was not included in the calculation.
Path B (Samples K2225-K803) and Path C (Samples KSH01-K802)
The two samples that constitute Path B are distinct from those that constitute Path A in that they are richer in CaO than Path A samples with similar Mg #. For the parent / daughter pair (K2225/K803) the general increase in the abundance of a wide variety of trace elements (except for the transition elements) suggests fractionation controlled by clinopyroxene ± minor olivine and spinel. A quantitative fractional crystallisation model for this pair (Table 12) has a sum of squares of 0.55, and a good agreement between calculated and observed trace element abundances (Figure 27).
Path C is represented by samples KSH01 (Mg # 46) and K802 (Mg # 38). A fractional crystallisation model that relates these two significantly evolved magmas requires clinopyroxene, olivine and spinel fractionation (Table 12). This suggestion is supported by the overall incompatible behaviour of Sr, Ba, Al, Th, Pb, Na, Er, U and K, and the compatible behaviour of Sc and V (indicating clinopyroxene, spinel ± olivine as fractionating phases). Trace element calculations overlap between calculated and observed values for almost all elements except Rb and Cs (Figure 28).
49
Table 12. Fractional crystallisation models and trace element calculations for Group 1 and 2 (Paths A, B and C), Group 3 and Group 4.
Fractional Crystallisation Models Group 1 and 2 PATH A KSH08-KSH03 KSH03-K679 K679-KSH02 Proportion of Phases Fractionated Cpx 77 KSH05 Cpx8c avg 69.7 KSH03 Cpx4b avg 78.7 KSH03 Cpx4b avg Ol 23 KSH05 Ol5c rim Fo82.2 16.0 KSH03 Ol3b avg Fo78.3 1.2 KSH03 Ol3b avg Fo78.3 Sp 14.3 KSH03 Cr Sp 20.1 KSH03 Cr Sp
F 0.562 0.335 0.203 R2 0.26 0.419 0.200
Major Element Calculations Daughter Parent Calc. Parent Daughter Parent Calc. Parent Daughter Parent Calc. Parent
SiO2 45.39 46.06 46.77 46.89 45.39 45.14 49.4 46.89 47.26 MgO 10.34 16.83 16.8 6.35 10.34 10.35 4.42 6.35 6.25 CaO 11.79 15.64 15.47 9.55 11.79 11.88 7.37 9.55 9.5
PATH B PATH C K2225-K803 KSH01-K802 Proportion of Phases Fractionated Proportion of Phases Fractionated Cpx 66.8 K2225 Cpx3b avg Cpx 66.6 KSH01 Cpx1b avg Ol 24.8 K2225 Ol3a avg Fo78.4 Ol 23.7 KSH01 Ol5b avg Fo64.9 Sp 8.4 KSH03 MgTiSp Sp 9.7 KSH01 MgTiSp
F 0.307 F 0.153 R2 0.552 R2 0.428
Major Element Calculations Major Element Calculations Daughter Parent Calc. Parent Daughter Parent Calc. Parent
SiO2 44.81 44.23 44.52 SiO2 48.11 46.29 46.87 MgO 5.7 8.64 8.6 MgO 3.71 5.64 5.6 CaO 10.29 13.4 13.34 CaO 7.96 9.26 9.13
Group 3 K813-K820 K820-K825 Proportion of Phases Fractionated Cpx 96.8 K811 Cpx4c rim 78.6 K820 Cpx1c rim Sp 3.2 K811 MgTiSp 21.4 K820 MgTiSp
F 0.27 0.176 R2 0.181 0.239
Major Element Calculations Daughter Parent Calc. Parent Daughter Parent Calc. Parent
SiO2 41.58 41.89 41.81 42.32 41.58 41.45 MgO 5.34 6.41 6.4 4.31 5.34 5.37 CaO 11.68 14.41 14.47 10.23 11.68 11.73
Group 4 K361-K897 K897-K894 Proportion of Phases Fractionated Cpx 29.8 K361 Cpx1a core 45.4 K894 Cpx3a core Plag 50.8 K894 Plag2 An56.9 20.9 K894 Plag2 An56.9 Ol 12.1 K361 Ol1a core Fo55 22.1 K361 Ol1b rim Fo49.8 Sp 7.3 K361 MgTi Sp 11.6 K894 Ti Sp
F 0.381 0.324 R2 0.298 0.355
Major Element Calculations Daughter Parent Calc. Parent Daughter Parent Calc. Parent
SiO2 44.04 44.73 44.98 45.84 44.04 44.23 MgO 5.65 6.16 6.29 3.31 5.65 5.73 CaO 10.32 11.53 11.39 8.58 10.32 10.21 F = amount of fractionation required R2 = sum of residuals squared
50
51
52
Group 3 (K813-K820-K825)
The trend of Group 3 samples, from the Shira Ridge, range in composition from Mg # 47 to 37. With respect to CaO versus Mg # (Figure 24), this group displays a slightly steeper trend that is richer in CaO than Group 1 and 2, and Group 4 samples of similar Mg #. This difference may reflect either a different composition of, or an increased proportion of clinopyroxene fractionation in the fractionating assemblage.
Sr abundances do not decrease with differentiation (decreasing Mg #) of Group 3 samples, indicating feldspar fractionation is negligible. Decreases in Cr and Sc abundances with decreasing Mg # support clinopyroxene (± olivine) as the fractionating assemblage with minor decreases in V and Ti indicating spinel fractionation.
Fractional crystallisation models for the parent/daughter pairs K813/K820 and K820/825 demonstrate that fractionation of clinopyroxene with minor spinel can account for the major element changes observed (Table 12). No olivine or plagioclase is required to create better quantitative models. Trace element abundances calculated for these models (Table 12) agree with observed values (except for Rb and Cs).
Group 4 (K361-K897-K894)
The highly evolved (Mg # 48 to 35) basanites from Platzkegel (Group 4) plot between Group 3 and Group 1 and 2 with respect to CaO versus Mg # (Figure 24). The overall trend on this diagram is similar to that of Group 1 and 2 samples suggesting a similar scenario of fractional crystallisation. Sr content decreases with decreasing Mg # in Group 4 samples suggesting feldspar fractionation, with similar decreases in V, Ti, Cr and Sc abundances supporting clinopyroxene, spinel (± olivine) fractionation.
Quantitative models of fractional crystallisation for the parent/daughter pairs K361/K897 and K897/K894 support fractionation of clinopyroxene, plagioclase, olivine and spinel (Table 12). Calculated abundances of trace elements for these models agree well with observed values (Figure 30).
53
54
55
Summary
The lithologic and chemical diversity observed in the Shira suite reflects low pressure fractional crystallisation of dominantly ferromagnesian phases from multiple, geochemically distinct ‘primary’ magmas. Clinopyroxene is the major fractionating phase, with olivine and spinel included in most fractionating assemblages. Plagioclase fractionation is negligible except in the evolution of Group 4 (Platzkegel).
The shallow fractionation of ferromagnesian minerals has been identified as the most important process during magma evolution in several East African Rift volcanic provinces as well as many intraplate volcanic provinces globally. Kabeto (2001) identified fractionation of olivine, clinopyroxene, apatite and Fe- Ti oxides as responsible for the evolution of mafic Samburu Hills rocks, with the addition of plagioclase to the fractionation assemblage in more evolved samples. Similarly, the evolution of nephelinitic magmas from East Africa nephelinitic volcanoes has been shown to incorporate significant fractionation of olivine, clinopyroxene, nepheline and Fe-Ti oxides, with clinopyroxene remaining on the liquidus over a wide range of melt temperatures (Peterson, 1989a;1989b). Shaw et al., (2003) and Ho et al., (2003) identified the fractionation of clinopyroxene ± olivine, spinel and plagioclase as the most important magmatic evolution processes within the intraplate volcanic provinces of Jordan and China respectively, whilst Al c et al., (2002) identified fractionation of dominantly olivine and clinopyroxene as responsible for major element trends in extension related alkalic volcanism in Turkey.
Crustal Contamination / Magma Mixing Models
The absence of xenocrysts or xenoliths of crustal materials in both field and petrographic observations, along with the absence of textural heterogeneities such as cognate inclusions, banded lavas and sieve textures or corroded cores in plagioclase phenocrysts (Gourgaud & Vincent, 2004) suggests that crustal contamination is not significant in the evolution of Shira samples. Ratios of specific trace elements also argue against crustal contamination in the Shira volcanic rocks; Zr/Hf ratios between 35 and 60 (Figure 31a) are consistent with mantle-derived intraplate basaltic rocks (Zr/Hf between 35 and 80) (Dupuy et al., 1992), that are not contaminated with crustal material. In the Shira volcanic suite, the highest Zr/Hf are observed in the most silica-undersaturated
56
nephelinites (Group 3) (~50 to 60) with the other groups having lower ratios (~35 to 50) but still considered as dominantly mantle-derived. Nb/Ta ratios are lower in Group 3 samples, than Group 1, 2 and 4 samples, however all values are between 14 and 18 (Figure 31a), and remain fairly constant within each group, for all derivative magmas (Figure 31b) (decreasing Mg #). Nb/Ta ratios for mantle–derived magmas range from 12.5 to 20 and are substantially different from Nb/Ta ratios of the crust (8 to 12.5) (Green, 1995). The Nb/Ta values of the Shira volcanic suite argue against crustal contamination.
60 A)
50
Zr/Hf
40
30 30 40 50 60 70 80 Mg/(Mg+Fe2+)
B) 19
17 a Ratios
Nb/Ta 15
Mantle-Derivied Nb/T
13 Crustal Nb/Ta Ratios (8-12.5)
30 40 50 60 70 80 Mg/(Mg+Fe2+ ) Figure 31. A) Plot of Zr/Hf versus Mg # showing ratios between 35 and 80, consistent with mantle derivation. B) Plot of Mg # versus Nb/Ta showing that Groups 1 and 2, Group 3 and Group 4 Nb/Ta ratios remain constant between 14 and 18, with evolution (decreasing Mg #).
57
A linear trend apparent on the Mg# versus CaO diagram that suggests magma mixing (Figure 32) (K804-KSH11-K2225). Petrographic, microprobe and LA- ICP-MS observations identify complex major and trace element zonation patterns in clinopyroxene (Figures 33 & 15) and plagioclase phenocrysts that imply complex crystallisation paths. Such complexity is compatible with magma chamber convection or magma mixing (Simonetti et al., 1996).
A successful magma mixing model was created that has a sum of squares value of 0.75 (using the same weightings as fractional crystallisation models) having slightly higher K2O, CaO, Na2O, SiO2 and MgO, and slightly lower Al2O3,
P2O5, FeO and TiO2 than the observed sample (Figure 34) (full results in Appendix B). This model suggests that it is possible to create a composition similar to KSH11 through the mixing of 16.3% of K2225 with 83.7% of K804 (Table 13). Trace element calculations performed using simple linear calculations broadly support mixing of K2225 and K804 (Figure 35), with minor discrepancies in the heavy REE and Sr results.
The complex zonation patterns and textures observed in clinopyroxene, as well as the reverse and oscillatory zonation of plagioclase phenocrysts in the same sample, can be attributed to decompression and complex crystallisation paths (Gençalio lu Ku cu & Floyd, 2001). Together with the magma mixing model suggested by the geochemical data, this evidence implies that some Shira samples reflect complex patterns of magma convection or magma mixing in sub-volcanic magma chambers.
16 KSH07KSH09 KSH08
KSH05 14 K813
K2225
K832A 12 KSH10K686 CaO K811 KSH03 K820 K361
K897 K803 10 K825 KSH04 K679 K832B KSH01 K895 KSH11 8 K894 K802 K804 KSH02 KSH06
40 50 60 70 80 Mg/(Mg+Fe2+ ) Figure 32. CaO versus Mg number plot showing the possible magma mixing path K804-KSH11-K2225.
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KSH05 clinopyroxene 1
A) B) KSH05 Clinopyroxene 1 Sc and Ce Variation
Sc Ce 140 35
120 30
1mm 100 25 eppm Ce 80 20
60 15 Sc ppm 40 10
20 5
0 0 abcdef ghi j k KSH05 Clinopyroxene 1 Core to Rim
13 X SEM Backscattered image - 15KV Working Distance = 38mm
C) KSH05 Clinopyroxene 1a REE Plot
100.00 KSH05-Cpx1a KSH05-Cpx1b KSH05-Cpx1c KSH05-Cpx1d KSH05-Cpx1e KSH05-Cpx1f KSH05-Cpx1g i KSH05-Cpx1h h,k KSH05-Cpx1i j KSH05-Cpx1j KSH05-Cpx1k f,g b progressive enrichment change from a c 10.00 Core Rim
e
REE/Chondrite (Sun & McDonough 1989) d
a-b-c-d-e-f-g-h-i-j-k 1.00 La* Ce* Pr* Nd* Sm* Eu* Gd* Tb* Dy* Ho* Er* Tm* Yb* Lu*
Figure 33. A) Back scanned image of clinopyroxene 1 in sample KSH05, showing analysed points and major element zonation. B) Sc and Ce variation from core to rim. C) LA-ICP-MS reduced quantitative data showing the change in degree of chondrite normalised (Sun & McDonough, 1989) REE enrichment from core to rim.
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Table 13. Magma mixing models for mixing path K804-KSH11-K2225.
Groups 1 and 2 Magma Mixing Models K804-KSH11-K2225 Evolved Magma Observed Magma Calculated Magma Primitive Magma Percent Mixing K804 KSH11 K2225 K2225 0.00% 16.30% 100% K804 100.00% 83.70% 0%
R2 0.753
Major Element Calculations (weight %) K804 KSH11 K2225 observed observed calculated observed
SiO2 49.61 47.79 48.75 44.23 MgO 4.35 4.97 5.05 8.64 CaO 8.05 8.69 8.93 13.4
2 R = sum of residuals squared
Major Element Magma Mixing Model
1.1
1.08
1.06
1.04
1.02
1
0.98
0.96
0.94
KSH11 Calculated / KSH11 Observed 0.92
0.9 K2O CaO Na2O SiO2 MgO MnO Al2O3 P2O5 FeO TiO2
calculated sample normalised observed KSH11 normalised
Trace Element Magma Mixing Model
1.5
1.4
1.3
1.2
1.1
1
0.9
0.8
0.7
KSH11 Calculated / KSH11 Observed 0.6
0.5 Y U V Er Hf Zr Sr Yb Lu Dy Th Eu La Ta Sc Rb Gd Nd Ce Nb Ba Pb Cs Sm
calculated sample normalised observed KSH11 normalised
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Figure 34. Comparison of major and trace element magma mixing models results normalised to KSH11 (the observed mixed magma)
1000 A 1989)
100
10 Concentration/Chondrite (Sun & McDonough
1 La Ce Nd Sm Eu Gd Dy Er Yb Lu
1000 K804-observed KSH11-observed KSH11-calculated K2225-observed
B
100
10 Concentration/Primitive Mantle (Sun & McDonough 1989)
1 Cs Rb Ba Th U Nb K La Ce Sr P Nd Zr Sm Eu Ti Dy Y Yb Lu
K804-observed KSH11-observed KSH11-calculated K2225-observed
Figure 35. A) Chondrite normalised (Sun & McDonough, 1989) REE diagram and B) Primitive mantle normalised (Sun & McDonough, 1989) multi element spider diagram for the magma mixing model K804-KSH11-K2225.
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Primitive Magmas, Melting and Sources
The lack of primitive and primary samples (aphanitic, Mg # 68-72, high Ni, Cr, Co and Sc) (Frey et al., 1978) in the Shira suite limits the determination of partial melting conditions and consequently, source characteristics. To overcome this limitation, two approaches are followed to calculate primary magma compositions. The first approach corrects non-primary magmas to ‘primary’ compositions by the addition of olivine in one percent increments until the magma has an Mg # of 71 (Frey et al., 1978). This approach requires 16% olivine addition for KSH03, 23% for K2225, 38% for K813 and 36% for K832a (Figure 36) (these samples are chosen as they are the most primitive of groups 1, 3 and 4). These ‘primary’ samples have low CaO abundances (8-10%), with two of the ‘primary’ basanites having higher CaO abundances than the ‘primary’ nephelinite. Trace element abundances are corrected using the Rayleigh fractional crystallisation rule (Figure 37) rather than by simple dilution.
The addition of such quantities of olivine is simplistic as the fractional crystallisation models calculated for the more evolved magmas indicate the importance of clinopyroxene and/or spinel fractionation. Thus a second method of fractionation correction uses phases theoretically determined to be on the liquidus of the chosen samples. Modelling of crystallisation histories for selected Shira samples involved the use of MELTS (Java applet version 1.1.0), a program written by Mark S. Ghiorso (Ghiorso & Sack, 1995; Asimow & Ghiorso, 1998; Ghiorso et al., 2002). Models were created for samples KSH03,
K2225, K813 and K832a using 0.2%, 1% and 2% H2O, 1 and 5 kb pressures, oxygen fugacity (fO2) of quartz-fayalite-magnetite (QFM) over temperatures from liquidus to 100°C below the liquidus in 5°C intervals. These models show that clinopyroxene is either on, or very close to the liquidus for all four modelled samples.
MELTS results thus indicate that both olivine and clinopyroxene probably fractionated from the primary magma to produce the analysed samples. The amount of clinopyroxene that fractionated is difficult to determine, and thus a database of primary and primitive melilitites, nephelinites, basanites and alkali- olivine basalts was compiled to establish the maximum and minimum CaO contents for a range of primitive magmas at Mg # 71 (Figure 38). These CaO values are used to determine the maximum and minimum amounts of
62
clinopyroxene fractionation (in addition to olivine) required to reach these limits. The compilation demonstrates that primary nephelinites and melilitites generally contain higher CaO than basanites and alkali-olivine basalts at Mg # 71. The Shira samples that are modelled are nephelinites and basanites; corresponding CaO limits at Mg # 71 are set at 11.75%-13% and 9.5%-11.75% respectively. The general direction of the olivine vector was known from the first approach. When combined with the minimum and maximum CaO end points determined from the database of primary and primitive samples, a point mid range is portrayed that has to be reached by clinopyroxene fractionation. The appropriate quantity of clinopyroxene (using appropriate phenocryst core compositions) was added in 1% increments until the olivine addition vectors graphically corresponded with the CaO limits set by the primary and primitive sample database (Figure 39). Although addition of clinopyroxene and olivine has been modelled as two separate stages, both were most likely fractionating concurrently. The resultant vector produced through the combination of the two separate stages reflects the more complex fractionation path.
Modelling of clinopyroxene and equilibrium olivine addition in order to reach the compositional limits of the ‘primary’ nephelinites and basanites show olivine is the dominant fractionating phase. Addition of between 12% and 18% clinopyroxene and 26% and 21% olivine is required for K813 to reach the upper and lower limits of the ‘primary’ nephelinite field. Sample KSH03 requires 0% to 10% clinopyroxene addition and 16% to 10% olivine addition to reach the limits of the ‘primary’ basanites field. Sample K2225 (basanite) requires 0% to 6% clinopyroxene addition, and 23% to 19% olivine addition, and sample K832a (basanite) requires 4 to 16% clinopyroxene addition and 32 to 21% olivine addition.
Trace element concentrations, corrected for both minimum and maximum clinopyroxene addition using Rayleigh fractional crystallisation, are nearly identical (possibly due to the low amount of clinopyroxene added relative to olivine) (Figure 40) (full results in Appendix C).
The fractionation model was then ‘checked’ by additional MELTS models to determine which model best represents ‘primary’ magmas for samples KSH03, K2225, K813 and K832a (Table 14). Crystallisation sequence modelling was performed utilising variables of 1, 5, 10 and 20 kb pressures, 0.2%, 1% and 2%
63
H2O, oxygen fugacity (fO2) of quartz-fayalite-magnetite (QFM) over temperatures from liquidus to solidus in 20°C intervals (Figure 41). Models were then qualitatively assessed through comparison of determined crystallisation paths and proportions of phases added to account for fractionation correction.
Low pressure models showed that olivine was generally the first phase fractionated, but was followed very closely by clinopyroxene. Increasing pressure did not significantly alter the olivine liquidus, however it did increase the temperature of the clinopyroxene liquidus such that the crystallisation sequence begins with clinopyroxene in models where the pressure was greater than 20kb. Increasing the H2O content had the opposite effect, reducing the temperature of the clinopyroxene liquidus far more than the olivine liquidus.
Four models are selected (one for each ‘primary’ magma) based on this method (Figure 41), which qualitatively represents the crystallisation sequence, proportion of phases added, and mineralogy of the original samples to which olivine and/or clinopyroxene has been added. The models which crystallised equivalent quantities of the phases added (clinopyroxene and olivine) before reaching compositions similar to the ‘uncorrected’ samples were selected. MELTS models show that sole olivine addition is too simplistic for samples K813 and K832a, as at the clinopyroxene liquidus only 20 to 25% olivine has crystallised, however 36% and 38% olivine has been added to reach the ‘primary’ compositions respectively. Models involving maximum clinopyroxene addition were also disregarded for samples K813 and K832a, as at the point where 18% and 16% clinopyroxene has crystallised (the quantity of clinopyroxene added respectively) the amount of olivine crystallised is 12 to 15%, whereas 21% olivine was added to both samples. The models involving minimum quantities of clinopyroxene addition were accepted for samples K813 and K832a, as the crystallisation sequences and proportion of phases crystallised were broadly comparable with the added phases. Both models for samples K2225 and KSH03 appear equally plausible, with crystallisation sequences and proportion of phases crystallised comparable to the amount of olivine, and olivine/clinopyroxene added. Due to the low volume of clinopyroxene added in these samples, and both models appearing equally plausible, the simplest method involving addition of olivine only has been accepted. In summation, the models which correlated closest with determined
64
crystallisation sequences were K2225+23% olivine, KSH03+16% olivine, K813+12% clinopyroxene + 26% olivine and K832a+4% clinopyroxene + 32% olivine, and will hence be used in partial melting models.
65
66
.
16
14 Melilitite
12 CaO Nephelinite
10 Basanite basalt
8 Alkali-olivine
6 40 50 60 70 80 Mg/(Mg+Fe2+ )
Primitive melilitites Primitive nephelinites Primitive basanites Primitive alkali olivine basalts
Figure 38. Compilation of primitive melilitites, nephelinites, basanites and alkali olivine basalts showing their CaO ranges at Mg number 71 (sourced from Brey, 1978; Frey et al., 1978; Phelps et al ., 1983; Kempton et al ., 1987; Jung, 1998; and Spath et al ., 2001).
67
68
Table 14. Corrected “primitive” magma major element compositions for both correction methods used in MELTS crystallisation sequence modelling (minimum clinopyroxene addition models for K2225 and KSH03 required no clinopyroxene and were identical to olivine addition model).
Sample K2225 K2225 KSH03 KSH03 K813 K813 K813 K832a K832a K832a Olivine added 23% 19% 16% 10% 38% ol 26% 21% 36% ol 32% 21% Clinopyroxene added 6% 10% 12% 18% 4% 16%
SiO2 42.68 43.38 43.85 45.04 40.42 41.87 42.68 43.49 43.95 45.44
Al2O3 10.25 10.03 11.31 10.81 9.64 9.57 9.38 10.08 9.88 9.66
TiO2 1.75 1.72 1.68 1.60 1.80 1.78 1.75 1.59 1.59 1.55
Fe2O3 2.23 2.13 2.14 1.99 2.47 2.23 2.12 2.44 2.37 2.14 FeO 11.35 10.84 10.94 10.16 12.58 11.40 10.80 12.46 12.06 10.89 MnO 0.17 0.16 0.18 0.17 0.20 0.20 0.19 0.15 0.15 0.15 MgO 16.28 15.61 15.63 14.47 18.53 16.49 15.66 18.11 17.45 15.54 CaO 10.42 11.35 9.83 11.51 9.55 11.64 12.69 8.71 9.38 11.53
Na2O 2.64 2.59 1.72 1.64 2.07 2.05 2.01 2.31 2.30 2.25
K2O 0.44 0.43 0.82 0.78 0.52 0.52 0.50 0.29 0.29 0.28
P2O5 0.39 0.38 0.37 0.36 0.51 0.51 0.50 0.38 0.38 0.37 Totals 98.56 98.59 98.44 98.52 98.18 98.20 98.24 99.92 99.74 99.75 Mg # 0.71 0.71 0.71 0.71 0.71 0.71 0.71 0.71 0.71 0.71
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70
Partial Melting Models
Compositional differences of primary/primitive alkalic rocks are traditionally attributed to differences in the degree of partial melting, source composition and crustal contamination (Frey et al., 1978). In a now classic study, Frey et al., (1978) concluded that a single enriched source composition could yield olivine melilitite (4-6% melting), olivine nephelinite to basanite (5-7% melting), alkali- olivine basalt (11-15% melting) and olivine basalt to olivine tholeiite (20-25% melting). Although contents of H2O and CO2 are critical factors for low degree melts and are important in the genesis of strongly silica-undersaturated olivine melilitites and nephelinites (Green, 1969; Brey & Green, 1976; 1977; Brey, 1978; Eggler, 1978; Frey et al., 1978; Wyllie, 1987), their precise effect is not quantified. The contribution of these volatile phases to the petrogenesis of the Shira volcanic rocks may therefore be important, but difficult to assess.
The trace element patterns of corrected ‘primitive’ samples show near parallel trends (Figures 37 & 40), differing only in degree of enrichment. Degree of enrichment increases from K832a (Group 4) to K2225 and KSH03 (Group 1), to K813 (Group 3), with abundances increasing from 128 to 219 times chondrite La, and 8 to 10.5 times chondrite Lu. These differences may be due to different degrees of partial melting, with nephelinite being produced by lower degrees of melting than basanite, resulting in greater incompatible element enrichment.
Negative K anomalies are observed for all samples including fractionation- corrected samples (Figures 20, 37 & 40). Potassium depletion relative to elements of similar incompatibility (e.g. Th, Nb and La) is common in intraplate volcanics, and is explained most easily by either fractionation of a K-bearing phase or retention of K in the source during partial melting. As no significant K- bearing phases are identified in Shira rocks, retention of a K-bearing phase in the source during partial melting is the most plausible explanation. Zhang and O’Reilly (1996) identified phlogopite retention as the most likely reason for similar characteristics in eastern Australia samples. Similarly, either residual amphibole or phlogopite is identified as the most likely cause of K depletions in Grand Comore magmas (Indian Ocean) (Deniel, 1998) and South African melilitites (Rogers et al., 1992). Amphibole and phlogopite retention in mantle source regions of alkalic basalts is postulated for many regions globally (e.g. Mertes & Schmincke, 1985; Wilson & Downes, 1991; Hoernle & Schmincke,
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1993; Class et al., 1994; Spath et al., 1996; Class & Goldstein, 1997) and is supported by both experimental evidence, and their occurrence in mantle derived xenoliths and as mantle xenocrysts in alkalic rocks (e.g. Best, 1974; Wilshire et al., 1980; Erlank et al., 1987; Gamble & Kyle, 1987; Harte et al., 1987).
Amphibole and phlogopite are identified in numerous mantle xenoliths from Kenya and northern Tanzania (Dawson et al., 1970; Dawson & Smith, 1973; Dawson & Smith, 1988; Henjes-Kunst & Altherr, 1992; Johnson et al., 1997). Primitive mantle normalised multi-element spider diagrams of Shira volcanic rocks are very similar to the neighbouring Chyulu Hills Volcanic Province (Spath et al., 2001). Spath et al., (2001) attributes the K depletion in the Chyulu Hills volcanic rocks to residual amphibole during non-modal equilibrium batch partial melting of an amphibole-bearing spinel lherzolite (53% olivine, 22% orthopyroxene, 18% clinopyroxene, 2% spinel, 5% amphibole melting in mode of 5%, 10%, 33%, 2% and 50% respectively). Forward modelling indicates that the relative K depletion and REE trends can not be accounted for by partial melting of a four phase spinel lherzolite (Spath et al., 2001).
As the negative K anomalies (relative to elements of similar incompatibility) observed in ‘corrected’ primary Shira samples cannot be accounted for through partial melting of a four phase spinel lherzolite, sources require residual phlogopite or amphibole. K, Rb and Ba concentrations are consistent with derivation from a kaersutitic or pargasitic amphibole rather than a phlogopite- bearing source (Beswick, 1976; Spath et al., 2001). The REE trends of ‘corrected’ primary Shira magmas are very similar, differing only in degree of enrichment. Chondrite normalised REE enrichment decreases from La to Er, then remains rather constant at 10 times chondritic LREE (Er to Lu) concentrations (Figures 37a & 40a). This style of REE enrichment is indicative of derivation form a spinel, rather than garnet lherzolite source, and when combined with K anomalies, suggest that the Shira lavas, like the nearvy Chyulu Hills lavas (Spath et al., 2001) are most likely sourced from an amphibole-bearing spinel lherzolite.
Fractional and/or continuous/dynamic melting (Langmuir et al., 1977; McKenzie, 1985; Albarede, 1995; Zou & Zindler, 1996; Shaw, 2000) of garnet-bearing and garnet-free sources with low residual source porosity produce REE trends
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significantly different to those observed in the Shira volcanic rocks. To produce compositions comparable to the Chyulu rocks, melting models that involve the pooling and accumulation of melt fractions (i.e. aggregated fractional melting and continuous melting with high residual source porosity) are required. These variations in melting models produce very similar results to simple batch melts (Spath et al., 2001; Maaløe & Pedersen, 2003). Therefore, although open to criticism on the grounds that the actual process may be too simplistic and therefore unreal, the minimal difference between results from simple batch partial melting calculations, and more complex models (Spath et al., 2001; Maaløe & Pedersen, 2003), coupled with the uncertainty inherent in using ‘corrected’ primary magmas justifies the use of simplistic batch melting models to speculate on the petrogenesis of the Shira rocks.
Reverse partial melting models of both modal and non-modal equilibrium batch melting (CL/CO=1/[DRS+F(1-DRS)]) of an amphibole-bearing spinel lherzolite were created (Figures 42 & 43). These models utilise the modal composition of Spath et al., (2001) (53% olivine, 22% orthopyroxene, 18% clinopyroxene, 2% spinel, 5% amphibole) and partition coefficients presented in Table 10. The models show that the production of fractionation-corrected ‘primary’ Shira samples can best be accounted for through modal equilibrium batch melting (Figure 42). Degrees of melting required to produce nephelinites and basanites from an enriched source are very similar to values calculated by Frey et al., (1978) for the alkalic rocks of south eastern Australia. Sample K813 (nephelinite) requires 4% partial melting, KSH03 and K2225 (basanites) require 7% partial melting, whereas K832a (basanite) requires 10% partial melting. The same degrees of melting also produce the closest correlation with non- modal equilibrium batch melting (Figure 43). Using these degrees of partial melting in non-modal batch melting calculations, produce source patterns similar in appearance to modal melting, but with a greater degree of variation in source concentrations (full results in Appendix D).
Forward modelling was used to confirm if the observed negative K anomalies could be created by retaining amphibole during partial melting. Sample K2225 was modelled as it is the least phyric Shira sample analysed. The calculated trace element source concentrations for corrected sample K2225 (7% modal equilibrium batch melting) and an assumed source K concentration of 0.2% were used (correlating with the degree of enrichment of elements with similar
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incompatibility (Nb, Th and La)). Modal equilibrium batch melting of between 1% and 10% was used with partition coefficient values adjusted such that the negative K anomaly observed could be reproduced (Figure 44). By using K D amphibole partition coefficients of between 3 and 15, (such that K preferentially remains in the solid during melting) similar sized anomalies were produced (full results in Appendix E).
These models support modal equilibrium batch melting of between 4 and 10% (calculated values for corrected samples) of an enriched amphibole-bearing spinel lherzolite, in which amphibole is residual to produce ‘primary’ Shira magmas.
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1000 1000 A A 1989) 1989)
100 100
10 10 Concentration/Chondrite (Sun & McDonough Concentration/Chondrite (Sun & McDonough
1 1 La Ce Nd Sm Eu Gd Dy Er Yb Lu La Ce Nd Sm Eu Gd Dy Er Yb Lu PM OIB AvSp Lherz PM OIB AvSp Lherz K813 + 12%cpx+26%ol source K813-4% melt KSH03 + 16%ol K813 + 12%cpx+26%ol source K813-4% melt KSH03 + 16%ol source KSH03-7% melt K2225 + 23%ol source K2225-10% melt source KSH03-7% melt K2225 + 23%ol source K2225-10% melt K832a + 4%cpx+32%ol source K832a-10% melt K832a + 4%cpx+32%ol source K832a-10% melt
1000 1000 B B 1989) 1989)
100 100
10 10 Concentration/Primitive Mantle (Sun & McDonough Concentration/Primitive Mantle (Sun & McDonough
1 1 Cs Rb Ba Th U Nb La Ce Sr Nd Zr Sm Eu Dy Y Yb Lu Cs Rb Ba Th U Nb La Ce Sr Nd Zr Sm Eu Dy Y Yb Lu OIB K813 + 12%cpx+26%ol source K813-4% melt OIB K813 + 12%cpx+26%ol source K813-4% melt KSH03 + 16%ol source KSH03-7% melt K2225 + 23%ol KSH03 + 16%ol source KSH03-7% melt K2225 + 23%ol source K2225-7% melt K832a + 4%cpx+32%ol source K832a-10% melt source K2225-7% melt K832a + 4%cpx+32%ol source K832a-10% melt Figure 42. Reverse modal equilibrium batch melting Figure 43. Reverse non-modal equilibrium batch melting models of corrected samples K813 (nephelinite), K2225, models of corrected samples K813 (nephelinite), K2225, KSH03 and K832a (basanites). Models calculated source KSH03 and K832a (basanites). Models calculated source trace element abundances by using variables of between trace element abundances by using variables of 0.001%, 0.001%, 0.01%. 0.1% and 1% to 20% (in 1% increments). 0.01%. 0.1% and 1% to 20% (in 1% increments). Models chosen are those in which the source trace Models chosen are those in which the source trace element abundances were as similar as possible. element abundances were as similar as possible. Models indicate that through modal batch melting of a Models indicate that through non-modal batch melting of a relatively homogenous source with LREE enrichment relatively homogenous source with LREE enrichment ~ 20 x chondrite, and HREE enrichment~3to4x ~ 20 to 30 x chondrite, and HREE enrichment~3to4x chondrite it is possible to reproduce the ‘corrected’ chondrite (with positive Eu, Dy and Yb anomalies) it is primary magma trace element concentrations through possible to reproduce the ‘corrected’ primary magma 4% melting for nephelinite (K813), 7% melting for Group trace element concentrations through 4% melting for 1 basanites (K2225 & KSH03) and 10% melting for Group nephelinite (K813), 7% melting for Group 1 basanites 4 basanite (K832a). A) Chondrite normalised (K2225 & KSH03) and 10% melting for Group 4 basanite (Sun & McDonough, 1989) REE diagram, B) Primitive (K832a). A) Chondrite normalised (Sun & McDonough, mantle normalised (Sun & McDonough, 1989) multi- 1989) REE diagram, B) Primitive mantle normalised element spider diagram (Mode = 53% olivine, 22% (Sun & McDonough, 1989) multi-element spider diagram orthopyroxene, 18% clinopyroxene, 2% spinel and 5% (Mode = 53% olivine, 22% orthopyroxene, 18% amphibole). clinopyroxene, 2% spinel and 5% amphibole melting in mode of 5%, 10%, 33%, 2% and 50% respectively).
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1000
100
10 Concentration/Primitive Mantle (Sun & McDonough 1989)
1 Cs Rb Ba Th U Nb K La Ce Sr Nd Zr Sm Eu Dy Y Yb Lu
1 3 5 7 10 K2225+23%ol K2225_Source_7%melt
Figure 44. Forward modal equilibrium batch melting of calculated source for corrected sample K2225 (assuming 7% partial melting required to produce this sample) showing the size of the K anomaly produced through between 1 and 10% partial melting in which K remains
K residual in amphibole (Damphibole = 6). (source K value was assumed at 0.2% in order to create a smooth trend with elements of similar incompatibility Nb, Th, and La) shown on a primitive mantle normalised (Sun & McDonough, 1989) multi-element spider diagram.
Source Characteristics and Formation Seismic, tomographic and petrologic studies of southeastern Kenya indicate average crustal thicknesses between 40km and 43km, with total lithospheric thicknesses of approximately 105km to 115km (Henjes-Kunst & Altherr, 1992; Novak et al.,1997a; 1997b; Ritter & Kaspar, 1997). Velocity perturbations observed between 40km and 70km are interpreted as partial melts within the upper mantle (Novak et al., 1997b).
As the spinel-garnet transition occurs between 21kb (1100°C) and 24kb (1300°C) (Green & Ringwood, 1967b, 1970), representing depths of
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approximately 65-80km, and the plagioclase-spinel transition occurs at approximately 10kb (30km) Shira sources are constrained to between 30 and 90km depth. Pargasitic or kaersutitic amphibole upper stability limits range from 21kbar to 30kbar (e.g. Green & Ringwood, 1970; Boettcher et al., 1975; Olafsson & Eggler, 1983; Mengel & Green, 1986) representing depths of approximately 70 to 90km. This stability range is consistent with the derivation of the Shira magmas from a spinel lherzolite source that is part of the sub continental lithosphere (to approximately 115km depth). Thus the Shira volcanic rocks reflect melting of an enriched sub continental lithosphere rather than an asthenospheric source.
Isotopic studies of numerous volcanic centres in northern Tanzania (Paslick et al., 1995) suggest ancient (>1Ga) underplating and metasomatism of the continental lithosphere by OIB melts is responsible for enrichment of the sub continental lithosphere. Megacryst vein studies identify metasomatism by alkaline silicate and possibly carbonatite melts beneath northern Tanzania (Johnson et al., 1997). No constraint is available for the timing of this metasomatic event. Asthenospheric-sourced carbonatite melts are identified as responsible for metasomatism of peridotite xenoliths from Olmani cinder cone, northern Tanzania (Rudnick et al., 1993) with numerous other authors postulating metasomatism of the lithospheric mantle in other East African regions (e.g. Vollmer & Norry, 1983a, 1983b; Cohen et al., 1984; Rogers et al., 1992; Furman, 1995). Fractionation-corrected Shira samples have trace element patterns similar to OIB-melts, although slightly less enriched and with K and Rb anomalies (Figure 45). An age of >1Ga for OIB-like continental lithosphere enrichment (Paslick et al., 1995) implies that source enrichment occurred long before the onset of rifting (Figure 46a). Although additional recent enrichment by plume-derived OIB melts may have occurred (i.e. Spath et al., 2001), the source of the Shira magmas probably acquired its enrichment signature from ancient (>1Ga) OIB underplating and metasomatism.
It is argued that ancient metasomatism created a sub continental lithosphere that was capable of producing highly-enriched silica under-saturated magmas by low degrees of partial melting. The ultimate cause of this melting event however, remains unclear. The EAR is the result of ‘active’ rifting processes in which a plume of asthenospheric mantle ascends, and undergoes decompression melting (Kampunzu & Mohr, 1991; Spath et al., 2001).
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Kilimanjaro is located well away from the rift axis, thus it is expected that the impact of the asthenosperic plume may be less. The major effect however, would be the provision of heat (Turner et al., 1996), and probably volatiles to the sub continental lithosphere to induce partial melting. In particular, the introduction of volatiles such as H2O and CO2 would both lower the solidus (Brey, 1969; Brey & Green, 1976) of the sub continental lithosphere and stabilise amphibole, as well as providing additional incompatible element enrichment. Thus while the Shira magmas are sourced from the sub continental lithosphere, the ultimate reason for their generation lies in the interaction between the upwelling asthenospheric mantle and the subcontinental lithosphere
Rock/OIB 10
1
.1 CsRbBaTh U Nb K LaCe Sr P Nd ZrSmEu Dy Y Yb Lu
Figure 45. Fractionation corrected “primitive” Shira samples normalised to Sun & McDonough (1989) OIB values.
Upon segregation from their source, melts follow structural weaknesses in the lithosphere (Figure 46b) before ponding at shallow depths (1-5kb) in small volume magma chambers, being subject to significant ferromagnesian fractionation (Figure 46c). Fractionation paths result from separate, geochemically distinct ‘primary’ magmas, in which the fractional crystallisation of basanites (samples K2225, KSH03 and K832a) is initially dominated by olivine, becoming clinopyroxene dominated between Mg # 45 and 60. Nephelinite fractional crystallisation is initially subject to both olivine and
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clinopyroxene fractionation, becoming increasingly dominated by clinopyroxene fractionation with evolution. Magma mixing by subsequent partial melts or convective currents within magma chambers is thought to occur episodically, resulting in oscillatory zoned phenocrysts.
The Shira volcanic centre has since collapsed, forming the present caldera, and been intruded by late stage dykes, sills and vent infill (Platzkegel) before cessation (Figure 46d).
Individual nephelinite cones in intraplate volcanic provinces (i.e. southeastern Australia, various regions in East Africa, New-Mexico etc.) may simply represent the extrusion of short-lived, single melts. Large mixed-association continental rift stratovolcanoes however may represent longer-lived polygenetic volcanoes, formed through the extrusion of many successive melts. Polygenetic volcanoes may form due to their positions relative to magmatic conduits (i.e. major faults in the case of Mt Kilimanjaro), raised geothermal gradients or source region differences (i.e. extent of metasomatism, mineralogy).
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A B 0km 0km Crust Crust
40km 40km SCLM Introduction of volatiles to the sub continental lithospheric mantle (SCLM), enriched by ancient OIB underplating and melts
Metasomatised SCLM Variable partial melts (4-10%) 105km 105km
Rising asthenospheric plume Rising asthenospheric plume Dehydration melting and structural weakening forming the initial depression and magmatic conduits to the crust (i.e. Kilimanjaro depression). C D Kibo Kibo Shira Shira Mawenzi Platzkegel Mawenzi
Crust Crust
Magma chambers (shallow fractional crystallisation) SCLM SCLM
Small volume magma chambers ponding and undergoing The collapse of Shira, followed by the eruption of Platzkegel extensive fractional crystallisation in the crust. Differring degrees of lavas and intrusion of various dykes/sills. This is in turn followed partial melting are responsible for nephelinites and basanites, with by the cessation of the Shira and Mawenzi volcanic centres cumulate samples erupted upon either emptying of magma chamber or resulting in Kilimanjaro’s present morphology as a result of magma mixing. Numerous partial melts (most likely of short lifespans) create a polygenetic volcano, resulting in the formation of Kilimanjaro and its three main volcanic centres (Shira, Kibo and Mawenzi). Figure 46. Proposed model for the genesis and evolution of Mt Kilimanjaro starting with A) Introduction of volatile phases to a previously enriched subcontinental lithospheric mantle resulting from upwelling asthenosphere. B) Lithospheric faulting and low degree partial melts of the SCLM following structural weaknesses, with larger polygenetic volcanoes occurring along major faults, where multiple partial melts may easily ascend and smaller monogenetic volcanoes occurring along minor faults or structural weaknesses. C) Fractional crystallisation of small volume melts occurring predominantly in small, shallow magma chamber, erupting to form Shira, Kibo and Mawenz. D) Formation of Shira caldera and Platzkegel followed by the cessation of Shira and Mawenzi resulting in the present morphology of Mt Kilimanjaro (not to scale).
CONCLUSION
The Shira volcanic suite consists of silica-undersaturated nephelinites, basanites, picro-basalts and hawaiites (Mg #’s ranging from 35.5 to 77.2). Groups identified on the basis of phenocryst assemblages and textures correlate with geographic location. Samples (East Shira Hill) contain olivine and clinopyroxene phenocrysts + microphenocrysts of plagioclase (Group 1), or plagioclase and clinopyroxene phenocrysts + microphenocrysts of olivine (Group 2). Samples with high Mg #’s contain abundant cumulate clinopyroxene and olivine (Fo92-Fo85). Group 3 samples (Shira Ridge) contain nepheline
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phenocrysts and Group 4 samples (Platzkegel) have distinct intergranular textures.
Trends on many geochemical diagrams identify fractional crystallisation paths reflecting fractionation of clinopyroxene ± olivine and spinel. Complex major and trace element zonation in clinopyroxene and feldspar phenocrysts suggest magma mixing.
Primitive samples corrected to ‘primary’ compositions indicate low degrees of partial melting of between 4% and 10%. Trace element abundances similar to OIB-melts, with negative K anomalies require retention of amphibole in the source during partial melting. REE trends are compatible with an amphibole- bearing spinel lherzolite source. When combined with local geophysical and mineralogical studies, this conclusion implies a source in the sub continental lithospheric mantle. The enrichment of this source reflects ancient (>1Ga) OIB under-plating and metasomatism. Rising asthenosphere provides a source of heat as well as volatiles, necessary in inducing melting and stabilising hydrous K-bearing phases.
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APPENDIX A
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APPENDIX A – FRACTIONAL CRYSTALLISATION MODELS Group 1 – Path A. Fractional crystallisation models and trace element calculations for Path A (KSH08-KSH03-K679-KSH02).
Groups 1 and 2 Fractional Crystallisation Models KSH08-KSH03 KSH03-K679 K679-KSH02 Proportion of Phases Fractionated Cpx 77 KSH05 Cpx8c avg 69.7 KSH03 Cpx4b avg 78.7 KSH03 Cpx4b avg Ol 23 KSH05 Ol5c rim Fo82.2 16.0 KSH03 Ol3b avg Fo78.3 1.2 KSH03 Ol3b avg Fo78.3 Sp 14.3 KSH03 Cr Sp 20.1 KSH03 Cr Sp
F 0.562 0.335 0.203 S2 0.26 0.419 0.200
Major Element Calculations Daughter Parent Calc. Parent Daughter Parent Calc. Parent Daughter Parent Calc. Parent
SiO2 45.39 46.06 46.77 46.89 45.39 45.14 49.4 46.89 47.26
TiO2 2.01 1.52 1.22 2.27 2.01 2.11 2.06 2.27 2.08
Al2O3 13.56 7.67 7.74 15.8 13.56 13.02 17.06 15.8 15.43 FeO 13.2 10.44 10.27 12.49 13.2 13.25 12.07 12.49 12.57 MnO 0.21 0.16 0.19 0.21 0.21 0.16 0.2 0.21 0.16 MgO 10.34 16.83 16.8 6.35 10.34 10.35 4.42 6.35 6.25 CaO 11.79 15.64 15.47 9.55 11.79 11.88 7.37 9.55 9.5
Na2O 2.06 1.06 0.95 3.64 2.06 2.42 4.43 3.64 3.53
K2O 0.98 0.38 0.49 2.06 0.98 1.4 2.25 2.06 1.82
P2O5 0.45 0.24 0.2 0.73 0.45 0.54 0.73 0.73 0.62 Trace Element Calculations Parent Daughter Calc. Daughter Parent Daughter Calc. Daughter Parent Daughter Calc. Daughter Sc 52.29 34.23 19.57 34.23 19.77 20.95 19.77 13.97 13.99 V 254.00 276.81 579.91 276.81 235.06 416.26 235.06 180.06 294.93 Cr 1571.65 316.44 72.59 316.44 99.07 68.73 99.07 35.39 39.53 Rb 11.09 22.35 24.75 22.35 54.02 32.66 54.02 62.75 66.44 Sr 329.97 671.84 677.92 671.84 877.47 926.59 877.47 945.01 1037.23 Y 14.82 22.64 19.06 22.64 28.34 26.04 28.34 29.74 30.00 Zr 105.55 194.15 196.51 194.15 296.65 216.31 296.65 279.91 299.32 Nb 33.64 63.50 76.41 63.50 107.72 93.09 107.72 97.42 132.59 Cs 0.11 0.24 0.24 0.24 0.54 0.35 0.54 0.56 0.67 Ba 188.02 455.77 416.12 455.77 690.43 660.69 690.43 781.96 844.50 La 24.83 46.78 52.09 46.78 71.85 67.00 71.85 75.85 87.32 Ce 45.89 85.65 98.71 85.65 131.61 110.64 131.61 137.00 147.27 Nd 22.11 39.39 43.56 39.39 55.55 49.35 55.55 58.10 61.06 Sm 4.32 7.15 7.42 7.15 9.85 8.60 9.85 9.68 10.59 Eu 1.29 2.13 2.02 2.13 2.72 2.67 2.72 2.64 3.04 Gd 3.85 5.99 6.16 5.99 7.53 7.69 7.53 7.60 8.55 Dy 2.97 4.47 4.67 4.47 5.52 5.69 5.52 5.65 6.24 Er 1.4 2.1 2.20 2.10 2.67 2.67 2.67 2.79 3.02 Yb 1.14 1.68 1.83 1.68 2.23 2.00 2.23 2.51 2.39 Lu 0.16 0.26 0.26 0.26 0.34 0.34 0.34 0.37 0.39 Hf 2.58 4.48 4.26 4.48 6.42 5.71 6.42 5.97 7.24 Ta 2.03 3.69 4.39 3.69 6.41 5.24 6.41 6.14 7.73 Pb 4.67 6.5 10.66 6.50 8.66 9.77 8.66 9.57 10.87 Th 2.8 4.89 6.13 4.89 9.38 7.14 9.38 8.83 11.54 U 0.58 0.88 1.26 0.88 1.12 1.28 1.12 1.65 1.37
F = amount of fractionation required S2 = sum of residuals squared
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Group 1 – Path B. Fractional Group 1 – Path C. Fractional crystallisation models and trace crystallisation models and trace element calculations for Path B element calculations for Path C (K2225-K803). (KSH01-K802).
Groups 1 and 2 Fractional Crystallisation Models Groups 1 and 2 Fractional Crystallisation Models K2225-K803 KSH01-K802 Proportion of Phases Fractionated Proportion of Phases Fractionated Cpx 66.8 K2225 Cpx3b avg Cpx 66.6 KSH01 Cpx1b avg Ol 24.8 K2225 Ol3a avg Fo78.4 Ol 23.7 KSH01 Ol5b avg Fo64.9 Sp 8.4 KSH03 MgTiSp Sp 9.7 KSH01 MgTiSp
F 30.7 F 0.153 S2 0.552 S2 0.428
Major Element Calculations Major Element Calculations Daughter Parent Calc. Parent Daughter Parent Calc. Parent
SiO2 44.81 44.23 44.52 SiO2 48.11 46.29 46.87
TiO2 2.51 2.26 2.41 TiO2 2.57 2.43 2.74
Al2O3 16.55 13.18 12.99 Al2O3 17.54 16.28 15.63 FeO 13.11 13.61 13.59 FeO 12.62 13.83 13.76 MnO 0.24 0.21 0.2 MnO 0.23 0.22 0.24 MgO 5.7 8.64 8.6 MgO 3.71 5.64 5.6 CaO 10.29 13.4 13.34 CaO 7.96 9.26 9.13
Na2O 4.37 3.4 3.03 Na2O 4.66 4.17 3.96
K2O 1.69 0.57 1.17 K2O 1.9 1.29 1.61
P2O5 0.75 0.5 0.54 P2O5 0.7 0.59 0.6
Trace Element Calculations Trace Element Calculations Parent Daughter Calc. Daughter Parent Daughter Calc. Daughter Sc 35.03 17.32 20.22 Sc 19.50 9.38 16.37 V 325.96 235.50 470.36 V 263.11 145.12 310.63 Cr 193.40 44.29 Cr 36.65 20.16 Rb 9.57 59.30 13.51 Rb 66.38 53.94 77.64 Sr 595.74 954.54 797.64 Sr 822.05 1000.05 942.40 Y 24.42 29.17 26.59 Y 27.84 29.68 29.65 Zr 193.67 328.48 222.27 Zr 258.20 262.01 277.51 Nb 71.26 107.97 101.10 Nb 90.25 88.29 105.77 Cs 0.28 0.49 0.40 Cs 0.36 0.76 0.43 Ba 366.92 703.35 514.84 Ba 634.96 710.36 740.68 La 48.82 74.34 67.21 La 67.88 69.41 78.73 Ce 88.91 136.24 115.00 Ce 121.85 124.89 137.53 Nd 39.90 58.48 49.77 Nd 52.98 53.15 59.03 Sm 7.31 10.11 8.66 Sm 9.44 9.46 10.33 Eu 2.16 2.98 2.58 Eu 2.84 2.63 3.12 Gd 6.53 8.02 7.95 Gd 7.53 7.41 8.36 Dy 4.79 5.80 5.78 Dy 5.37 5.64 5.94 Er 2.39 2.71 2.88 Er 2.53 2.80 2.80 Yb 1.91 2.37 2.22 Yb 2.16 2.48 2.34 Lu 0.28 0.32 0.34 Lu 0.31 0.38 0.35 Hf 4.61 6.82 5.63 Hf 5.61 5.66 6.22 Ta 4.25 6.48 5.85 Ta 5.14 4.99 5.96 Pb 6.00 7.49 8.65 Pb 5.45 8.48 6.43 Th 5.96 7.68 8.38 Th 7.50 8.99 8.77 U 1.34 1.55 1.88 U 1.40 1.89 1.63
F = amount of fractionation required F = amount of fractionation required S2 = sum of residuals squared S2 = sum of residuals squared
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Group 3. Fractional crystallisation models and trace element calculations for the Group 3 fractionation path (K813-K820-K825).
Group 3 Fractional Crystallisation Models K813-K820 K820-K825 Proportion of Phases Fractionated Cpx 96.8 K811 Cpx4c rim 78.6 K820 Cpx1c rim Sp 3.2 K811 MgTiSp 21.4 K820 MgTiSp
F 0.27 0.176 S2 0.181 0.239
Major Element Calculations Daughter Parent Calc. Parent Daughter Parent Calc. Parent
SiO2 41.58 41.89 41.81 42.32 41.58 41.45
TiO2 2.46 2.71 2.69 2.32 2.46 2.76
Al2O3 15.71 14.54 14.06 17.23 15.71 15.43 FeO 16.29 15.07 15.09 15 16.29 16.23 MnO 0.32 0.3 0.29 0.29 0.32 0.29 MgO 5.34 6.41 6.4 4.31 5.34 5.37 CaO 11.68 14.41 14.47 10.23 11.68 11.73
Na2O 4.58 3.12 3.43 5.88 4.58 4.87
K2O 1.13 0.78 0.87 1.5 1.13 1.25
P2O5 0.9 0.77 0.66 0.94 0.9 0.77
Trace Element Calculations Parent Daughter Calc. Daughter Parent Daughter Calc. Daughter Sc 13.25 9.37 7.61 9.37 6.67 6.95 V 335.77 278.34 459.96 278.34 253.13 337.79 Cr 19.27 13.06 5.03 13.06 4.12 5.95 Rb 126.66 63.60 171.38 63.60 55.26 75.82 Sr 912.24 1182.32 1182.07 1182.32 1151.69 1361.45 Y 36.28 39.22 37.70 39.22 37.90 41.16 Zr 385.83 360.25 465.20 360.25 353.74 359.82 Nb 121.95 138.41 166.13 138.41 148.53 165.09 Cs 0.66 0.57 0.89 0.57 0.61 0.69 Ba 532.86 616.73 718.21 616.73 762.33 731.69 La 81.92 90.80 107.66 90.80 92.75 107.19 Ce 150.89 162.13 196.68 162.13 165.72 177.49 Nd 67.70 68.37 84.74 68.37 67.35 73.75 Sm 12.09 11.87 14.22 11.87 11.46 12.57 Eu 3.62 3.32 4.13 3.32 3.32 3.65 Gd 9.98 9.61 11.54 9.61 9.38 10.72 Dy 7.26 7.42 8.33 7.42 7.17 8.24 Er 3.43 3.68 3.93 3.68 3.63 4.09 Yb 2.90 3.15 3.32 3.15 3.16 3.33 Lu 0.39 0.48 0.46 0.48 0.43 0.54 Hf 7.57 6.52 8.87 6.52 6.02 7.22 Ta 8.51 10.03 11.31 10.03 10.62 11.75 Pb 5.75 5.65 7.88 5.65 5.44 6.86 Th 8.50 9.98 11.41 9.98 10.62 11.91 U 1.85 2.60 2.47 2.60 2.78 3.09
F = amount of fractionation required S2 = sum of residuals squared
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Group 4. Fractional crystallisation models and trace element calculations for the Group 4 fractionation path (K361-K897-K894).
Group 4 Fractional Crystallisation Models K361-K897 K897-K894 Proportion of Phases Fractionated Cpx 29.8 K361 Cpx1a core 45.4 K894 Cpx3a core Plag 50.8 K894 Plag2 An56.9 20.9 K894 Plag2 An56.9 Ol 12.1 K361 Ol1a core Fo55 22.1 K361 Ol1b rim Fo49.8 Sp 7.3 K361 MgTi Sp 11.6 K894 Ti Sp
F 0.381 0.324 S2 0.298 0.355
Major Element Calculations Daughter Parent Calc. Parent Daughter Parent Calc. Parent
SiO2 44.04 44.73 44.98 45.84 44.04 44.23
TiO2 2.47 2.21 2.53 2.13 2.47 2.66
Al2O3 15.82 16.39 16.31 18.63 15.82 15.99 FeO 15.22 14.14 14.02 12.59 15.22 15.14 MnO 0.23 0.2 0.22 0.24 0.23 0.37 MgO 5.65 6.16 6.29 3.31 5.65 5.73 CaO 10.32 11.53 11.39 8.58 10.32 10.21
Na2O 4.18 3.59 3.55 5.14 4.18 3.8
K2O 1.44 0.65 1.01 2.57 1.44 1.8
P2O5 0.63 0.39 0.42 0.97 0.63 0.7
Trace Element Calculations Parent Daughter Calc. Daughter Parent Daughter Calc. Daughter Sc 20.56 13.98 20.77 13.98 4.25 11.53 V 317.62 268.64 513.12 268.64 133.06 397.40 Cr 0.00 24.13 0.00 24.13 8.96 Rb 13.81 27.59 19.95 27.59 78.61 38.69 Sr 813.04 734.79 648.53 734.79 1232.35 819.86 Y 20.56 27.23 28.77 27.23 29.89 33.90 Zr 165.97 228.59 220.64 228.59 274.90 268.60 Nb 52.07 83.86 82.64 83.86 152.97 121.51 Cs 0.29 0.44 0.45 0.44 0.75 0.63 Ba 439.70 546.68 606.57 546.68 904.48 751.24 La 38.08 60.24 58.22 60.24 95.73 85.41 Ce 70.29 108.49 101.13 108.49 161.86 141.83 Nd 32.13 46.76 45.80 46.76 61.69 60.16 Sm 6.17 8.36 8.67 8.36 9.95 10.53 Eu 1.99 2.49 2.78 2.49 2.82 3.22 Gd 5.18 6.98 7.59 6.98 7.77 9.29 Dy 4.07 5.24 5.95 5.24 5.68 6.95 Er 1.93 2.58 2.82 2.58 2.90 3.42 Yb 1.73 2.09 2.43 2.09 2.31 2.62 Lu 0.24 0.32 0.36 0.32 0.33 0.43 Hf 3.94 4.98 5.77 4.98 4.65 6.58 Ta 3.11 5.00 4.83 5.00 9.17 7.08 Pb 6.11 7.53 9.87 7.53 10.39 11.14 Th 4.49 7.53 7.05 7.53 12.71 10.86 U 1.07 1.74 1.67 1.74 2.89 2.50 F = amount of fractionation required S2 = sum of residuals squared
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APPENDIX B
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APPENDIX B – MAGMA MIXING MODEL Magma mixing models for mixing path K804-KSH11-K2225.
K804-KSH11-K2225 Evolved Magma Primitive Magma Percent Mixing K804 KSH11 K2225 K2225 0.00% 16.30% 100% K804 100.00% 83.70% 0% S2 0.753 Major Element Calculations observed observed calculated observed
SiO2 49.61 47.79 48.75 44.23
TiO2 2.22 2.42 2.22 2.26
Al2O3 17.27 17.2 16.61 13.18 FeO 11.82 12.74 12.11 13.61 MnO 0.18 0.19 0.19 0.21 MgO 4.35 4.97 5.05 8.64 CaO 8.05 8.69 8.93 13.4
Na2O3.443.383.44 3.4
K2O2.4522.15 0.57
P2O5 0.6 0.62 0.59 0.5 Trace Element Calculations observed observed calculated observed Sc 15.04 14.43 18.30 35.03 V 210.71 246.89 229.50 325.96 Cr 69.24 89.48 193.40 Rb 116.09 89.15 98.73 9.57 Sr 1316.21 881.56 1198.77 595.74 Y 39.01 30.29 36.64 24.42 Zr 325.54 270.48 304.04 193.67 Nb 94.29 92.91 90.54 71.26 Cs 0.38 0.71 0.36 0.28 Ba 704.94 695.26 649.84 366.92 La 78.98 72.09 74.06 48.82 Ce 143.20 129.50 134.35 88.91 Nd 60.55 53.39 57.18 39.90 Sm 10.80 9.75 10.23 7.31 Eu 2.91 2.71 2.79 2.16 Gd 8.78 7.66 8.41 6.53 Dy 7.24 5.88 6.84 4.79 Er 3.83 2.89 3.59 2.39 Yb 3.40 2.48 3.16 1.91 Lu 0.51 0.37 0.47 0.28 Hf 7.24 6.04 6.81 4.61 Ta 5.18 5.08 5.02 4.25 Pb 8.78 9.56 8.32 6.00 Th 10.52 9.13 9.78 5.96 U 2.60 1.02 2.40 1.34 S2 = sum of residuals squared
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APPENDIX C
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APPENDIX C – PRIMARY MAGMA COMPOSITIONS Major and trace element compositions of ‘fractionation corrected’ primary/primitive magmas. Models include olivine only addition, clinopyroxene and olivine addition required to reach minimum CaO content as determined from primary/primitive sample database, and clinopyroxene and olivine addition required to reach maximum CaO content determined from database.
Sample K2225 K2225 KSH03 KSH03 K813 K813 K813 K832a K832a K832a Ol Added 23% 19% 16% 10% 38% 26% 21% 36% 32% 21% Cpx Added 6% 10% 12% 18% 4% 16% Major elements (weight percent)
SiO2 42.68 43.38 43.85 45.04 40.42 41.87 42.68 43.49 43.95 45.44
Al2O3 10.25 10.03 11.31 10.81 9.64 9.57 9.38 10.08 9.88 9.66
TiO2 1.75 1.72 1.68 1.60 1.80 1.78 1.75 1.59 1.59 1.55
Fe2O3 2.23 2.13 2.14 1.99 2.47 2.23 2.12 2.44 2.37 2.14 FeO 11.35 10.84 10.94 10.16 12.58 11.40 10.80 12.46 12.06 10.89 MnO 0.17 0.16 0.18 0.17 0.20 0.20 0.19 0.15 0.15 0.15 MgO 16.28 15.61 15.63 14.47 18.53 16.49 15.66 18.11 17.45 15.54 CaO 10.42 11.35 9.83 11.51 9.55 11.64 12.69 8.71 9.38 11.53
Na2O 2.64 2.59 1.72 1.64 2.07 2.05 2.01 2.31 2.30 2.25
K2O 0.44 0.43 0.82 0.78 0.52 0.52 0.50 0.29 0.29 0.28 P2O5 0.39 0.38 0.37 0.36 0.51 0.51 0.50 0.38 0.38 0.37 Totals 98.56 98.59 98.44 98.52 98.18 98.20 98.24 99.92 99.74 99.75 Mg # 0.71 0.71 0.71 0.71 0.71 0.71 0.71 0.71 0.71 0.71 Trace elements (ppm) Sc 28.05 32.93 29.52 38.06 8.89 13.17 15.93 19.70 22.47 33.01 V 250.99 244.47 232.52 221.45 209.86 208.18 204.82 186.01 186.01 183.10 Cr 309.59 385.76 433.10 624.98 44.91 66.45 82.98 135.94 153.88 230.45 Rb 7.41 7.22 18.84 17.98 79.91 79.40 78.21 13.54 13.55 13.36 Sr 461.12 453.75 566.32 548.38 575.53 583.22 580.23 370.80 373.38 375.47 Y 18.85 19.53 19.05 20.05 22.78 25.85 27.25 17.07 17.84 20.08 Zr 151.88 150.58 165.09 161.87 249.21 256.09 256.77 127.39 128.85 131.44 Nb 55.01 53.58 53.43 50.89 76.58 75.91 74.67 40.03 40.02 39.37 Cs 0.22 0.21 0.20 0.19 0.42 0.42 0.41 0.29 0.29 0.28 Ba 284.75 277.77 384.85 367.47 337.77 335.65 330.65 287.65 287.80 283.86 La 37.69 37.02 39.36 38.01 51.44 51.97 51.61 30.14 30.32 30.39 Ce 68.56 67.19 72.02 69.27 94.57 95.05 94.15 55.58 55.82 55.66 Nd 30.77 30.45 33.12 32.35 42.43 43.54 43.59 26.48 26.77 27.26 Sm 5.64 5.66 6.01 6.02 7.58 8.04 8.18 4.89 4.99 5.25 Eu 1.67 1.69 1.80 1.82 2.28 2.47 2.54 1.48 1.52 1.63 Gd 5.04 5.10 5.04 5.10 6.27 6.75 6.93 4.32 4.44 4.74 Dy 3.70 3.75 3.76 3.82 4.57 4.94 5.07 3.33 3.42 3.67 Er 1.85 1.87 1.77 1.80 2.17 2.34 2.41 1.62 1.67 1.79 Yb 1.49 1.50 1.42 1.44 1.85 1.98 2.03 1.40 1.43 1.52 Lu 0.22 0.22 0.22 0.22 0.25 0.26 0.27 0.20 0.20 0.21 Hf 3.58 3.60 3.78 3.80 4.80 5.11 5.22 2.89 2.96 3.13 Ta 3.29 3.22 3.11 2.99 5.37 5.38 5.31 2.41 2.42 2.40 Th 4.61 4.51 4.12 3.95 5.36 5.35 5.28 3.31 3.31 3.28 U 1.04 1.02 0.74 0.71 1.17 1.17 1.16 0.76 0.76 0.75
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APPENDIX D
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APPENDIX D – REVERSE PARTIAL MELTING MODELS Source trace element concentrations calculated by reverse partial melting models of modal and non-modal equilibrium batch partial melts. Calculations are performed on ‘fractionation’ corrected samples which best represent primary/primitive magmas for samples K832a, K2225, KSH03 and K813. The degree of melting required to produce these primary/primitive samples is shown, corresponding with trace element abundances for the source required to produce them.
Source Concentrations
Modal Non-Modal Sample K832a K2225 KSH03 K813 K832a K2225 KSH03 K813 Olivine Added 32 23 16 26 32 23 16 26 Clinopyroxene Added 412412
Degree of Batch Melting to Produce Fractionation 10% 7% 7% 4% 10% 7% 7% 4% Corrected Magmas
Trace Elements (ppm) Sc 22.96 28.68 30.18 13.47 53.47 68.04 71.60 32.55 V 173.76 233.90 216.69 193.55 190.78 257.64 238.68 213.87 Cr 669.35 1381.21 1932.26 303.88 427.13 877.65 1227.79 192.31 Rb 1.96 0.86 2.19 6.97 4.34 2.20 5.60 21.81 Sr 64.42 66.84 82.09 68.46 124.51 143.52 176.26 168.56 Y 5.44 5.31 5.37 6.68 9.59 9.84 9.94 13.10 Zr 41.03 44.91 48.82 69.91 81.60 94.32 102.52 155.91 Nb 6.06 6.78 6.58 7.21 8.51 10.25 9.96 12.16 Cs 0.04 0.02 0.02 0.03 0.03 0.02 0.02 0.02 Ba 44.05 35.55 48.04 32.42 99.75 92.49 125.01 101.72 La 3.99 3.87 4.04 3.83 4.86 4.99 5.21 5.42 Ce 9.12 9.29 9.76 10.23 11.07 11.76 12.35 13.76 Nd 5.04 4.97 5.34 5.85 7.26 7.60 8.18 9.69 Sm 1.12 1.11 1.19 1.38 1.75 1.86 1.98 2.48 Eu 0.35 0.34 0.37 0.44 0.62 0.64 0.69 0.91 Gd 0.94 0.94 0.94 1.08 1.21 1.26 1.26 1.52 Dy 0.84 0.82 0.83 0.97 1.54 1.59 1.62 2.03 Er 0.42 0.42 0.40 0.47 0.49 0.50 0.48 0.58 Yb 0.47 0.46 0.44 0.57 0.85 0.86 0.82 1.12 Lu 0.06 0.06 0.06 0.06 0.06 0.06 0.06 0.06 Hf 0.68 0.73 0.77 0.90 1.43 1.67 1.76 2.29 Ta 0.34 0.37 0.35 0.45 0.53 0.64 0.60 0.90 Th 0.45 0.50 0.45 0.42 0.50 0.56 0.50 0.50 U 0.10 0.11 0.08 0.09 0.12 0.14 0.10 0.13
Mode Non-Modal Melting Proportions 53% Olivine 5% 22% Orthopyroxene 10% 18% Clinopyroxene 33% 2% Spinel 2% 5% Amphibole 50%
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APPENDIX E
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APPENDIX E – FORWARD PARTIAL MELTING MODELS Trace element results for forward modal batch melting models from the calculated source for 'Fractionation Corrected' Sample K2225.
Forward Modelling Trace Element Concentrations Source Degree of Melting 0% 0.01% 0.10% 1% 2% 3% 4% 5% 7% 10% Sc 28.68 28.00 28.00 28.01 28.02 28.02 28.03 28.04 28.05 28.07 V 233.90 252.37 252.36 252.18 251.98 251.78 251.58 251.38 250.99 250.40 Cr 1381.21 292.53 292.74 294.83 297.19 299.59 302.03 304.51 309.59 317.53 Rb 0.86 17.27 16.98 14.53 12.52 11.00 9.81 8.85 7.41 5.95 Sr 66.84 828.38 819.97 744.41 675.27 617.88 569.48 528.12 461.12 387.41 Y 5.31 23.33 23.25 22.57 21.85 21.18 20.54 19.95 18.85 17.42 Zr 44.91 185.00 184.48 179.46 174.19 169.22 164.52 160.08 151.88 141.05 Nb 6.78 118.28 116.56 101.71 89.11 79.28 71.40 64.95 55.01 44.74 Cs 0.02 0.64 0.62 0.50 0.41 0.35 0.30 0.27 0.22 0.17 Ba 35.55 601.94 593.42 519.91 457.01 407.69 367.97 335.31 284.75 232.23 La 3.87 109.89 107.24 86.43 71.11 60.39 52.49 46.41 37.69 29.40 Ce 9.29 131.68 130.14 116.49 104.34 94.48 86.32 79.46 68.56 56.87 Nd 4.97 50.50 50.09 46.30 42.71 39.63 36.97 34.65 30.77 26.35 Sm 1.11 8.12 8.07 7.64 7.22 6.83 6.49 6.18 5.64 4.99 Eu 0.34 2.35 2.34 2.22 2.11 2.00 1.91 1.82 1.67 1.48 Gd 0.94 7.50 7.45 7.01 6.58 6.20 5.87 5.56 5.04 4.42 Dy 0.82 5.03 5.00 4.78 4.56 4.36 4.17 4.00 3.70 3.32 Er 0.42 2.49 2.47 2.37 2.26 2.17 2.08 2.00 1.85 1.67 Yb 0.46 1.79 1.79 1.74 1.69 1.65 1.61 1.57 1.49 1.39 Lu 0.06 0.27 0.27 0.26 0.25 0.25 0.24 0.23 0.22 0.20 Hf 0.73 5.08 5.05 4.80 4.54 4.31 4.10 3.91 3.58 3.18 Ta 0.37 8.08 7.93 6.70 5.71 4.98 4.41 3.96 3.29 2.62 Th 0.50 12.16 11.91 9.87 8.30 7.15 6.29 5.61 4.61 3.64 U 0.11 2.90 2.83 2.31 1.92 1.64 1.44 1.28 1.04 0.82 K 0.20 0.67 0.67 0.65 0.64 0.62 0.61 0.60 0.57 0.54
Modal Melting Source Compositions CPX 18% SP 2% OL 53% OPX 22% AMPH 5%
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