MAGMATIC EVOLUTION OF THE SHIRA VOLCANICS, MT KILIMANJARO ... MAGMATIC EVOLUTION OF THE SHIRA...

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Transcript of MAGMATIC EVOLUTION OF THE SHIRA VOLCANICS, MT KILIMANJARO ... MAGMATIC EVOLUTION OF THE SHIRA...

  • School of Natural Resource Sciences

    Queensland University of Technology

    MAGMATIC EVOLUTION OF THE

    SHIRA VOLCANICS, MT

    KILIMANJARO, TANZANIA

    By

    Stephen John Hayes

    B.App.Sc. (QUT)

    2004

    Supervisor:

    Associate Professor David A. Gust

    Queensland University of Technology

    A Thesis submitted for the degree of Master of Applied Science

    (Queensland University of Technology)

  • KEYWORDS

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

    evolution, fractional crystallisation

  • I

    ABSTRACT

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

  • II

    TABLE OF CONTENTS

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

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

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

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

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

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

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

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

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

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

  • III

    LIST OF FIGURES

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

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

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

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

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

  • IV

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

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

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

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

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

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

    LIST OF TABLES

    Table 1. Analytical precision of EDS microprobe results 15 Table 2. Analytical precision of ICP-AES major element results