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THESES SIS/LIBRARY TELEPHONE: +61 2 6125 4631 R.G. MENZIES LIBRARY BUILDING NO:2 FACSIMILE: +61 2 6125 4063 THE AUSTRALIAN NATIONAL UNIVERSITY EMAIL: [email protected] CANBERRA ACT 0200 AUSTRALIA USE OF THESES This copy is supplied for purposes of private study and research only. Passages from the thesis may not be copied or closely paraphrased without the written consent of the author. COMPARATIVE GEOCHEMISTRY OF SOME VOLCANIC SUITES OF THE SOLOMON ISLANDS AND BOUGAINV ILLE: IMPLICATIONS FOR METALLOGENESIS CROMWELL QOPOTO BSc. University of Papua New Guinea, Pon Moresby, Papua ew Guinea A THESIS SUBMITTED FOR THE DEGREE OF MASTER OF SCIENCE OF THE AUSTRALIAN NATIONAL UN IVERSITY November 2002 STATEMENT All the results presented in this thesis, and the conclusions drawn from them, are the author's own, except where otherwise indicated. The various persons who have assisted and guided the author are decailed in the acknowledgments, as are those who have helped in the preparation of this thesis. All sources are acknowledged in this thesis. I certify that the substance of this thesis has not already been submitted for any degree, and is not currently being submitted for any degree or qualification. Cromwell Qopoto II COMPARATIVE GEOCHEMISTRY OF SOME VOLCANIC SUITES OF THE SOLOMON ISLANDS AND BOUGAINV ILLE: lMPLICA TIONS FOR METALLOGENESIS TAB LE OF CONTENTS Page number Statement II Table of Contents 111 List of Figures VI List of Tables IX Acknowledgements x Abstract Chapter 1: Introduction 3 I .0 Previous work done 7 1. l Tectonic Settings and Regional Geology 7 i Te1Tane Model l I ii. Tectonic Style 12 iii. Metallogenesis 12 1.2 Local Geology 14 l.2. 1 Bougainville 14 1.2.2 Fauro Island Group 15 1. 2.3 Choiseul 17 1.2.4 Guadalcanal 19 1.3 Petrologic and Geochemical Studies 21 1.3.J Petrologic classification of the Solomon Arc 2 1 1. Pre-Miocene Volcanic Basement 22 11. Ultramafic Basement 22 iii. Intrusive Bodies 23 iv. Plio-Pleistocene: calc-alkaline volcanic rocks 23 1.3.2 Solomon Islands Volcanic Suites r_.) l.3.3 Bougainville Volcanic Groups 26 iii C ha pter 2: Geochemical Studies of the Solomon Isla nds Arc Systems 27 2.1 Geochemistry of the Solomon Arc 27 2. 1. l Solomon Islands Arc geochemical studies 27 i. Harker Diagrams 27 ii. AFM Diagram 31 2. 1.2 Bougainvillc Volcanic Suites Geochemical Studies 31 2.2.1 Whole rock (bulk) Geochemistry 31 2.2.2 Trace Element (bulk) Geochemistry 45 2.2.2.1 alkalies, alkaline earths. transition clemen ts, and rare ea rth elements 45 2.2.2.2 REE: Chondrite-Normalised pauerns 47 2.2.2.3 Primitive Mantle-Normalised "Spider" diagrams 49 Chapter 3: Melt Inclusion Studies 3. 1 Introduction 55 3.2 Evolution and Origin of Melt Inclusions 55 3.3 Rock Associations and Petrographic Studies of M ls 59 3.4 Different Types of Mls 60 3.5 Types of data recoverable from the analysis of Mis 60 i. Trapping and closure temperature 60 ii. Constraints on maxi mun and minimum cooling rates 61 iii. Bulk composition and liquid line of descent 61 iv. Volatile content 61 3.6 Mis resulting from melt-fluid immiscibility 61 3.7 Studies of M Js in Selected Solomon Islands Volcanic Suites 62 3.7.1 Bougai nville Volcanic Suites 62 3.7.2 Major Element Compositions of M is and Host Minerals 64 3.7.3 Fauro Volcanic Suites 65 3.7.4 Major Element Compositions of MJs and Host Minerals 72 3.7.5 Choiscul Volcanic Suites 82 i. Maetambe 82 ii. Kumboro 88 iv ii. Kumboro 88 3.7.6 Major Element Compositions of Mls and Host Minerals 88 3.7.7 Gold Ridge (Guadalcanal) Volcanics 92 3.7.8 Major Element Compositions of Mls and Host Minerals 92 3.8 Comparison of major element compositions of the Mis and host crystals for Bougainville, Faure, Choiseul, and Gold Ridge 97 Chapter 4: Discussion, Conclusions, and Further Studies 4.1 The texture and nature of the Mls and their fractionation trends 105 4.2 Geochemical compositions of the Mis and matrix glasses 105 4.3 Whole rock geochemical compositions 106 4.4 Trace element geochemical compositions 106 4.5 Conclusions 107 4.6 Fu11her Studies 108 References 109 Appendix 1 119 Appendix 2 120 v List of Figures Page Number Figure l. Map showing the location of the Solomon Islands. Specific areas of study for this thesis are indicated by the aITows. 4 Figure 2. P.J. Coleman's province model for the Solomon Islands (e.g., Coleman 1965; 1966; 1970). 8 Figure 3. Terrane model of the Solomon Islands (after M. G. Petterson, 1999). 10 Figure 4. Generalised geological map of Bougainville and Buka Islands after Rogerson et al. (1989). l3 Figure 5. Aerial view and general geological map of the Fauro I sland Group. 16 Figure 6. A general geological map of Choiseul. 18 Figure 7. Map showing the location of the Gold Ridge Volcanics and Gallego. 20 Figure 8. Petrographic features of the volcanic rocks. 24 Figure 9. Harker diagrams for the Fauro sui te (after Turner and Ridgway, 1982). 28 Figure 10. Harker diagrams for selected volcanic basements of different I slands of the Solomon Islands. 30 Figure 11. A-F-M diagram for selected rock suites from the Solomon Island arc 32 Figure 12. Relationship between K 20 and Si02 wt% and locations of individual volcanoes on Bougainville. 33 Figure 13. Harker diagrams for selected volcanic suites from Savo, Gallego, Fauro, Choiseul , and Gold Ridge. 37 Figure 14. Plots of MgO wt% vs other major oxides for selected bulk analyses of suites from Savo, Gallego, Fauro, Choiseul, and Gold Ridge. 38 Figure 15. Plots of K20 wt% vs major oxides for selected bulk analyses of sui tes from Savo, Gallego, Fauro, Choiseul, and Gold Ridge. 39 Figure 16. Plots of K 20 + Na20 wt% vs major oxides for selected bulk Analyses of sui tes from Savo, Gallego. Fauro, Choiseul, and Gold Ridge. 40 Figure 17. Abundances of selected large ion lithophile, ferromagnesian and chalcophile trace elements plotted against Si02 wt%. 41 Figure 18. Abundances of selected rare earth and high field strength elements plotted against Si02 wt%. 42 Figure 19. K 20 vs. Si02 wt% for selected volcanic suites from the Solomon Islands and Bougainville. 43 Figure 20. Total alkalies vs. Si02 wt% for selected volcanic suites from the VI Solomon ls lands and Bougainvi lie. 44 Figure 21. Rare earth element abundances normalised to chondritic meteorite values for selected rock types of the Fauro volcano. 46 Figure 22. Rare earth element abundances normalised to chondritic meteorite values for the calc-alkaline volcanoes of Choiseul (Kumboro and Maetambe) and si .licified volcanic breccias from the Gold Ridge Volcanics. 46 Figure 23. Rare earth element abundances normalised to chondritic meteorite Values for selected volcanic rocks from Bougainville. 48 Figure 24. Trace element abundances no1malised to "primitive mantle" for selected volcanic frocks from Fauro, Choiseul, and Gold Ridge. 5 J Figure 25. Trace element abundances no1malised to "primitive mantle" for selected volcanic frocks from Savo. 52 Figure 26. Trace element abundances normalised to " primitive mantle" for selected volcanic frocks from Gallego. 53 Figure 27. Trace element abundances normalised to "primitive mantle" for selected volcanic frocks from Bagana (Bougainville). 54 Figure 28. Photomicrographs of Mls in volcanics from Choiseul. 56 Figure 29. Photomicrographs of Mis in volcanics from Choiseul. 57 Figure 30. Photomicrographs of plagioclase- and clinopyroxene-hosted Mi s in volcanics from Choiseul. 58 Figure 31. Hypothetical outline of possible routes of magmatic differentiation. as interpreted from magmatic inclusion of melts. fluid and vapour. 63 Figure 32. Compatison of whole rocks and Mls hosted by va1ious phenocryst Phases projected in terms of total alkalies vs Si02 wt%. 70 Figure 33 . Major oxides of Mls plotted against silica for various Bougainville volcanics. 71 Figure 34. Two well developed glass inclusions embedded inside an Fe-Ti oxide crystal. 73 Figure 35 . Mis trapped in a quartz phenocrysr. 73 Figure 36. Comparison of whole rocks, M i s hosted by various phenocryst phases, and manix glasses projected in terms of total alkalies vs Si01 wt%, for Fauro. 80 Figure 37. Major oxide concentrations of Mis and mat1ix glasses from Fauro plotted against silica concentrations. 81 VII Figure 38. Mis hosted by various crystals wi thin a coarse grained pegmatite 87 Figure 39. MJs trapped in a plagioclase host (Maetambe). 87 Figure 40. Mis and vari ous crystalline phases (including Fe-Ti ox ides) trapped in phenocrysts of hornblende, biotite, and clinopyroxene. 89 Figure 41. Plots of major ox ide concentrations vs . silica of Mis and matrix glasses in volcanic rocks from Choiseul. 90 Figure 42. Comparison of whole rocks, Mls hosted by various phenocryst phases, and matrix glasses projected in terms of total alkalies vs SiO ~ wt%, fo r Choiseul. 9 l Figure 43. Major oxide compositions of Mis and matrix glasses plolted against si lica for Gold Ridge. 95 Figure 44. Comparison of whole rocks, Mis hosted by various phenocryst phases, and mat1ix glasses projected in terms of total alkal ies vs Si02 wt%, for Gold Ridge. 96 Figure 45. Pl ots of major oxides vs SiO ~ wt% for Mls hosted by clinopyroxene. 98 Figure 46. Pl ots of major ox ides vs Si02 wt% for Mls hosted by plagioclase. 99 Figure 47. Pl ots of major ox ides vs Si02 wt% fo r Mis hosted by Fe-Ti oxides. 100 Figure 48. Plots of major oxides vs Si02 wt% fo r Mls hosted by quartz. lO I Figure 49.

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