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The Geochemical Evolution of Alkaline Magmas from the Crary Mountains, Marie Byrd Land, Antarctica

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The Geochemical Evolution of Alkaline Magmas from the Crary Mountains, Marie Byrd Land, Antarctica THE GEOCHEMICAL EVOLUTION OF ALKALINE MAGMAS FROM THE CRARY MOUNTAINS, MARIE BYRD LAND, ANTARCTICA SUVANKAR CHAKRABORTY A Thesis Submitted to the Graduate College of Bowling Green State University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE May 2007 Committee: Kurt Panter, Advisor John Farver Thomas Wilch © 2007 Suvankar Chakraborty All Rights Reserved iii ABSTRACT Kurt Panter, Advisor Late Cenozoic alkaline volcanism in the Crary Mountains, Marie Byrd Land, Antarctica is associated with the West Antarctic rift system. More than 400 km 3 (LeMasurier et al., 1990) of alkaline magmas were erupted from four major volcanic centers. Based on previous field and dating studies the volcanism occurred between ~9 and <1 Ma and the activity migrated to the south at a rate of ~0.7 cm/yr. The Crary Mountains represent one of four ranges of volcanoes in Marie Byrd Land that show this type of age progression; younging away from the center of the province (LeMasurier and Rex, 1989; Panter et al., 2000). The volcanic deposits at the Crary Mountains consist of lavas and interbedded agglutinated scoria deposits as well as thick hydrovolcanic sequences (hyaloclastites, pillow breccia, tuff) all of which range in composition from basanite to intermediate types to phonolite, trachyte, and rhyolite. This study focuses exclusively on lava samples that are fresh and unaltered. Basaltic rocks are consisting of olivine, clinopyroxene, plagioclase and titanomagnetite phenocrysts. Phenocrysts found in intermediate to felsic compositions include of olivine, clinopyroxene, alkali amphibole, alkali and plagioclase feldspars, nepheline, apatite, titanomagnetite, and aenigmatite. Most of the phenocrysts of different rock types are either unzoned or normally zoned and disequilibrium textures are rare. Stratigraphic sections of Mount Steer and Rees show a complex petrographic history; however, the overall compositions of the Crary Mountains show a change to mafic composition with progression in age. The evolution trends and magma differentiation processes were determined based on geochemistry and geochemical relationships for these alkaline volcanic rocks. Overall, the iv alkaline lavas from the Crary Mountains show trends of increasing SiO 2, Na 2O, K 2O with decreasing MgO and CaO. On other hand highly incompatible trace elements (e.g., Th, La, Zr) show an increase in concentration with magmatic differentiation. On the basis of geochemical studies trachytes and phonolites are divided into two groups each; nepheline-normative trachytes ( Ne-trachyte) and quartz-normative trachytes ( Qtz -trachyte); low- alkali (Na 2O + K 2O <13.46 wt% ) high silica (> 60 wt%) phonolites and high-alkali (Na 2O + K 2O > 15.47 wt%) low silica (<56 wt%) phonolites. Using least-squares mass balance and Rayleigh fractionation models, the Ne-trachytes can best be explained by a single differentiation trend generated by fractional crystallization of a basanite magma. An explanation for the generation of quartz-normative trachytes ( Qtz -trachyte) and rhyolite can be generated by the assimilation of granitoid country rocks by mugearite magma coupled with fractional crystallization (AFC processes). Two different fractionation schemes are responsible for the production of phonolitic rocks; one that can explain the evolution of intermediate mugearite magmas to low-alkali phonolites and another from basanite to high-alkali phonolites. The modeling results indicate that the evolution of alkaline magmas erupted in the Crary Mountains is complex and these alkaline magmas were produced by a combination of magmatic differentiation processes. v It is theory that decides what can be observed Albert Einstein vi ACKNOWLEDGMENTS The author is deeply indebted to Dr. Kurt S Panter, Associate Professor, Department of Geology, Bowling Green State University for his supervision and guidance throughout this thesis. Secondly, the author would like to thank the Geology Department at Bowling Green State University for their challenging program from which he has learned so much. The author is grateful to Dr. Nelia Dunbar at New Mexico Institute of Mining and Technology for conducting microprobe analysis of the thin sections. Dr. Thomas I Wilch Associate Professor of the Department of Geology, Albion College provided geochemical data, maps and valuable basic geological information of the Crary Mountains. The author expresses heartiest thanks to Dr. John Farver, Department of Geology, Bowling Green State University for supporting him during the completion of the thesis work. The author gives his heartiest thanks to Mr. Bill Butcher and Mr. Jagannath Paul for their moral support during the completion of the dissertation work. The author remembers, with particular pleasure, the help, co-operation and comments offered by dearest friend Jaydeep Ghosh. Least but not the least the author wishes to recall the inspiration and blessing of his parents and his wife during the whole session of M.S. vii TABLE OF CONTENTS Page INTRODUCTION ................................................................................................................. 1 GEOLOGICAL SETTING .................................................................................................... 7 Regional Geology ...................................................................................................... 7 Volcanic Geology…………………………………………………………... 8 PETROLOGICAL DATA ..................................................................................................... 14 Petrographic types...................................................................................................... 14 Mineralogical Features............................................................................................... 16 Mineral Compositions……………………………………………………… 16 Analytical Methods………………………………………………… 16 Olivine................................................................................................ 16 Clinopyroxene.................................................................................... 21 Feldspar and feldspathoids.………………………………………. 25 Fe-Ti oxides………………………………………………………. 30 Amphibole………………………………………………………… 30 Aenigmatite, apatite and quartz…………………………………… 36 WHOLE-ROCK GEOCHEMISTRY .................................................................................... 41 Major Elements.......................................................................................................... 41 Trace Elements........................................................................................................... 50 DISCUSSION ………........................................................................................................... 55 Magmatic Evolution by Fractional Crystallization………………………………… 55 Ne-Trachyte Series………………………………………………………….. 59 viii Phonolite Series……………………………………………………………. 65 Qtz-Trachyte Series…………………………………………………………. 69 Magmatic Evolution by Assimilation of Crust……………………………………… 73 CONCLUSIONS…………………………………………………………………………… 79 REFERENCES ...................................................................................................................... 82 APPENDIX 1. PETROGRAPHY ........................................................................................ 96 APPENDIX 2.1. OVLIVINE CHEMISTRY......................................................................... 109 APPENDIX 2.2. CLINOPYROXENE CHEMISTRY…………………………………….. 112 APPENDIX 2.3 FELDSPAR CHEMISTRY……………………………………………... 120 APPENDIX 2.4. Fe-Ti OXIDE CHEMISTRY…………………………………………… 125 APPENDIX 2.5. AMPHIBOLE CHEMISTRY…………………………………………... 127 APPENDIX 2.6. APATITE CHEMISTRY……………………………………………….. 130 APPENDIX 2.7. AENIGMATITE CHEMISTRY………………………………………… 132 APPENDIX 3. MAJOR AND TRACE ELEMENT CHEMISTRY………………………. 134 ix LIST OF FIGURES Figure Page 1a Regional location map of the Marie Byrd Land volcanic province of West Antarctica (after Panter et al., 2000)............................................................................................ 4 1b An outcrop map of the Crary Mountains, Marie Byrd Land. .................................... 5 2a Cartoons simplified stratigraphic cross-sections showing major compositional rock types at Mounts Steere and Rees. …………………………………………………. 11 2b Detailed stratigraphic section of the Lie Cliff (Mount Steere) .................................. 12 2c Variation in compositions of different rock types of the Crary Mountains with age (Ma)……………………………………………………………………………. 13 3 Total alkali (wt%) versus SiO 2 (wt%) diagram (after Le Bas et al., 1986) ............... 15 4 Photomicrograph of olivine phenocrysts within felsic groundmass.......................... 19 5 Clinopyroxene quadrilateral showing compositions of olivine and clinopyroxene... 20 6 Photomicrograph of clinopyroxene phenocryst in phonolite………………………. 23 7a Back scatter image (BSE) of Ti-rich clinopyroxenes in hawaiite………………….. 24 7b Photomicrograph of Ti-rich clinopyroxene in hawaiite……………………………. 24 8 Classification of cores and rims of feldspar………………………………………… 28 9a BSE of rimmed plagioclase feldspar in benmorite…………………………………. 29 9b Photomicrograph of plagioclase feldspar in Benmorite……………………………. 29 10 Variation in magnetite phenocrysts composition…………………………………... 32 11 BSE of titanomagnetites phenocrysts in phonolite………………………………… 32 12 Photomicrograph of interstitial amphibole grain in phonolite…………………….. 35 13 BSE of fayalite (Fa 98 ) inclusion within amphibole………………………………… 35 x 14 Variations in amphibole composition……………………………………………..
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