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Mechanism of Formation and Age of the Ayyarmalai A-Type Charnockite

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Mechanism of Formation and Age of the Ayyarmalai A-Type Charnockite Mechanism of Formation and Age of the Ayyarmalai A-type Charnockite – Granite association from the South-Eastern Palghat- Cauvery Shear System, Southern India Stephanie. E. Rowe Tectonics Resources and Exploration (TRaX), Department of Geology and Geophysics, School of Earth and Environmental Sciences, University of Adelaide, South Australia. E-Mail: [email protected] OCTOBER 2010 S. Rowe, 2010 Table of Contents ABSTRACT................................................................................................................................................. 3 1. INTRODUCTION................................................................................................................................... 5 1.1. Charnockite Definition & Problem ................................................................................................ 5 1.2. Characteristics of Igneous and Metamorphic Charnockites........................................................ 6 2. GEOLOGICAL SETTING .................................................................................................................... 8 2.1. Geology of India ............................................................................................................................... 8 2.1.1. Dharwar Craton & The Northern Granulite Terrain ................................................................. 8 2.1.2. Southern Granulite Terrain ........................................................................................................ 9 2.2. Palghat-Cauvery Shear System .................................................................................................... 10 3. FIELD RELATIONS & SAMPLE PETROLOGY ........................................................................... 11 3.1.Field Relationships ...................................................................................................................... 12 3.2.Ayyarmalai Quarry Massif Charnockite .................................................................................. 12 3.2.1. Garnet & Hornblende Rich Charnockites ............................................................................. 12 3.2.2. Mafic Enclaves....................................................................................................................... 14 3.2.3. Charncokite Gneiss Zones ..................................................................................................... 14 3.3. Ayyarmalai Granite ..................................................................................................................... 14 3.3.1. Pink Alkali Granite ................................................................................................................... 14 3.3.2. Grey Granite............................................................................................................................. 15 3.3.3. Dehydration Zones.................................................................................................................... 15 3.3.4. Pegmatite .................................................................................................................................. 15 4. ANALYTICAL PROCEDURES ......................................................................................................... 16 4.1. Petrography & Mineral Chemistry ............................................................................................. 16 4.1.1 Pressure- Temperature Estimates.............................................................................................. 16 4.2. Whole rock Major and Trace Elements....................................................................................... 17 4.2.1 Trace Elements......................................................................................................................... 17 4.3. U-Pb Zircon Dating........................................................................................................................ 18 4.4. Whole Rock Isotopic Analyses ..................................................................................................... 19 4.4.1. Radiogenic Isotopes.................................................................................................................. 19 4.4.2. Stable Isotopes .......................................................................................................................... 19 1 S. Rowe, 2010 5. RESULTS .............................................................................................................................................. 20 5.1. Mineral Chemistry......................................................................................................................... 20 5.1.1. Ayyarmalai Charnockite .......................................................................................................... 20 5.1.1.1. Garnet .......................................................................................................................... 20 5.1.1.2. Pyroxenes..................................................................................................................... 21 5.1.1.3. Retrograde Minerals.................................................................................................... 21 5.1.2.P-T Estimate (Charnockite) ....................................................................................................... 22 5.2. Whole Rock Geochemistry............................................................................................................ 22 5.2.1. Ayyarmalai Charnockites......................................................................................................... 22 5.2.2. Ayyarmalai Pink and Grey Alkali Granites ............................................................................. 23 5.3. U-Pb Zircon Geochronology.......................................................................................................... 24 5.3.1. Charnockite Zircons (SR-6 and SR-57) ................................................................................... 25 5.3.2. Ayyarmalai Granites (SR-68H and SR-63).............................................................................. 26 5.4. Isotopes............................................................................................................................................ 27 5.4.1. Radiogenic Isotopes................................................................................................................. 27 5.4.1.1 Sm-Nd............................................................................................................................... 27 5.4.1.2. Rb-Sr................................................................................................................................ 27 5.4.1.3. Pb-Pb................................................................................................................................ 28 5.4.2. Stable Isotopes .......................................................................................................................... 28 5.4.2.1. δ18O isotopes.................................................................................................................... 28 6. DISCUSSION........................................................................................................................................ 29 6.1. Petrographic and Geochemical Interpretation............................................................................ 29 6.3. Timing and significance of charnockite and felsic magmatism in the PCSS............................ 31 6.4. Protolith and Crustal Evolution ................................................................................................... 33 7. CONCLUSION ..................................................................................................................................... 36 8. ACKNOWLEDGEMENTS.................................................................................................................. 37 9. REFERENCES...................................................................................................................................... 37 10. FIGURE CAPTIONS ......................................................................................................................... 45 11. APPENDIX A – Supplementary Data Tables 12. APPENDIX B – Supplementary Figures 2 S. Rowe, 2010 MECHANISM OF FORMATION AND AGE OF THE AYYARMALAI A-TYPE CHARNOCKITE – GRANITE ASSOCIATION FROM THE SOUTH-EASTERN PALGHAT-CAUVERY SHEAR SYSTEM, SOUTHERN INDIA ABSTRACT The Ayyarmalai A-type charnockite and A-type alkali granite lies on the south-eastern margin of the Palghat-Cauvery Shear System and provides an example of co-magmatism that was later overprinted with granulite facies metamorphism at ~2.45-2.5Ga. The Palghat-Cauvery Shear System represents an intriguing zone with Neoproterozoic aged granulites (~800-500 Ma) to the south and Archaean granulites (~3000-2500 Ma) to the north; the origins of which are still often disputed. This study presents whole rock major and trace element compositions, mineral chemistry, pressure-temperature estimates and whole rock Sm-Nd, Rb-Sr, Pb-Pb and δ18O isotopic compositions of this A-type charnockite-granite association found at Ayyarmalai, Tamil Nadu, Southern India. The subsequent data from this study suggests that: (1) the Ayyarmalai charnockites from the Palghat-Cauvery Shear System have zircon ages that
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