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研究生: 阮氏妙
Nguyen Thi Dieu
論文名稱: Geochronology and geochemical characteristics of Precambrian granite in the Main Islands, Republic of Seychelles
Geochronology and geochemical characteristics of Precambrian granite in the Main Islands, Republic of Seychelles
指導教授: John Gregory Shellnutt
John Gregory Shellnutt
學位類別: 碩士
Master
系所名稱: 地球科學系
Department of Earth Sciences
論文出版年: 2019
畢業學年度: 107
語文別: 英文
論文頁數: 119
中文關鍵詞: Seychellesferroan granitepost-collisonal extensionzircon geochronology
英文關鍵詞: Seychelles, ferroan granite, post-collisonal extension, zircon geochronology
DOI URL: http://doi.org/10.6345/THE.NTNU.DES.001.2019.B07
論文種類: 學術論文
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  • The Seychelles microcontinent located in western Indian Ocean consists of a large number of granitic islands. The oldest known rocks of the Seychelles microcontinent are Neoproterozoic and comprise the Main Islands (Mahé and Praslin group). Previous studies demonstrated that the emplacement age of the Neoproterozoic granites is ~750 Ma although the age of Île aux Récifs is ~50 million years older (808.8±1.9 Ma). New weighted-mean zircon 238U/206Pb ages for rocks collected from northern Mahé (750.6±4.1 Ma, 756.9±4.5 Ma, 756.4±5.1 Ma) and western Praslin (761±12 Ma, 754.2±6.9 Ma, 753.8±4.9 Ma) are reported in this study. The new radioisotopic ages overlap with previous results but a significant number of inherited zircons were identified that fall within two major groups: ~790 Ma to ~820 Ma and ~850 Ma to 880 Ma. Geochemically, the Seychelles granites are characterized by high SiO2 and are metaluminous to peraluminous, alkali-calcic to calc-alkalic, and ferroan. The negative correlation of SiO2 with Al2O3, Fe2O3, CaO, TiO2, MgO and P2O5 indicates that the rock experienced crystal fractionation either during or after emplacement. They also show the enrichment in light rare earth elements compared to heavy rare earth elements with distinct negative Eu anomalies (0.1-0.84). Mahé samples illustrate positive εNd(t) = +1.85 to +2.83, and ISr values from 0.7022 to 0.7031 while those of Praslin shows negative εNd(t) = -1.52 to -1.29 and extremely low ISr values. Consequently, it appears that the Mahé and Praslin granites are derived from different sources. Tectonomagmatic discrimination of the granites is compatible with within-plate or post-orogenic environment. The ~50 million year gap between the oldest and youngest rocks, and their compositional (within-plate vs. volcanic-arc) dichotomy are difficult to reconcile. It is likely that Neoproterozoic granites of the Seychelles microcontinent were not associated with a volcanic-arc but rather related to post-collision extension or rifting. Furthermore, it is possible that the age of Île aux Récifs may be indicative of inherited zircon and not the emplacement age.

    The Seychelles microcontinent located in western Indian Ocean consists of a large number of granitic islands. The oldest known rocks of the Seychelles microcontinent are Neoproterozoic and comprise the Main Islands (Mahé and Praslin group). Previous studies demonstrated that the emplacement age of the Neoproterozoic granites is ~750 Ma although the age of Île aux Récifs is ~50 million years older (808.8±1.9 Ma). New weighted-mean zircon 238U/206Pb ages for rocks collected from northern Mahé (750.6±4.1 Ma, 756.9±4.5 Ma, 756.4±5.1 Ma) and western Praslin (761±12 Ma, 754.2±6.9 Ma, 753.8±4.9 Ma) are reported in this study. The new radioisotopic ages overlap with previous results but a significant number of inherited zircons were identified that fall within two major groups: ~790 Ma to ~820 Ma and ~850 Ma to 880 Ma. Geochemically, the Seychelles granites are characterized by high SiO2 and are metaluminous to peraluminous, alkali-calcic to calc-alkalic, and ferroan. The negative correlation of SiO2 with Al2O3, Fe2O3, CaO, TiO2, MgO and P2O5 indicates that the rock experienced crystal fractionation either during or after emplacement. They also show the enrichment in light rare earth elements compared to heavy rare earth elements with distinct negative Eu anomalies (0.1-0.84). Mahé samples illustrate positive εNd(t) = +1.85 to +2.83, and ISr values from 0.7022 to 0.7031 while those of Praslin shows negative εNd(t) = -1.52 to -1.29 and extremely low ISr values. Consequently, it appears that the Mahé and Praslin granites are derived from different sources. Tectonomagmatic discrimination of the granites is compatible with within-plate or post-orogenic environment. The ~50 million year gap between the oldest and youngest rocks, and their compositional (within-plate vs. volcanic-arc) dichotomy are difficult to reconcile. It is likely that Neoproterozoic granites of the Seychelles microcontinent were not associated with a volcanic-arc but rather related to post-collision extension or rifting. Furthermore, it is possible that the age of Île aux Récifs may be indicative of inherited zircon and not the emplacement age.

    ACKNOWLEDGEMENT ii Table of contents iii List of figures vi List of pictures ix List of tables x Abbreviations xi Abstract 1 CHAPTER 1. INTRODUCTION 3 1.1. Continental crust 3 1.2. Super continental cycles 4 1.3. Relationship between Rodinia and Gondwana 5 1.4. East African Orogen 8 1.5. The Seychelles microcontinent 12 1.6. Purpose of the study 14 CHAPTER 2. GEOLOGICAL BACKGROUND 18 2.1. Mahé group island 19 2.2. The Praslin - La Digue island group 22 2.3. Sampling 24 CHAPTER 3. PETROGRAPHY 28 3.1. Mahé granites 28 3.2. Praslin granites 31 CHAPTER 4. METHODS 36 4.1. Zircon U-Pb dating by LA-ICPMS 36 4.1.1. Zircon preparation 36 4.1.2. Principles of zircon U-Pb geochronology 37 4.1.3. Procedure of U-Pb geochronology 38 4.2. Whole rock major elements by XRF 39 4.2.1. Principles of X-ray Fluorescence spectrometry 39 4.2.2. Procedure 39 4.3. Whole rock trace elements by ICP-MS 41 4.3.1. Principles of ICP-MS 41 4.3.2. Procedure 42 4.4. Whole rock Sr-Nd isotopes by TIMS 43 4.4.1. Principles of TIMS operation 43 4.4.2. Procedure 43 CHAPTER 5. RESULTS 46 5.1. Zircon geochronology 46 5.1.1. Zircons morphology 46 5.1.2. Zircon U-Pb ages 49 5.2. Geochemistry 54 5.2.1. Major elements geochemistry 54 5.2.2. Trace element geochemistry 58 5.2.3. Sr-Nd Isotopes 61 CHAPTER 6. DISCUSSION 64 6.1. Geochronology of the Seychelles microcontinent 64 6.1.1. Age of emplacement 64 6.1.2. Age of inheritance 65 6.1.3. Regional temporal correlation 67 6.2. Petrogenesis 68 6.3. Tectonic setting of the Seychelles microcontinent 70 6.3.1. Tectonic setting 70 6.3.2. Regional tectonic correlation 74 6.4. Tectonomagmatic evolution of the Seychelles microcontinent 76 CONCLUSIONS 78 REFERENCES 79 Appendix A 92 Appendix B 99

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