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Chronological and geochemical studies of granite and enclave in Baimashan pluton,Hunan, South China |
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| Authors: | Chen?WeiFeng |
| Institution: | 1. State Key Laboratory of Mineral Deposit Research,Department of Earth Sciences,Nanjing University,Nanjing 210093,China 2. Research Institute No.230,CNNC,Changsha 410011,China 3. State Key Laboratory of Mineral Deposit Research,Department of Earth Sciences,Nanjing University,Nanjing 210093,China;Guangzhou Institute of Geochemistry,Chinese Academy of Sciences,Guangzhou 510640,China |
| Abstract: | Zircon LA-ICP-MS U-Pb dating reveals that the Baimashan Pluton is composed mainly of late Indosinian (204.5±2.8 Ma-209.2±3.8 Ma) biotite granodiorites/monzonitic granites (LIGs) and early Yanshanian (176.7±1.7 Ma) two-micas monzonitic granites (EYGs), and the coeval (203.2±4.5 Ma-205.1±3.9 Ma) mafic microgranular enclaves (MMEs) are generally found in the former. In addition, the ages of cores within zircons from LIGs and MMEs ranging from 221.4±4.0 Ma to 226.5±4.1Ma provide evidence of multistage magma intrusion during Indosinian in the study area. Measured 3010±20.6 Ma of inherited zircon age suggests that there may be recycling Archaean curstal material in existence in this area. LIGs and EYGs share some similar geochemical features: subalkaline and peraluminous granites, enrichment of Th, U, K, Ta, Zr, Hf and LREE but depletion of Ba, Nb, P, Ti and Eu, low ɛ Nd(t) values but high (87Sr/86Sr)i ratios, and old T 2DM (ca. 1.9–2.0 Ga). The behaviors of incompatible elements and REE are mainly dominated by fractional crystallization of plagioclase, K-feldspar, ilmenite and apatite, but that of Sr isotope mainly controlled by EC-AFC. They are crust-sourced and derived from partial melting of paleo-Proterozoic metagreywackes and related to biotite dehydration melting. LIGs are formed in post-collisional tectonic setting as crustal local extension and thinning during late Indosinian. But EYGs may be evolved products of congeneric granitic magma with LIGs formed in late Indoinian, which were emplaced again when crust underwent extensive thinning and extension in post-orogenic tectonic setting during Yanshanian in SC after undergoing EC-AFC. MMEs should be cognate enclaves and derived from liquid immiscibility of host magma. Supported by the National Natural Science Foundation of China (Grant No. 40372036) and the Key Project of Ministry of Education of China (Grant No. 306007) |
| Keywords: | granite mafic microgranular enclave zircon LA-ICP-MS dating lithogenesis EC-AFC |
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