| 晚期折返过程含石墨流体包裹体及其意义：以印度两个不同地体为例 |
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| 作者姓名： | M.K.Panigrahi S.Gupta |
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| 作者单位： | Department of Geology ＆ Geophysics, L L T. , Kharagpur, 721302, lndia |
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| 基金项目： | We acknowledge financial support from the Department of Science and Technology, Government of India, Grants No ESS/16/116/98, ESS/16/1999/2003 and SR/FST/ESI-015/200, the last one providing instrumental support for the fluid inclusion microthermometry and LASER Raman Spectrometry.MKP acknowledges financial support from the host Institute for attending the first meeting of ACR0FI at Nanjing. |
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| 摘 要： | Archean greenstone belts and Proterozoic granulite mobile belts are products of fundamentally different tectonic processes that culminated in different levels of crustal incision.The present study focuses on graphite-bearing fluid inclusions from two such terrains in India,the Angul domain of Eastern Ghats Mobile Belt and Hutti-Maski schist belt of the eastern Dharwar greenstone-granite belt.In beth cases,a high population of such inclusions within the fluid inclusion assemblage rules out the possibility of graphite being a captive phase,and instead confirms that it was deposited by the fluid within the inclusion cavity.Graphite is usually observed to be occurring with either pure water or a pure carbonic( CO_2 only)liquid,or with a CH_4 dominated carbonic liquid without vapor at room temperature.Graphite precipitation in inclusions is brought about by reaction of the CO2 and CH4 trapped as a homogeneous fluid to give rise to H_2O and C(graphite).Molar volume calculations for the CO_2-CH_4 mixture assuming an appropriate PVTX relationship indicates that there is a substantial increase in volume with decreasing pressure at a given temperature.The reaction producing graphite and H_2O from CH_4 and CO_2 involves substantial volume reduction,and hence would be favored when the rock undergoes rapid exhumation.Graphite-beating inclusions in quartz in a late-stage leucosome from migmatites in the Angul domain of the EGMB are accompanied by other fluid inclusion evidence for isothermal decompression.In the Hutti-Maski schist belt of the eastern Dharwar Craton,graphite-bearing inclusions occur in structurally controlled quartz veins(often auriferous)within metamorphosed mafic volcanics(schists and amphibolites).The Raman spectra indicate that graphites in fluid inclusions from the Hutti-Maski schist belt have both ordered(O)and the disordered(D)peaks,whereas those from the Angul area of EGMB lack the disordered(D)peaks, with both having perfectly symmetrical‘S’peak.This implies that in both belts,exhumation from the burial depth maxima was a rapid process.However,the Hutti-Maski schist belt experienced a lower amount of uplift than the Angul domain,where the driving mechanism led to a deeper level of incision.This difference in the extent and rate of exhumation is speculated to be related to a fundamental difference in the nature of tectonism.A more detailed comparative study of the fluid inclusion characteristics would possibly throw more light on the changing tectonic style from the Archean to the Proterozoic,a topic that is extensively debated.
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| 关 键 词： | 印度 晚期折返过程 石墨 流体包裹体 意义 |
| 文章编号： | 1000-0569/2007/023（01）-0053-64 |
| 修稿时间： | 2006-08-01 |
Graphite bearing fluid inclusions and their implications to late-stage exhumation processes: Case studies from two disparate terrains in India |
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| M.K.Panigrahi S.Gupta.Graphite bearing fluid inclusions and their implications to late-stage exhumation processes: Case studies from two disparate terrains in India[J].Acta Petrologica Sinica,2007,23(1):53-64. |
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| Authors: | M K Panigrahi S Gupta |
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| Abstract: | Archean greenstone belts and Proterozoic granulite mobile belts are products of fundamentally different tectonic processes that culminated in different levels of crustal incision. The present study focuses on graphite-bearing fluid inclusions from two such terrains in India, the Angul domain of Eastern Ghats Mobile Belt and Hutti-Maski schist belt of the eastern Dharwar greenstone-granite belt. In both cases, a high population of such inclusions within the fluid inclusion assemblage rules out the possibility of graphite being a captive phase, and instead confirms that it was deposited by the fluid within the inclusion cavity. Graphite is usually observed to be occurring with either pure water or a pure carbonic ( CO2 only) liquid, or with a CH4 dominated carbonic liquid without vapor at room temperature. Graphite precipitation in inclusions is brought about by reaction of the CO2 and CH4 trapped as a homogeneous fluid to give rise to H2O and C (graphite). Molar volume calculations for the CO2-CH4 mixture assuming an appropriate PVTX relationship indicates that there is a substantial increase in volume with decreasing pressure at a given temperature. The reaction producing graphite and H2O from CH4 and CO2 involves substantial volume reduction, and hence would be favored when the rock undergoes rapid exhumation. Graphite-bearing inclusions in quartz in a late-stage leucosome from migmatites in the Angul domain of the EGMB are accompanied by other fluid inclusion evidence for isothermal decompression. In the Hutti-Maski schist belt of the eastern Dharwar Craton, graphite-bearing inclusions occur in structurally controlled quartz veins (often auriferous ) within metamorphosed mafic volcanics (schists and amphibolites). The Raman spectra indicate that graphites in fluid inclusions from the Hutti-Maski schist belt have both ordered (O) and the disordered (D) peaks, whereas those from the Angul area of EGMB lack the disordered (D) peaks, process. However, the Hutti-Maski schist belt experienced a lower amount of uplift than the Angul domain, where the driving mechanism led to a deeper level of incision. This difference in the extent and rate of exhumation is speculated to be related to a fundamental difference in the nature of tectonism. A more detailed comparative study of the fluid inclusion characteristics would possibly throw more light on the changing tectonic style from the Archean to the Proterozoic, a topic that is extensively debated. |
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| Keywords: | Graphite Fluid inclusions Exhumation processes Disparate terrains India |
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