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首页> 外文期刊>Chemical geology >Petrology and U-Th-Pb geochronology of the sapphirine-quartz-bearing metapelites from Rajapalayam, Madurai Block, Southern India: Evidence for polyphase Neoproterozoic high-grade metamorphism
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Petrology and U-Th-Pb geochronology of the sapphirine-quartz-bearing metapelites from Rajapalayam, Madurai Block, Southern India: Evidence for polyphase Neoproterozoic high-grade metamorphism

机译：印度南部马杜赖区块拉贾拉帕拉姆邦含蓝宝石-石英的变质岩的岩石学和U-Th-Pb年代学：多元新元古代高变质证据

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摘要

Strongly migmatitic sapphirine-bearing granulites occur as dm- to m-sized enclaves in garnetiferous leucogranites in the southeastern part of the Madurai Block (MB), the largest section of Neoproterozoic crust in the Southern Granulite Terrain of India (SGT). Early mineral assemblages (biotite-sillimanite-quartz, garnet-biotite-quartz) break down via successive dehydration melting reactions to high-T and high-P phase assemblages (e.g. garnet-melt, orthopyroxene-sillimanite-melt, orthopyroxenecordierite-melt). Melt-consuming reactions along the retrograde path led to partial resorption of garnet to orthopyroxenesillimanite, orthopyroxene-sapphirine and/or orthopyroxene-cordierite as well as orthopyroxene-sillimanite assemblages to cordierite-biotite-quartz symplectites. The occurrence of UHT metamorphism is indicated by the presence of small inclusions of sapphirine-quartz intergrowths in garnet. This reaction sequence is interpreted in a KFMASH partial petrogenetic grid taking both melt-producing and melt-consuming reactions into account which yields a clockwise beating-cooling-decompression P-T path culminating at 8-11 kbar and 1000-1100 degrees C. In-situ U-Th-Pb monazite dating has been performed to more precisely constrain the ages given by chemical electron-microprobe on monazite in various textural positions. Monazite inclusions in garnet often yield a bimodal distribution of early (950-850 Ma) and late Neoproterozoic, (600-500 Ma) ages which is in good agreement with the results of U-Pb and EPMA zircon dating in other places of the southeastern Madurai Block. By contrast, monazite inclusions in other mafic minerals (orthopyroxene, cordierite, biotite) or as interstitial phase exclusively yield ages between 550 and 480 Ma. The early Neoproterozoic population is interpreted to reflect the timing of migmatisation and peak UHT metamorphism, whereas the late Neoproterozoic ages are inferred to date cooling and exhumation with granitic magmas to a higher crustal level of the Madurai Block. (c) 2007 Elsevier B.V. All rights reserved.
机译：在印度南部花岗岩地区（SGT）的新元古代地壳的最大部分，马杜赖块体（MB）东南部的石榴石状隐花花岗岩中，具有强烈隐伏性蓝宝石的花岗岩以dm至m大小的飞地形式出现。早期矿物组合（黑云母-硅线石-石英，石榴石-黑云母-石英）通过对高T和高P相组合的连续脱水熔化反应而分解（例如，石榴石熔体，邻辉石-硅线石熔体，邻辉石-堇青石-熔体）。沿逆行路径的消耗熔体的反应导致石榴石部分吸收为邻辉石-硅锰铁矿，邻辉石-蓝宝石和/或邻辉石-堇青石以及邻辉石-硅线石组合体为堇青石-黑云母-石英手榴石。石榴石中存在蓝宝石-石英共生物的小包裹体表明了UHT变质的发生。在KFMASH部分成岩网格中解释了该反应顺序，同时考虑了产生熔体和消耗熔体的反应，从而产生了顺时针跳动-冷却-减压PT路径，最终在8-11 kbar和1000-1100℃达到终点。进行U-Th-Pb独居石定年可以更精确地限制化学电子微探针在独居石上各种结构位置给出的年龄。石榴石中的独居石夹杂物经常在新元古代（950-850 Ma）和新元古代（600-500 Ma）晚期产生双峰分布，这与东南部其他地方的U-Pb和EPMA锆石测年结果非常吻合。马杜赖街区。相比之下，其他镁铁质矿物（邻辉石，堇青石，黑云母）或间隙相中的独居石夹杂物仅产生550-480 Ma的年龄。新元古代早期的种群被解释为反映了迁移的时间和UHT变质的高峰，而据推测，新元古代晚期是由花岗岩岩浆降温和发掘到马杜赖地块较高的地壳。 （c）2007 Elsevier B.V.保留所有权利。

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