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首页> 外文期刊>Contributions to Mineralogy and Petrology >Post-peak, fluid-mediated modification of granulite facies zircon and monazite in the Trivandrum Block, southern India
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Post-peak, fluid-mediated modification of granulite facies zircon and monazite in the Trivandrum Block, southern India

机译：峰后，流体介导的印度南部特里凡得琅地块中的粒状锆石和独居石的变质作用

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The quarry at Kottavattom in the Trivandrum Block of southern India contains spectacular examples of fluid-assisted alteration of high-grade metamorphic rocks. Garnet-biotite gneiss has undergone a change in mineral assemblage to form submetre scale orthopyroxene-bearing patches, later retrogressed to form an amphibole-bearing lithology. These patches, often referred to as arrested or incipient charnockite, crosscut the original metamorphic foliation and are typically attributed to passage of a low aH_2O fluid through the rock. Whilst this conversion is recognised as a late stage process, little detailed chronological work exists to link it temporally to metamorphism in the region. Zircon and monazite analysed from Kottavattom not only record metamorphism in the Trivandrum Block but also show internal, lobate textures crosscutting the original zoning, consistent with fluid-aided coupleddissolution-reprecipitation during formation of the orthopyroxene-bearing patches. High-grade metamorphism at the quarry occurred between the formation of metamorphic monazite at -585 Ma and the growth of metamorphic zircon at -523 Ma. The fluid-assisted alteration of the garnet-biotite gneiss is poorly recorded by altered zircon with only minimal resetting of the U-Pb system, whereas monazite has in some cases undergone complete U-Pb resetting and records an age for fluid infiltration at -495 Ma. The fluid event therefore places the formation of the altered patches at least 25 Myr after the zircon crystallisation in the garnet-biotite gneiss. The most likely fluid composition causing the modification and U-Pb resetting of zircon and monazite is locally derived hypersaline brine.
机译：印度南部特里凡得琅地块Kottavattom的采石场包含了高品位变质岩流体辅助蚀变的壮观例子。石榴石-黑云母片麻岩经历了矿物组成的变化，形成了亚米级规模的含邻苯二甲酚的斑块，随后逐渐退缩，形成了具有角闪石的岩性。这些斑块通常被称为滞留的或初期的菱镁矿，可切穿原始的变质叶面，通常归因于低aH_2O流体通过岩石。虽然这种转换被认为是后期过程，但很少有详细的时间顺序工作可以暂时将其与该地区的变质联系起来。从Kottavattom分析的锆石和独居石不仅记录了Trivandrum区块的变质作用，而且还显示出内部，呈叶状的纹理横切了原始地带，这与含邻苯二酚的斑块形成过程中的流体辅助耦合溶解-再沉淀相一致。采石场的高级变质发生在-585 Ma的变质独居石的形成与-523 Ma的变质锆石的生长之间。锆石蚀变记录的石榴石-黑云母片麻岩的流体辅助蚀变很难记录，仅对U-Pb系统进行了最小程度的复位，而独居石在某些情况下经历了完全的U-Pb复位并记录了-495的流体渗透年龄。嘛。因此，在石榴石-黑云母片麻岩中的锆石结晶后，液体事件使至少25 Myr的蚀变斑块形成。引起锆石和独居石的改性和U-Pb重置的最可能的流体成分是局部衍生的高盐盐水。

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