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首页> 外文期刊>Journal of Asian earth sciences >The Nagercoil Granulite Block, southern India: petrology, fluid inclusions and exhumation history
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The Nagercoil Granulite Block, southern India: petrology, fluid inclusions and exhumation history

机译：印度南部的纳格柯尔花岗岩块：岩石学，流体包裹体和发掘史

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The Nagercoil Granulite Block (NGB) constitutes the southernmost domain in the high-grade metamorphic terrain of southern India and dominantly comprises orthopyroxene-K-feldspar-plagioclase-quartz-biotite-ilmenite bearing massive charnockites. Where they are mixed with aluminous supracrustals, the charnockites contain garnet, often-displaying typical decompression textures with coronal orthopyroxene and plagioclase surrounding relict garnet grains. Orthopyroxene is preserved in near-pristine state in the charnockite suggesting low water activity. Garnet-orthopyroxene thermometry of NGB charnockites yields temperatures of 691-934 ℃. Garnet-biotite thermometry yield values that broadly overlap (538-864 ℃) with the above estimate, although some lower values indicative of retrograde Fe-Mg exchange are also obtained. The presence of mesoperthite in the rock and high fluorine content in biotites also suggest high metamorphic temperatures. Charnockite barometry using garnet-orthopyroxene-plagioclase-quartz assemblage yields 4.0-6.3 kbar. An overall drop in pressure by over 2 kbar correlate with the isothermal decompression history inferred from textural criteria. Detailed fluid inclusion petrographic, microthermometric and laser Raman spectroscopic data presented here allow an evaluation of the fluid evolution history of NGB charnockites. We distinguish four groups of inclusions, with the earliest generation characterised by CO_2 with variable but minor proportion of H_2O. Subsequent generations are dominated by CO_2. Locally, CH_4 and N-2 occur as trace components. CO2-rich inclusions dominate in all minerals: garnet, K-feldspar, plagioclase and apatite. Homogenisation temperatures assign densities of 0.92-0.93 g/cm~3 for the CO_2 involved in charnockite formation. A close correspondence of the isochores for CO_2 + H_2O and CO_2 inclusions with P-T data derived from mineral phase equilibria suggests fluid entrapment at peak metamorphic conditions. The metamorphic evolution of NGB and its exhumation path characterised by isothermal decompression are comparable with those of charnockites from Pan-African terrains elsewhere in East Gondwana.
机译：Nagercoil花岗岩块（NGB）构成了印度南部高等变质带中最南端的区域，主要由邻辉石-K-长石-斜长石-石英-黑云母-钛铁矿组成，具有块状菱铁矿。混合有铝质表壳的菱形岩中含有石榴石，通常表现出典型的减压纹理，冠状邻苯二甲ene和斜长石环绕在残lic榴石晶粒周围。邻苯二甲酚以近原始状态保存在长方岩中，表明水活度低。 NGB霞石的石榴石-邻二甲苯测温温度为691-934℃。石榴石-黑云母的测温值与上述估计值大致重叠（538-864℃），尽管也获得了一些较低的值，表明铁-镁的交换。岩石中有白云母和黑云母中氟含量高也表明变质温度很高。使用石榴石-邻苯二酚-斜长石-石英组合的霞洛克气压计可产生4.0-6.3 kbar。总体压力下降超过2 kbar与根据构造标准推断的等温减压历史有关。此处提供的详细的流体包裹体岩石学，显微热学和激光拉曼光谱数据可以评估NGB霞石的流体演化历史。我们区分了四类夹杂物，其中最早的一类是CO_2，而H_2O的含量却有所不同。后续世代以CO_2为主导。 CH_4和N-2在本地作为痕量成分出现。富含CO2的内含物在所有矿物中均占主导地位：石榴石，钾长石，斜长石和磷灰石。均质化温度为形成霞石的CO_2分配了0.92-0.93 g / cm〜3的密度。 CO_2 + H_2O和CO_2夹杂物的等时线与源自矿物相平衡的P-T数据密切对应，表明在峰值变质条件下流体截留。 NGB的变质演化及其以等温减压为特征的发掘路径与东冈瓦纳其他地区的泛非洲地形上的菱角岩具有可比性。

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