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首页> 外文期刊>Contributions to Mineralogy and Petrology >The igneous charnockite—high-K alkali-calcic I-type granite—incipient charnockite association in Trivandrum Block, southern India
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The igneous charnockite—high-K alkali-calcic I-type granite—incipient charnockite association in Trivandrum Block, southern India

机译：印度南部特里凡得琅区块的火成块状辉绿岩—高钾碱钙I型花岗岩—初生闪锌矿联合体

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摘要

The Pan-African (640 Ma) Chengannoor granite intrudes the NW margin of the Neoproterozoic high-grade metamorphic terrain of the Trivandrum Block (TB), southern India, and is spatially associated with the Cardamom hills igneous charnockite massif (CM). Geochemical features characterize the Chengannoor granite as high-K alkali-calcic I-type granite. Within the constraints imposed by the high temperature, anhydrous, K-rich nature of the magmas, comparison with recent experimental studies on various granitoid source compositions, and trace- and rare-earth-element modelling, the distinctive features of the Chengannoor granite reflect a source rock of igneous charnockitic nature. A petrogenetic model is proposed whereby there was a period of basaltic underplating; the partial melting of this basaltic lower crust formed the CM charnockites. The Chengannoor granite was produced by the partial melting of the charnoenderbites from the CM, with subsequent fractionation dominated by feldspars. In a regional context, the Chengannoor I-type granite is considered as a possible heat source for the near-UHT nature of metamorphism in the northern part of the TB. This is different from previous studies, which favoured CM charnockite as the major heat source. The occurrence of incipient charnockites (both large scale as well as small scale) adjacent to the granite as well as pegmatites (which contain CO2, CO2-H2O, F and other volatiles), suggests that the fluids expelled from the alkaline magma upon solidification generated incipient charnockites through fluid-induced lowering of water activity. Thus the granite and associated alkaline pegmatites acted as conduits for the transfer of heat and volatiles in the Achankovil Shear Zone area, causing pervasive as well as patchy charnockite formation. The transport of CO2 by felsic melts through the southern Indian middle crust is suggested to be part of a crustal-scale fluid system that linked mantle heat and CO2 input with upward migration of crustally derived felsic melts and incipient charnockite formation, resulting in an igneous charnockite – I-type granite – incipient charnockite association.
机译：泛非（640 Ma）的Chenannoor花岗岩侵入印度南部特里凡得琅区块（TB）的新元古代高品位变质带的西北边缘，并且在空间上与小豆蔻丘陵火成的霞石地块（CM）相连。地球化学特征将成安努尔花岗岩表征为高钾碱钙I型花岗岩。在岩浆的高温，无水，富含钾的性质所施加的约束条件下，与近期对各种花岗石源组成的实验研究以及微量元素和稀土元素的模拟相比较，Chengannoor花岗岩的独特特征反映了火成岩性质的烃源岩。提出了一种成岩模型，其中存在一段时期的玄武岩底层。该玄武质下地壳的部分融化形成了CM菱镁矿。 Chengannoor花岗岩是由CM的Charnoenderbites部分熔化而产生的，随后的分馏以长石为主。在区域范围内，由于结核北部的变质具有近UHT的性质，Chengannoor I型花岗岩被认为是可能的热源。这与以前的研究不同，后者的研究倾向于使用CM霞石作为主要热源。与花岗岩以及伟晶岩（含有CO2 ，CO2 -H2 O，F和其他挥发物）相邻的初期霞石岩（无论大小） ，表明从碱性岩浆中排出的流体在凝固时通过流体引起的水活度降低而产生了初期的菱镁矿。因此，花岗岩和相关的碱性伟晶岩充当了Achankovil剪切区区域中热量和挥发物传递的管道，导致普遍形成的以及斑片状的方铁矿。据推测，长英质熔体通过南部印度中部地壳输送CO2 是地壳规模流体系统的一部分，该系统将地幔热和CO2 输入与地壳衍生的长英质熔体和早期沉积的向上迁移联系在一起。钙长石的形成，形成火成的钙长石– I型花岗岩–初始钙长石联合体。

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