Controls of mineral reactions in high‐grade garnet‐wollastonite‐scapolite‐bearing calcsilicate rocks: an example from Anakapalle, Eastern Ghats, India
Controls of mineral reactions in high‐grade garnet‐wollastonite‐scapolite‐bearing calcsilicate rocks: an example from Anakapalle, Eastern Ghats, India
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高品位石榴石-硅灰石-方柱石含钙硅酸盐岩石中矿物反应的控制：以印度东高止山脉 Anakapalle 为例
DOI：
10.1111/j.1525-1314.1997.tb00635.x
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发表时间：
1997
影响因子：
3.4
通讯作者：
J. Ehl
中科院分区：
文献类型：
--
作者：
P. Sengupta;Sanjoy Sanyal;S. Dasgupta;M. Fukuoka;J. Ehl
研究了东高止山脉的一组含石榴石-硅灰石-方石石-钙硅酸盐麻粒岩，以记录深陆地壳变质演化过程中矿物反应的控制。所研究的岩石在毫米级范围内显示出矿物学和相组成的非均质性。在复杂的自然系统中，斜辉石对石榴石的形成和组成有很强的影响。使用涉及石榴石的蒸汽不相关平衡的P - T估计确定了从摄氏850C到摄氏5.5-5.2 kbar的近等压冷却路径。推导出的轨迹与使用校准良好的温度计从伴生岩石中推断出的整个逆行P - T路径的末端段吻合得很好。钙硅酸盐主体中硅灰石和方石的普遍存在表明，在变质峰期，aCO2较低。受含石榴石组合的矿物-流体平衡约束的流体组成显示出固体相组成的区域变化，例如石榴石和斜辉石。我们构建了一个定量的log/CO2‐log/O2图来描述不同钙硅酸盐组合的稳定性，以及这些逃逸物种的组成和行为。矿物-流体平衡和定量流体/岩石比计算的结果，结合拓扑约束，暗示了所研究岩石中缺乏蒸汽的元形态。认为变质峰期的fO2是由环境fO2监测的。随后，在回退过程中，不同的区域独立演化，流体成分由矿物-流体平衡控制。
ABSTRACT A suite of garnet‐wollastonite‐scapolite‐bearing calcsilicate granulites from the Eastern Ghats has been investigated to document the controls of mineral reactions during the metamorphic evolution of the deep continental crust. The rocks studied show heterogeneity in modal mineralogy and phase compositions in millimetre‐sized domains. Textural relations, and the compositional plots of the phases, established that the clinopyroxene exerts a strong influence on the formation and composition of garnet in the complex natural system. P‐T estimates using the vapour‐independent equilibria involving garnet define a near isobaric cooling path from c. 850C at c. 5.5–5.2 kbar. The deduced trajectory tallies well with the terminal segment of the overall retrograde P‐T path construed from the associated rocks using well‐calibrated thermobarometers. The ubiquitous occurrence of wollastonite and scapolite in the main calcsilicate body suggests low aCO2 during peak metamorphic condition. Fluid compositions constrained from mineral‐fluid equilibria of the garnet‐bearing assemblages show domainal variations as a function of the compositions of the solid phases, e.g. garnet and clinopyroxene. A quantitative log/CO2‐log/O2 diagram has been constructed to depict the stability of the different calcsilicate assemblages as functions of the compositions and the behaviour of these fugitive species. The results of the mineral‐fluid equilibria and the quantitative fluid/rock ratio calculations, in conjunction with the topological constraints, imply vapour‐deficient meta‐morphism in the rocks studied. It is argued that fO2 during peak metamorphism was monitored by the ambient fO2. Subsequently, during retrogression, different domains evolved independently, whereas the fluid composition was controlled by the mineral‐fluid equilibria.