Competing roles of evolving P-T conditions, equilibration volume and chemical potential landscape in the formation of corona texture: a case study from the Southern Granulite Terrane, India

IF 3.5 2区 地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS
Anindita Dey, Sirina Roy Choudhury, Pulak Sengupta
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引用次数: 0

Abstract

Corona texture is defined by the development of partial or complete rim of one or more minerals around a central reactant mineral depicting limited mass transport (local equilibration) in the length scale of the coronitic layers. The mineral assemblages forming corona texture in a metamorphic rock are commonly used to trace the P-T-X conditions through which the rock evolved during various tectonic processes. However, without a proper assessment of the changes in the equilibration volume (EV) and its effect on the mineralogy, any petrological interpretation deduced from the coronitic texture may be incorrect. In this study, we demonstrate that the double corona texture, observed in a suite of Mg-Al rich ortho-amphibole cordierite-bearing rock from the Cauvery Shear Systemin (Southern Granulite Terrane, India), developed in response to the continuously evolving EV. The studied rock contains aluminosilicate porphyroblasts that are set in a matrix of ortho-amphibole ± quartz. The aluminosilicate porphyroblasts are rimmed successively by an inner symplectic corona of sapphirine + cordierite, and an outer mono-mineralic corona of cordierite (near ortho-amphibole). Locally, patches of corundum with a rind of cordierite grow preferentially along the interface of aluminosilicate and the inner symplectic corona. Based on detailed petrography and mineral composition analyses, the corona textures are interpreted to have formed through a sequence of different chemical reactions that occurred in local micro-domains. We calculated quantitative P-T pseudosection in a NCFMASHT (Na2O-CaO-FeO-MgO-Al2O3-SiO2- H2O-TiO2) system and activity-adjusted P-T petrogenetic grid in a MASH (MgO-Al2O3-SiO2- H2O) system which, together, suggest that the coronitic assemblages were formed in response to a steeply decompressive retrograde P-T path from >8.8 kbar to <6 kbar, at a nearly constant temperature of ~700°C. Changes in EV in response to the limited transport of chemical components during the formation of corona texture were investigated through isothermal P-μMgO, P-μSiO2, and P-μMgO-μSiO2 MASH diagrams. Our results quantitatively model the continuously changing chemical potential landscape (P-μMgO- μSiO2 evolution path) around the central aluminosilicate porphyroblast within the corona-bearing micro-domain. The path demonstrates that a gradually shrinking EV around the central aluminosilicate during retrogression led to the sequential change of mineral reactions and equilibrium mineral assemblages, and resulted in the formation of multiple coronae. Unavailability of fluids and/or rapid exhumation is considered as the most dominant factors responsible for the decreasing elemental mobility and the consequent shrinking in EV in the studied rock.
演化的P-T条件、平衡体积和化学势景观在日冕结构形成中的相互竞争作用:以印度南部麻粒岩地体为例
日冕结构是由一种或多种矿物在中心反应物周围的部分或完整边缘的发育来定义的,它描绘了在日冕层的长度尺度上有限的质量运输(局部平衡)。在变质岩中形成日冕结构的矿物组合通常用于追踪岩石在不同构造过程中演化的P-T-X条件。然而,如果没有对平衡体积(EV)的变化及其对矿物学的影响进行适当的评估,任何由冕状结构推断的岩石学解释都可能是不正确的。在这项研究中,我们证明了在印度南部麻粒岩地体Cauvery剪切系统中观察到的一套富镁铝的含堇青石的正角闪孔岩石中观察到的双日冕结构是对不断演化的EV的响应。所研究的岩石含有铝硅酸盐卟啉母细胞,它们被设置在正角闪孔±石英的基质中。铝硅酸盐卟啉母细胞的内环依次为蓝宝石+堇青石的辛日冕,外环依次为堇青石的单矿物日冕(近正角闪孔)。局部地,带有堇青石外壳的刚玉斑块优先沿着硅酸铝和内部辛日冕的界面生长。根据详细的岩石学和矿物成分分析,日冕结构被解释为通过在局部微域中发生的一系列不同的化学反应形成的。我们计算了NCFMASHT (na20 - cao - feo -MgO-Al2O3-SiO2- H2O- tio2)体系的定量P-T伪剖面和MASH (MgO-Al2O3-SiO2- H2O)体系的活度调整P-T成岩格网,两者共同表明,冠状组合是在~700°C近乎恒定的温度下,在从8.8 kbar到6 kbar的急剧解压缩的P-T路径下形成的。通过等温P-μMgO、P-μSiO2和P-μMgO-μSiO2的MASH图,研究了电晕织构形成过程中化学成分有限输运的EV变化。我们的研究结果定量模拟了在含日冕微域中,围绕中心铝硅酸盐卟岩的连续变化的化学势景观(P-μ mgo - μSiO2演化路径)。该路径表明,在退积过程中,中心铝硅酸盐周围的EV逐渐缩小,导致矿物反应和平衡矿物组合的顺序变化,并导致多个日冕的形成。流体的缺乏和(或)快速挖掘被认为是造成所研究岩石中元素迁移率下降和EV随之萎缩的最主要因素。
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来源期刊
Journal of Petrology
Journal of Petrology 地学-地球化学与地球物理
CiteScore
6.90
自引率
12.80%
发文量
117
审稿时长
12 months
期刊介绍: The Journal of Petrology provides an international forum for the publication of high quality research in the broad field of igneous and metamorphic petrology and petrogenesis. Papers published cover a vast range of topics in areas such as major element, trace element and isotope geochemistry and geochronology applied to petrogenesis; experimental petrology; processes of magma generation, differentiation and emplacement; quantitative studies of rock-forming minerals and their paragenesis; regional studies of igneous and meta morphic rocks which contribute to the solution of fundamental petrological problems; theoretical modelling of petrogenetic processes.
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