The determination of metamorphic pressure–temperature (P–T) conditions of granulites provides us a natural window into the composition, structure, and P–T conditions of the lower crust and process of continental growth. Here, we constrain the P–T evolution of a suite of mafic granulite from the eastern part of Madurai Block. Integrated results from mineral reactions, conventional thermobarometry, and pseudosection analysis suggest that the studied mafic granulites, with a peak assemblage of coarse-grained garnet + coarse-grained clinopyroxene + plagioclase + quartz + [rutile], were metamorphosed under granulite facies conditions of 830 ± 50°C and 9.5 ± 1 kbar. Subsequently, the rocks underwent near isothermal decompression leading to the formation of symplectic assemblages of clinopyroxene + orthopyroxene + plagioclase and orthopyroxene + ilmenite, within a P–T range of 9.0–5.5 kbar and 750–800°C. The estimated peak temperature conditions are somewhat lower than those obtained using characteristic ultra-high temperature (UHT) mineral assemblages and non-conventional thermometers (~1,000°C at ~10 kbar) from the co-metamorphosed metapelitic granulites of the Madurai Block. This may be attributed to the high diffusion rate of divalent Fe and Mg, which often results in retrograde re-equilibration leading to considerable underestimation of peak temperatures in mafic granulites. To overcome this problem, we have additionally determined peak metamorphic temperatures of the studied rocks using a recently developed garnet–clinopyroxene rare earth element (REE)-based thermometer that takes advantage of much lower diffusion rate of trivalent REEs. Using the REE thermometer, we have obtained a peak temperature of 1,050 ± 50°C for the studied rocks, which is in reasonable agreement with the peak metamorphic temperature obtained from previous studies. The constrained P–T conditions and the clockwise P–T path suggest that the studied rocks were buried to lower crustal depths at UHT conditions in a convergent margin setting. The near isothermal decompression suggests rapid exhumation, most likely due to an extensional event. The peak UHT metamorphic conditions could have been attained at the core of a long-lived orogenic plateau, as suggested by other studies.

中文翻译：

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来自印度南部马杜赖区块的超高温镁铁质麻粒岩：常规热气压测量法、假截面分析和基于稀土元素的温度测量法的限制

麻粒岩变质压力-温度（P-T）条件的测定为我们了解下地壳的成分、结构和 P-T 条件以及大陆生长过程提供了一个自然窗口。在这里，我们限制了来自马杜赖地块东部的一套镁铁质麻粒岩的 P-T 演化。矿物反应、常规热压法和假截面分析的综合结果表明，所研究的镁铁质麻粒岩具有粗粒石榴石+粗粒斜辉石+斜长石+石英+[金红石]的峰组合，在830 ± 50°C 和 9.5 ± 1 kbar。随后，岩石经历近等温减压，形成单斜辉石+斜辉石+斜长石和斜辉石+钛铁矿的辛组合，在 9.0-5.5 kbar 和 750-800°C 的 P-T 范围内。估计的峰值温度条件略低于使用特征超高温 (UHT) 矿物组合和非常规温度计（~1,000°C，~10 kbar）从马杜赖区块的共变质变泥质麻粒岩获得的温度条件。这可能归因于二价 Fe 和 Mg 的高扩散速率，这通常会导致逆行重新平衡，从而导致严重低估镁铁质麻粒岩的峰值温度。为了克服这个问题，我们还使用最近开发的基于石榴石-单斜辉石稀土元素 (REE) 的温度计，利用三价 REE 的扩散速率低得多的优势，另外确定了所研究岩石的峰值变质温度。使用 REE 温度计，我们获得了所研究岩石的峰值温度 1,050 ± 50°C，这与先前研究中获得的峰值变质温度合理一致。受约束的 P-T 条件和顺时针 P-T 路径表明，在 UHT 条件下，在收敛边缘设置中，研究的岩石被埋藏到较低的地壳深度。接近等温的减压表明快速折返，很可能是由于伸展事件。正如其他研究所表明的那样，在长期存在的造山带高原的核心可能已经达到了峰值 UHT 变质条件。受约束的 P-T 条件和顺时针 P-T 路径表明，在 UHT 条件下，在收敛边缘设置中，研究的岩石被埋藏到较低的地壳深度。接近等温的减压表明快速折返，很可能是由于伸展事件。正如其他研究所表明的那样，在长期存在的造山带高原的核心可能已经达到了峰值 UHT 变质条件。受约束的 P-T 条件和顺时针 P-T 路径表明，在 UHT 条件下，在收敛边缘设置中，研究的岩石被埋藏到较低的地壳深度。接近等温的减压表明快速折返，很可能是由于伸展事件。正如其他研究所表明的那样，在长期存在的造山带高原的核心可能已经达到了峰值 UHT 变质条件。