Metamorphic phase equilibria modelling and zircon U–Pb geochronology of ultrahigh-temperature cordierite granulites from the Madurai Block, India: implications for hot Gondwana crust

The Madurai Block (MB) is the largest Precambrian crustal block in the Southern Granulite Terrane (SGT) of India and hosts rare cordierite- and orthopyroxene-bearing granulites. Investigations based on field study, petrology, metamorphic P–T estimation, and detrital zircon geochronology of these granulites are crucial for understanding the ultrahigh-temperature (UHT) metamorphism and crustal evolution in this block. Here we investigate the petrology and zircon U–Pb geochronology of two new localities of cordierite granulites at Kottayam (southern MB; SMB) and Munnar (central MB; CMB). Petrographic observations and phase equilibria modelling results indicate that these rocks experienced UHT metamorphism with the peak temperature exceeding 950℃ and involving clockwise P–T paths. The prograde mineral assemblages define the P–T conditions of 6.8–8.7 kbar and 750–875℃. The peak conditions are estimated using pseudosection modelling and geothermometry, which yield P–T estimates of 7.1–9.1 kbar and 955–985℃. The retrograde cooling and decompression are inferred at 860–790℃ and <6.5 kbar, respectively. Partial melting played an important role during metamorphism and contributed to the overgrowth around detrital zircons. The melt production process was probably related to biotite dehydration melting, and was mainly triggered by heating, with or without the effect of decompression. Detrital zircons in cordierite granulite samples from the two localities show similar age distributions and have dominantly Neoproterozoic ages (1024–760 Ma). The zircon cores show oscillatory zoning with a wide range of Th/U ratios (0.01–0.96), implying complex protoliths from multiple Neoproterozoic provenances from both southern and central domains of the MBs. Zircon rims and homogeneous bright zircons yield mean ages of 549 ± 5 Ma, 536 ± 6 Ma, and 544 ± 6 Ma, which are interpreted to represent zircon overgrowths during the post-peak cooling and decompression process. The timing of peak UHT metamorphism is constrained as 549–599 Ma, which coincides with the assembly of the Gondwana supercontinent.

马杜里地块（Madurai Block, MB）是印度南部麻粒岩地体（Southern Granulite Terrane, SGT）中规模最大的前寒武纪地壳地块，产出罕见的含堇青石与斜方辉石的麻粒岩。针对这类麻粒岩开展的野外地质调查、岩石学研究、变质压力-温度（P-T）估算以及碎屑锆石年代学研究，对于理解该地块的超高温（ultrahigh-temperature, UHT）变质作用与地壳演化至关重要。本文对采自科塔亚姆（Kottayam，位于马杜里地块南部，简称SMB）和穆纳尔（Munnar，位于马杜里地块中部，简称CMB）两处新发现的堇青石麻粒岩产地开展了岩石学与锆石U-Pb年代学研究。岩相学观察与相平衡模拟结果显示，这些岩石经历了峰期温度超过950℃的超高温变质作用，并呈现顺时针的P-T演化轨迹。进变质矿物组合对应的P-T条件为6.8~8.7千巴（kbar）、750~875℃。峰期P-T条件通过假相截面模拟与地质温压计估算得到，结果为7.1~9.1千巴、955~985℃。退变质阶段的冷却与减压分别被限定在860~790℃与<6.5千巴的条件下。部分熔融作用在变质过程中发挥了重要作用，并促成了碎屑锆石的边部增生。熔体形成过程可能与黑云母脱水熔融相关，主要由升温触发，或伴随减压作用的影响。两处产地的堇青石麻粒岩样品中的碎屑锆石具有相似的年龄分布特征，主体为新元古代年龄（1024~760 Ma）。锆石核部具有震荡环带，Th/U比值范围宽泛（0.01~0.96），指示其原岩来源复杂，源自马杜里地块南部与中部多个新元古代物源区。锆石边部与均质亮锆石的平均年龄分别为549±5 Ma、536±6 Ma与544±6 Ma，被认为代表了峰期后冷却与减压过程中形成的锆石增生边。超高温变质峰期的时限被限定为549~599 Ma，这与冈瓦纳超大陆的聚合时间相吻合。