Tectonometamorphic Evolution of High-Grade Lower Crustal Rocks in the Madurai Block of Southern Granulite Terrane, India: Constraints from Petrology, Geochemistry, and Geochronology

Tiwari, Ashish Kumar (2025) Tectonometamorphic Evolution of High-Grade Lower Crustal Rocks in the Madurai Block of Southern Granulite Terrane, India: Constraints from Petrology, Geochemistry, and Geochronology. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Ashish Kumar Tiwari (17RS005)) 17RS005.pdf - Submitted Version Restricted to Repository staff only Download (48MB)

Abstract

The exposed lower crustal rock ensembles serve as key archives for understanding the longterm processes that govern lower crustal dynamics and crust-mantle interactions. These rocks provide a window into the thermal, chemical, and tectonic evolution of the Earth's deep crust, offering clues about the mechanisms of continental growth and the development of ancient mountain belts. We studied high-grade lower crustal rocks from the Madurai Block in the granulite terrane of southern India to reconstruct their pressure-temperature-time (P-T-t) evolution by integrating petrography, mineral chemistry, thermobarometry, pseudosection modelling, and texturally controlled age dating. We characterized the high-grade metamorphism from various lithologies, including semipelites, mafic granulites and charnockites, demonstrating that the rocks attainted peak ultrahigh-temperature (UHT) (T>900 °C ) conditions in a clockwise P-T loop with decompression-cum-cooling following the peak. The UHT conditions were likely attained in a long-lived collisional orogeny, with collision initiation at ca. 590 Ma, attainment of peak UHT conditions at ca. 550 Ma and extensional collapse in the sustained UHT at ca. 510-475 Ma. The prolonged UHT metamorphism (at least 40 MYr) was primarily driven by heat produced form radioactive decay within the crust, alongside contributions from mantle heat supply. Correlating the timing and durations of this metamorphism with other continental fragments, we argue for the existence of a long-lived hot orogen in the Madurai Block associated with the assembly of the Gondwana supercontinent. We also revisit the petrogenesis of the charnockites, the most dominant lithology of the exposed rock ensemble. By integrating petrological, geochemical, and geochronological data with a complementary forward modelling approach, we demonstrate that the charnockites were likely formed by partial melting of a heterogenous lower crust, with their compositional diversity explained by various in-source processes. Our finding points to an extensional tectonic setting in the Madurai Block during the mid-Neoproterozoic as opposed to arc-setting.

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