Characterizing the pressure-temperature conditions and age of metamorphism of metapelitic granulites from the northeastern part of the Madurai Block, Southern Granulite Terrane

Ishram, Ramswroop (2023) Characterizing the pressure-temperature conditions and age of metamorphism of metapelitic granulites from the northeastern part of the Madurai Block, Southern Granulite Terrane. Masters thesis, Indian Institute of Science Education and Research Kolkata.

Text (MS dissertation of Ramswroop Ishram (18MS049)) Thesis_18MS049.pdf - Submitted Version Restricted to Repository staff only Download (4MB)

Abstract

The Granulite Terrane of South India, lying south of the Dharwar craton, is an extensive granulite massif that preserves rock record of more than 2.5 billion years from the Mesoarchean to Cambrian. It has been divided into two units. The Northern Granulite Terrane, lying immediately south of the Dharwar craton and extending up to the Moyar-Bhawani Shear Zone, is commonly interpreted as the granulite facies equivalent of the Dharwar craton. The Southern Granulite Terrane lies south of the Moyar-Bhawani Shear Zone and extends till the continental margin. The Southern Granulite Terrane is sub- divided into three geographic divisions, namely Nilgiri- Namakkal Block, Madurai Block and Trivandrum Block, separated from each other by the Palghat- Cauvery Shear Zone and Achankovil Shear Zone. The Madurai Block is the largest block of the Southern Granulite Terrane and preserves geological evolutionary history from ~2500 to 500 Ma. The domain is dominated by charnockites and metapelites, some of which preserves record of ultrahigh-temperature metamorphism. The ultrahigh-temperature metamorphic rocks have been extensively studied in this region; however, these rocks are restricted in space and time compared to the vastness of the Madurai Block. The ultrahigh-temperature rocks are dominantly preserved in the central part of Madurai Block and formed during the Pan-African orogeny (~600-500 Ma). To understand the tectono-metamorphic evolution of the entire domain throughout the Proterozoic, the other rocks become important, which are still not well studied. In this study, the petrochronological evolution of the metapelites from the northeastern part of the Madurai Block has been reported. These exposed rocks show leucosome and melanosome layers with the mineral assemblage of garnet + K-feldspar + plagioclase + biotite + sillimanite + quartz + spinel. The P-T conditions for the peak metamorphism (T=820-840°C and P=7 kbar) are constrained using pseudosection modelling. The retrograde P-T conditions (T=650°C and P=5kbar) are obtained by utilizing Fe-Mg exchange thermometry in the garnet + retrograde biotite assemblage and garnet- sillimanite-plagioclase-quartz barometry. Obtained retrograde P-T conditions indicates that these rocks have cooled from a relatively high temperature-pressure conditions. Texturally controlled in-situ monazite dating in the metapelites yields two significant metamorphic zones. The Th-poor darker cores, yield an age of 787±13Ma and the relatively Th-rich brighter rims, yield an age of 564±49 Ma. Therefore, the peak P-T condition has were most likely attained as a result of the Cryogenian metamorphism (ca. 800Ma). The Cryogenian granulite facies metamorphism of the Madurai Block has been tentatively correlated with the Rodinia supercontinent breakup. The Ediacaran metamorphic overprint has been correlated with the Ediacaran-Cambrian metamorphism during the Pan-African Orogeny, which is largely reported from the Madurai Block.

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