Tectono-metamorphic Evolution of the Madurai and Trivandrum Blocks of Southern Granulite Terrane, South India: Relevance to Gondwanaland Assembly

Abstract

The southern part of Southern Granulite Terrane is separated into Trivandrum Block (TB) in the south from Madurai Block (MB) to the north by the Pan-African Achankovil shear zone system (AKSZ). A combined structural, metamorphic, geochemical and geochronological data is presented here from the common lithologies of the MB and TB to understand the tectono metamorphic evolutionary history of this terrane. Detailed field-based analysis of contrasting mesoscale structures and deformation kinematics in the granulite facies rocks across the AKSZ show distinct differences in the geometry of superposed fold structures between the MB and TB prior to their accretion along the AKSZ. Deformation microstructures and garnet-bearing melt-hosted syn-D3 extensional shears attest to the persistence of high temperature during D2 D3 deformations. The metamorphic temperatures and pressures estimated from different lithologies of the MB and TB using various thermobarometers are of the same range i.e. 600–880 °C and 5–8 kbar. The similar clockwise P-T path retrieved by phase equilibrium modelling from the MB and TB represents the last deformation metamorphism shared between the MB and TB during their accretion along the AKSZ. The distinct evolutionary history of the MB and TB is supported by the chronological contrasts between them e.g. the prominent Mid Neoproterozoic (~ 750 Ma) dates in the MB are rare in the TB, and the Paleoproterozoic (~2200 Ma) dates in the TB is rare in the MB. The strong imprints of 500–600 Ma monazite ages shared between the TB, MB and AKSZ attest to the deformation-metamorphism occurred during their Late-Neoproterozoic to Cambrian accretion along the AKSZ. The geochemical data from various lithounits suggest post-Archean granitoid affinity and subduction settings in both the blocks. The structural discordances along with the differences in metamorphic P-T conditions, geochemistry and chronological contrasts suggest that both the blocks have distinct crustal evolutionary histories prior to their accretion along the AKSZ. From the new data, a southward subduction model is proposed which provides insight into the subduction–accretion–collision tectonics associated with the Late-Neoproterozoic to Cambrian evolutionary history of this region. Integration of structural-metamorphic and chronological data from Madurai and Trivandrum Blocks (India) and south-central Madagascar suggests that the TB is equivalent to Androyan and Anosyan domains in southern Madagascar; by contrast, the MB is tectonically equivalent to central Madagascar which includes the Antananarivo and Itremo-Ikalamavony Blocks. Thus, it is concluded that the block north of the Pan-African Achankovil Shear Zone Ranotsara Shear Zone comprising the Madurai Block (India) and central Madagascar share a common history, but this is different from the southern block comprising the Trivandrum Block (India) and the high-grade gneisses and felsic intrusives of southern Madagascar. The present study propose that the sinistral steep-dipping Ranotsara Shear Zone separating the southern and central Madagascar continues into India as the AKSZ – the accretion boundary between the Trivandrum and the Madurai Blocks.