Decoding Rodinia events in South India: Evidences from late-Tonian ages within granulites

Madurai Block (MB), the largest crustal block of Southern Granulite Terrain (SGT), in South India occupies central position in East Gondwana (EG) reconstructions and preserves evidences for multiple magmatic-metamorphic episodes viz., (a) Neoarchean to Siderian, (b) Orosirian (c) Stenian to Early Tonian and (d) Late Tonian. Among these, Late-Tonian magmatic and associated high-T metamorphic records have drawn significant attention as these are coeval with breakup of Rodinia. However, detailed petrological and geochronological studies have been lacking to establish the link between late-Tonian magmatism and associated metamorphism. Within MB, rift related associated alkaline plutons have been reported that are aligned along the transcrustal Suruli Shear Zone (SSZ) within the Kambam Ultrahigh temperature Belt (KUB). In the present study, we focus on hitherto unreported locations along SSZ with evidences for HT-UHT metamorphism coeval with magmatism due to continental extension (rift). LA-ICPMS U-Pb dating of metamorphic zircons from charnockites and garnet-biotite gneiss yields concordant U-Pb ages of ca. 757 Ma, 766 Ma and 819 Ma with Ti-in-zircon temperatures greater than 700⁰C with Garnet-Biotite exchange thermometry values close to 900⁰C. Additionally, detrital monazites from UHT metapelites also yield concordant U-Pb ages of 752 Ma and 739 Ma pointing to similar metamorphic signature. Hence these results call for the interpretation of SSZ as a late-Tonian rift zone associated with prominent HT-UHT metamorphic event. This stage was followed by the Ediacaran-Cambrian thermal event in which, SSZ evolved as a major suture zone within the SGT, evidenced by 550 Ma U-Pb ages from zircon cores and monazite within granites and metapelites, respectively. The close correlation between late-Tonian magmatism and metamorphism in MB with other EG fragments such Eastern Ghats, Antarctica and Sri Lanka proposes late-Tonian magmatism and HT-UHT metamorphism as a pervasive thermal event in EG during Rodinia breakup. This point to the conclusion that SGT was an integral component in Rodinia supercontinent cycle and hence the present tectonic model proposed for SGT can be re-evaluated.