The P-T-t architecture of a Gondwanan suture: REE, U-Pb and Ti-in-zircon thermometric constraints from the Palghat Cauvery shear system, South India

Abstract

Understanding the relationship between accessory mineral growth and the evolution of silicate mineral assemblages along the entirety of a P-T-t path is a critical step in developing models for evolving tectonic systems. Here we combine U–Pb age data (for zircon and monazite), rare earth element (REE) data and compositionally specific phase diagrams (P-T pseudosections) for the rocks of the Palghat Cauvery shear system (PCSS), Southern Indian order to constrain the periodicity of heating/cooling and burial/exhumation events during the Ediacaran/Cambrian amalgamation of Gondwana. HREE data from zircon are consistent with zircon grow that 672–724 °C during the breakdown of garnet in the kyanite stability field at 535.0 ± 4.9 Ma. This represents a cooling that punctuates the P-T-t path. Subsequent monazite growth and symplectite formation occurred at 920 °C and 7.5 kbar, 10 Ma after zircon growth which reflects a period of reheating and decompression related to delamination and the collapse of the East African orogen. The REE chemistry of the monazite is consistent with the system having undergone partial melting prior to monazite growth, thereby altering the bulk rock chemistry. The periodicity of the heating and cooling cycles (10 Ma) from this study is consistent with recently proposed tectonic switching models for the formation of granulite metamorphism in accretionary/collisional tectonic settings. The elevated heat flows required to generate the UHT metamorphism are achievable in the proposed back-arc setting for the PCSS during Gondwana amalgamation.