Metamorphic P-T-t constraint on the Bantimala garnet-epidote-glaucophane schist and its implication on Mesozoic evolution of Western Sulawesi, Indonesia

Bantimala Complex of Western Sulawesi contains a mixed lithology of blueschist and eclogite, which are the ideal rock assemblage for understanding the geodynamic process of the Tethys subduction system. However, previous studies only reported rough P-T ranges and failed to present a systematic P-T evolutionary history for blueschists of this area. In this study, we carried out detailed petrographic, phase equilibrium modelling, and U-Pb and Ar-Ar geochronological analysis on two garnet-epidote-glaucophane schists samples (18SS24-1, 18SS24-2) to construct their metamorphic histories. Results suggest that both samples underwent eclogite-facies metamorphism with peak conditions of 27-28.5 kbar/615-640 degrees C (18SS24-1) and 23-28 kbar/565-630 degrees C (18SS24-2). Their P-T paths show hairpin patterns with geothermal gradients of 7-8 degrees C/km at peak stages. 40Ar/39Ar dating yielded an age of 125.0 +/- 0.4 Ma, which is interpreted to postdate HP metamorphism. Compared with other HP eclogite with similar peak conditions worldwide, Bantimala garnet-epidote-glaucophane schists are significantly low in bulk CaO content. This geochemical feature enables them to preserve garnet-glaucophane-dominated peak mineral assemblages instead of garnet-omphacite-dominated ones, though they were subducted over 80 km depth and metamorphosed up to eclogite-facies. The discovery of garnet-epidote-glaucophane schists of eclogite-facies metamorphism corroborated the existence of Franciscan-type metamorphism that is characterized by the cold deep subduction of an oceanic lithosphere between the Sundaland landmass and east Java-west Sulawesi terrane during the Early Cretaceous.