Constraining The Thermal History Of An Ultra-hot Orogen From Metamorphic Reaction History And Garnet-orthopyroxene Diffusion Modelling Studies

Abstract

The southern margin of the Central Indian Tectonic Zone experienced a history of metamorphism at different sets of conditions during the Proterozoic. A primary, high temperature assemblage of aluminous orthopyroxene (XMg= 0.585) + calcic plagioclase (An49-52) + magnesian ilmenite (XMgTiO3 = 0.06-0.07) experienced three recrystallization events (M1 with a TMax of ~1000 °C at 9.5 kbar and M2 at 900°C, 6.7 kbar and M3 at 770°C, 7.5 kbar) to produce two generations of garnet, orthopyroxene and biotite. We have integrated observations on metamorphic reaction textures, mineral compositional zonation, calculated pseudosections, available geochronological data, and diffusion modelling of compositional profiles in garnet and orthopyroxene to constrain the timescale of thermal evolution for this complex sequence. We find that at least a three stage thermal history is necessary to account for all observations consistently. Cooling rates on the order of 10’s of °C/ my indicate that high temperatures of M1-3 metamorphism were sustained for tens of million years, providing evidence of a long lasting (i.e. > 60 my) regional metamorphic event.