Low temperature eclogite facies rocks discovered in the Eastern Himalayan Syntaxis: Poly‐cyclic metamorphic evolution and implications

Abstract

AbstractWe report the first occurrence of poly‐cyclic high‐pressure low‐temperature (HP‐LT) rocks from the easternmost Indus‐Yarlung suture zone, formed during subduction of Neo‐Tethyan oceanic lithosphere. Petrology, mineral composition and P–T pseudosection modelling reveal two low‐temperature eclogite facies metamorphic events with an initial high‐pressure P–T condition of 16.4–18.7 kbar and 510–520°C, exhumation to 10.5–12.0 kbar and 580–590°C and a subsequent second high‐pressure P–T condition of ~16 kbar and ~560°C and exhumation to ≤9 kbar and ≤600°C. This history implies a complex ‘yo‐yo type’ P–T path. In situ monazite dating and textural relationships show that late‐stage exhumation, cooling and garnet breakdown occurred at c. ~25–22 Ma. We interpret the first burial event to represent subduction of the Neo‐Tethys Ocean at the eastern Indus‐Yarlung suture zone. Initial exhumation, reburial and final exhumation represent material transport in a large‐scale convective circulation system in the subduction channel. Convective overturn in the subduction channel evidently serves both as a mechanism to produce poly‐cyclic metamorphism and to exhume LT eclogite facies rocks.