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

We 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 degrees C, exhumation to 10.5-12.0 kbar and 580-590 degrees C and a subsequent second high-pressure P-T condition of similar to 16 kbar and similar to 560 degrees C and exhumation to <= 9 kbar and <= 600 degrees 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. similar to 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.