WHAT DO COESITE ECLOGITES TELL US ABOUT THE EARLIEST STAGES OF INDIA ASIA COLLISION?

Coesite eclogites outcrop at two localities in the NW Himalaya: Tso Morari in NW India and Kaghan in N Pakistan. Both lie south of the, blueschist bearing, India-Asia suture and are hosted in rocks of the Indian Plate continental crust. Robust age data, indicate a peak age of UHP metamorphism at ca. 47 Ma for both localities. Diffusion modelling of garnet zonation patterns shows that peak metamorphic temperatures of about 600^oC could not be maintained for > 400000 years before initiation of cooling linked to rapid uplift. Cooling through 500^oC was by 42 Ma. UHP metamorphism was thus short-lived and followed by very rapid exhumation to mid crustal, greenschist facies conditions. PTt paths mapping exhumation are deduced from post-peak metamorphic assemblages and textures. In Pakistan, field data show initial stages of exhumation to be accommodated by synchronous N-vergent extensional shear zones above and S-vergent thrusts below. Exhumation mechanisms are less clear at Tso Morari. For the Himalaya, rates of crustal subduction to UHP conditions, of UHP metamorphism and of subsequent exhumation to mid crustal levels implies that initial stages of India-Asia collision cannot have pre-dated 50 Ma. In Pakistan, exhumation involved thrusting UHP rocks onto MP Barrovian sequences with identical metamorphic ages to the eclogites. Thus, at early stages of continent-continent collision characterised by rapid convergence, regional Barrovian type metamorphism may be as rapid as UHP metamorphism. The structural geometry of the North Indian plate implies that the angle of continental subduction decreased with time, that UHP rocks were subsequently underplated as more Indian Plate crust entered the collision zone, and that break-back thrusts placed some of the later arriving material on top of the UHP sequences. This, together with Miocene extensional excision along the Main Mantle Thrust, suggests that UHP rocks do not necessarily precisely locate the position of a primary suture.