Inverted metamorphism within the Tibetan slab of Bhutan; evidence for a tectonically transported heat-source

Abstract

Within the central portion of the Tibetan slab of Bhutan, early mineral assemblages that formed at model conditions of about 480 - 500'C and 7 kbar were overprinted by a high-temperature penetrative fabric. This fabric, which probably formed during convergence of India with Asia, was' later overprinted by mineral assemblages that formed at temperatures up to about 640'C, at about 6 kbar. Above the central ponion, in the upper part of the Tibetan slab, syn- to postdeformational mineral assemblages equilibrated at 4 kbar and 620'C after cooling from temperatures of about 700oC. Reaction textures and geothermobarometry indicate high-temperature decompression of the upper portion of the Tibetan slab to about 4 kbar from a higher pressure. The upper portion contains leucogranites that intruded into rocks that previously had been heated past the second sillimanite isograd and had been penetratively deformed. The leucogranites therefore were not the source of heat for the high-temperature upper portion of the slab. Our data from Bhutan suggest that the central portion of the slab was heated by the migmatitic upper portion, We propose that this migmatite was uplified from lower crustal depths and thrust over the central portion. The proposed thrust-shear probably predates motion on the Main Central thrust. The entire hot package, including the central and upper portions of the Tibetan slab, was rapidly exhumed during and after thrusting.