Burial and exhumation history of a Lesser Himalayan schist: Recording the formation of an inverted metamorphic sequence in NW India

Coupled analysis of the pressure-temperature (PT) evolution and accessory phase geochronology of a single sample reveals the burial-uplift history of part of the Lesser Himalaya during the Middle Miocene. Phase-equilibria calculations indicate that a peak temperature of 600-640 degrees C followed burial to approximately 25 km depth. Laser-ablation monazite geochronology yields a weighted mean Pb-206*/U-238 age of 11.1 +/- 2.0 Ma and a Tera-Wasserburg Concordia intercept age of 10.6 +/- 0.9 Ma, with no distinguishable age difference between matrix and inclusion grains. Considerations of the likelihood of excess Pb-206 further suggest that the crystallization age lies in the range 9-10 Ma. Textural analysis suggests that monazite grew during prograde metamorphism. Peak metamorphic conditions were followed by exhumation and cooling, forming a distinctively tight PT path closure. Both the shape of this path and its relatively young prograde phase distinguish Lesser Himalayan evolution from that typically infer-red for the High Himalaya, and allow exploration of the thermal mechanisms that operated in the western Himalaya during the interval ca. 23-6 Ma. The PTt history is characteristic of footwall heating due to rapid overthrusting of hot rock (the Higher Himalaya), followed by incorporation into a thrust sheet that exhumed the sequence rapidly enough to preserve an inverted metamorphic gradient. (c) 2007 Elsevier B.V. All rights reserved.