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

Abstract

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 °C followed burial to approximately 25 km depth. Laser-ablation monazite geochronology yields a weighted mean ²⁰⁶Pb^∗/²³⁸U 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 ²⁰⁶Pb 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 inferred 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.