Tectonothermal evolution of the High Himalayan Crystalline Sequence, Langtang Valley, northern Nepal

The High Himalayan Crystalline Sequence in north‐central Nepal is a 15‐km‐thick pile of metasediments that is bound by the Main Central Thrust to the south and a normal fault to the north. The Langtang section through the metasediments shows an apparent inversion of metamorphic isograds with high‐ P , kyanite‐grade rocks exposed beneath low‐ P , sillimanite‐grade rocks. Textural evidence confirms that the observed inversion is a result of a polyphase metamorphic history and phase equilibria studies indicate that thermal decoupling has occurred within a mechanically coherent section of crust. Rocks now exposed at the base of the High Himalayan thrust sheet underwent Barrovian regional metamorphism (M1) prior to 34 Ma in the early stages of the Himalayan orogeny, recording metamorphic conditions of T = 710 ± 30° C, P = 9 ± 1 kbar. After the activation of the Main Central Thrust, which emplaced these metapelites southwards onto the lower grade Lesser Himalayan formations, the upper part of the thrust sheet was overprinted by a second heating event (M2), resulting in sillimanite‐grade metamorphism and anatexis of metapelites at T = 760 ± 30° C, P = 5.8 ± 0.4 kbar between 17 and 20 Ma. Crustally derived, leucogranite magmas have been emplaced into low‐grade Tethyan sediments on the hangingwall of the normal fault that bounds the northern limit of the metapelitic sequence. The cause of the selective heating of the upper section of the metasediments during M2 cannot be reconciled with either post‐thrusting thermal relaxation or advection models. The cause of M2 remains problematical but it is suggested that heat focusing has occurred at the top of the High Himalayan Crystalline Sequence as a result of movement on the normal fault blanketing metapelites of high heat productivity with low‐grade sediments of low thermal conductivity. This model implies that the normal fault was active before M2, consistent with decompression textures that formed during, or shortly after, sillimanite‐grade metamorphism.