High-precision geothermobarometry across the High Himalayan metamorphic sequence, Langtang Valley, Nepal

One of the long recognized features of Himalayan geology is the apparent inversion of metamorphic sequences, as evidenced in both metamorphic parageneses and thermobarometric data. With the aid of an extended thermobarometric dataset from the Langtang Valley section of the Higher Himalayan Crystallines, it can be demonstrated that the relatively large uncertainties associated with traditional thermobarometric techniques severely limit the tectonic interpretation of metamorphic gradients across the Himalayas. We apply the recently developed Delta PT approach, which significantly improves the precision to which pressure and temperature differences between samples may be calculated. High-precision thermobarometric data reveal an isothermal, rather than inverted, temperature array at Langtang, while the pressure data suggest significant structural complexity, with the Higher Himalayan Crystallines in the Langtang section comprising two distinct, possibly duplicated sequences, each having experienced considerable structural attenuation following metamorphism.