Thermobarometric constraints on the thermal history of the Main Central Thrust Zone and Tibetan Slab, eastern Nepal Himalaya

Abstract

ABSTRACT The Main Central Thrust (MCT) south of Mt Everest in eastern Nepal is a 3 to 5km thick shear zone separating chlorite‐bearing schist in the lower plate from sillimanite‐bearing migmatitic gneiss in the overlying Tibetan Slab. The metamorphic grade increases through the MCT zone toward structurally higher levels. Previous workers have suggested that either post‐ or synmetamorphic thrust movement has caused this inversion of metamorphic isograds. In an effort to quantify the increase in grade and to constrain proposed structural relations between metamorphism and slip on the fault, four well‐calibrated thermobarometers were applied to pelitic samples collected along two cross‐strike transects through the MCT zone and Tibetan Slab. Results show an increase in apparent temperature up‐section in the MCT zone from 778 K to 990 K and a decrease in temperature to ∼850 K in the lower Tibetan Slab, which is consistent with synmetamorphic thrust movement. A trend in calculated pressures across this section is less well‐defined but, on average, decreases up‐section with a gradient of ∼28MPa/km, resembling a lithostatic gradient. Pressure‐temperature paths for zoned garnets from samples within the MCT zone, modelled using the Gibbs' Method, show a significant decrease in temperature and a slight decrease in pressure from core to rim, which might be expected for upper plate rocks during synmetamorphic thrust movement. Samples from the uppermost Tibetan Slab yield higher temperatures and pressures than those from the lower Tibetan Slab, which may be evidence for later‘resetting’ of thermobarometers by intrusion of the large amounts of leucogranite at that structural level.