Orogenesis in the Eastern Himalayas: A study of structure, geochronology and metamorphism in Bhutan

This study investigates the tectonics of the eastern Himalaya using geological mapping, geochemistry, geochronology and metamorphism. In a revised geological map of Bhutan the expanse of Tethyan sediments in central Bhutan is increased and the extent of leucogranite bodies better defined. 500 meter scale north-verging folds, comparable to those recognized in Nepal, are identified in the allochthonous Tethyan sediments in central Bhutan. A cross-cutting dyke at 17.4 Ma constrains the minimum age of folding. Tethyan sediments overlie higher-grade Greater Himalaya Series rocks on a shear zone wherever observed, rather than a combination of tectonic and depositional contacts as suggested in the literature. Leucogranites (21 – 18 Ma) in central Bhutan typically intruded as sills along the contact between the Greater Himalayan Series and Tethyan klippen. They have a muscovite-rich source (~30%), a maximum melt fraction of 0.18 and melted under vapour-absent muscovite breakdown conditions. Their ages indicate that the eastern Himalaya experienced both the main Himalayan leucogranite phase (24 – 18 Ma) and a younger phase at ~12 Ma recorded in northern Bhutan. Two subgroups of the high grade Greater Himalayan Series, at higher and lower structural levels with peak temperatures of ~650 °C and 700 °C respectively, are interpreted to represent bimodal flow within an extruding channel. Chemically and texturally distinct monazites from pelitic Greater Himalayan Series record prograde burial and decompression stages, with monazite ages constraining peak pressures conditions to 21 – 19 Ma. Burial (1.4 ± 0.2 mm/yr), heating (4.5 ± 1.8 °C/Ma) and decompression (3.5 ± 0.7 mm/yr) rates are calculated from a P-T-t path generated from a pseudosection for a pelitic sample from the Greater Himalayan Series, yielding the first known burial rate for amphibolitegrade Greater Himalayan Series rocks and indicating that rates of heating and decompression in Bhutan are comparable to other parts of the orogen.