Thermal Evolution of the Mid-Crust From the Himalayan Orogen

Chambers, Jennifer Ann (2008). Thermal Evolution of the Mid-Crust From the Himalayan Orogen. PhD thesis The Open University.



The Haimanta Group, at the top of the exhumed mid-crust in the western Himalaya (Sutlej valley), underwent prograde metamorphism to depths of 25 to 30 km at c. 30 Ma. Initial exhumation of the Haimanta Group was accompanied by further heating, from 610-620 °C up to 660 °C at c. 23 Ma; further exhumation (at c. 1.3 mm/yr) to a depth of c. 7 km during the Early Miocene resulted in cooling to 300-350 °C. Meanwhile, the underlying Greater Himalayan Sequence (GHS) was rapidly exhumed due to coeval extension on the South Tibetan Detachment above and compression on the Main Central Thrust below. The Jutogh Group, with Palaeoproterozoic (c. 1810 Ma) leucogranites characteristic of the Lesser Himalayan Sequence, was buried and metamorphosed beneath the overthrust GHS, and involved the partial recrystallisation of uraninite in leucogranite at 11 Ma.

The Chekha Formation (Radi klippe) in the eastern Himalaya (Bhutan), in a similar structural position to the Haimanta Group, also reveals a history of Oligocene prograde metamorphism, to c. 7 kbars and 550-620°C at c. 22 Ma, synchronous with prograde metamorphism in the underlying GHS (peak metamorphism, c. 24 Ma). Comparable metamorphism and deformation in the Haimanta Group and Chekha Formation, distinct from that in the underlying GHS, is the result of a common tectono-thermal process in the early evolution of the orogen. Pressure-temperature-time data from the metamorphic core are generally consistent with the channel flow model, but may be equally compatible with contesting models.

Uranium(-thorium)-lead isotope analysis of monazite (which formed above 580 °C after, or during the final stages of garnet growth, and probably via the breakdown of allanite and apatite) provided a robust method for dating prograde-peak metamorphism. Samarium-neodymium garnet geochronology was hampered by Ca-bearing Nd-rich garnet inclusions (e.g. allanite).

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