Contrasting CW and CCW tectono-metamorphic belts in the eastern Himalayan syntaxis: quantification of P–T–t paths and tectonic interpretation

In the eastern Himalayan syntaxis, the Indus–Tsangpo Suture Zone separates the Himalaya to the south from the southern Lhasa terrane to the north. In this study, we combine petrology, zircon U–Pb geochronology and phase equilibrium modeling to quantify the P–T–t paths of metapelites from the Namche Barwa complex, part of the Himalaya, and the Nyingchi complex, part of the southern Lhasa terrane. The Namche Barwa complex is interpreted to record slow cooling across a HP granulite facies P–T field at >12 kbar from 790 °C to the solidus at 750 °C between c. 50 Ma and c. 17 Ma, followed by decompression with slight heating to a P–T field at 5.5–8 kbar and 790–825 °C at c. 3 Ma, consistent with the post-peak segment of a CW P–T–t path. By contrast, the Nyingchi complex records a CCW P–T–t path characterized by compression through peak granulite facies P–T conditions of 7.5–9 kbar at 760–780 °C, before re-crossing the solidus at P of ～11 kbar at c. 35–31 Ma. Based on the youngest published spot date from a detrital core in zircon from metapelite, the maximum depositional age for the protoliths of the Nyingchi complex is c. 53 Ma. We propose a tectonic scenario in which the Nyingchi complex (CCW P–T–t path) was buried in the interval c. 53–35 Ma synchronous with heating then compression during ongoing convergence related to the India–Asia collision. By contrast, the Namche Barwa complex (CW P–T–t path), which represents the subducted Indian continental lithosphere, records relatively fast exhumation from c. 17 to c. 3 Ma that was initiated by crustal-scale folding linked to orogen-scale gravitational spreading and thrusting, enhanced by surface processes.