Exhumation of the Tso Morari eclogite, northwestern Himalayas

 The coesite-bearing eclogites of the Tso Morari dome in northwestern Himalayas occur as boudins within felsic gneisses. They represent exhumed Indian continental crust, and they originated through subduction of the northern continental margin of India during its early Eocene collision with the Kohistan-Ladakh arc. The process of exhumation of continental ultrahigh-pressure (UHP) rocks such as eclogites is not well understood, though several geodynamic mechanisms such as crustal stacking, channel flow, transmantle diapirs, and incorporation within a collisional orogen have been proposed. According to early researchers, eclogites are exhumed to the base of the lower crust through the "subduction channel", a wedge-shaped low-viscosity zone along the interface between the subducting slab and the overlying mantle wedge. However, subduction channels are characterized by a thermal gradient, and the Tso Morari UHP rocks show isothermal decompression in their P-T paths of metamorphism. Furthermore, the lower pressure conditions of isothermal decompression do not exist along the top surface of subducting slabs. Thus, P-T paths based on metamorphic equilibria are not consistent with exhumation of the Tso Morari eclogites through a subduction channel. On the other hand, more recent numerical simulations, petrological modeling, and field evidence from Papua New Guinea have shown the possibility of exhumation of continental UHP rocks through diapirism within the mantle wedge, or through the formation of a continental collisional orogen.

We present new constraints on the P-T evolution of hydrous and carbonate-rich samples of the Tso Morari eclogite between 2.2-2.3 GPa/400-425 oC and ~0.4 GPa/450 oC using thermobarometry and calculated P-T-MCO2 phase equilibria. Our results indicate that the eclogites were strongly heated at high pressures from 400-425 oC at 2.2-2.3 GPa to 670-720 oC at 1.80-1.9 GPa during the early stages of exhumation. Diffusion modeling of Ca variation across the core-rim interface of garnet indicates that the heating stage lasted only <0.1 Myr, in accordance with geochronological constraints and fast exhumation rates. Our P-T path is at odds with exhumation of the eclogites along a subduction channel as model calculations indicate that the intermediate P-T conditions of 1.8-1.9 GPa/670-720 oC are not achieved along the subducting slab. Instead, the constrained P-T conditions are consistent with heating within the mantle wedge overlying the subducting slab. Therefore, we conclude that the Tso Morari eclogites were possibly exhumed as part of a low-density, felsic diapir rising through the mantle wedge. Based on low viscosity values of mantle wedges associated with modern subduction zones, the calculated exhumation rate for the Tso Morari eclogite is extremely fast (29-147 mm/yr) and at par with that constrained for other northwestern Himalayan UHP rocks.

Publications:

  1. Chatterjee, N. and Jagoutz, O. (2015) Exhumation of the UHP Tso Morari eclogite as a diapir rising through the mantle wedge. Contributions to Mineralogy and Petrology, 169:3. doi: 10.1007/s00410-014-1099-y