P-T evolution across the Main Central Thrust Zone (Eastern Nepal): hidden discontinuities revealed by petrology

Identifying the location and nature of theMain CentralThrust Zone (MCTZ) is a major challenge in most of the Himalayan chain. As a contribution to clarifying this conundrum, in eastern Nepal a number of metapelite samples were selected for petrological study along a transect on the eastern flank of the ArunTectonicWindow. Both to the west and east of the study area, the Makalu and Kangchendzonga transects show metamorphic units characterized by a well-documented inverted metamorphism, with metamorphic grade increasing northward from lower (Lesser Himalaya) to higher (High Himalayan Crystallines) structural levels across the MCTZ. A detailed petrological study of the selected metapelite sequence allowed us to recognize three superimposed tectonometamorphic units, which are separated by cryptic and transitional metamorphic discontinuities. These units are characterized by different P^T evolutions, peculiar zoning styles of garnets and contrasting T/depth ratios. Specifically: (1) the structurally lowest sample shows a prograde P^T path characterized by an increase in both P and T, up to peak metamorphic conditions of 5508C and 065 GPa; (2) two structurally intermediate samples preserve relics of a prograde history characterized by heating and decompression up to peak metamorphic conditions of 600^6508C and 085^095 GPa; (3) the structurally highest samples consist of mostly unzoned minerals and assemblages, interpreted as chemically equilibrated, that do not preserve relics of their prograde metamorphic history. Peak metamorphic conditions of 6558C, 075 GPa (still inside the white mica stability field), and a minimum temperature of 7908C at 105 GPa (beyond the stability limit of white mica) have been determined for these samples. These data provide new constraints on the location, tectonic setting and metamorphic evolution of the Himalayan units in a hitherto poorly known area.