Tectono-metamorphic evolution of the Karakoram Terrane: Constrained from P–T–t–fluid history of garnet-bearing amphibolites from trans Himalaya, Ladakh, India

Abstract

This study presents new data sets on the petrological, fluid inclusion and U-Pb zircon geochronological aspects of the garnet-bearing amphibolites from the eastern Karakorum (Pangong-Tso), Ladakh. to delineate the P-T-t-fluid evolution and its tectonic implications.The calculated phase equilibria in the model system NCKFMASHTO reveal that the peak metamorphism occurred at 8.5–9.5 kbar, 630–655 °C, and retrograde metamorphism crossed a stage at 5.5–5.7 kbar, 620–630 °C. Monophase primary and secondary carbonic and carbonic-aqueous fluid inclusions occur in garnet and quartz. Trapping of the primary inclusion occurred at pressures lower than the estimated peak P-T conditions (5.8–6.1 kbar, 629–638 °C) indicative of partially re-equilibrated fluid inclusions during exhumation. The primary inclusions were original as carbonic-aqueous fluid derived from prograde decarbonation of the amphibolite’s protolith or adjacent carbonate-rich rocks and H2O leached out during deformation, leaving a pure CO2 fluid.U–Pb dating of zircons yielded two age groups: (i) at 128 to 131 Ma interpreted as the age of detrital zircons of sedimentary derivation, and constrain the maximum depositional age of the sedimentary protolith at 131 Ma and (ii) at 114 ± 0.38 Ma is interpreted as the age of peak metamorphism remarkably similar to the metamorphic age of the surrounding metapelites. This metamorphic stage corresponds to the well-known Cretaceous phase of crustal thickening and heating along the active continental margin of South Asia.