Mineralogical and fluid inclusion constraints on the formation of the Karakorum Migmatite: implications for H.sub.2O-fluxed melting and exhumation of the South Tibetan Crust

This study presents new petrological and fluid inclusion datasets of migmatites from the Karakorum Shear Zone, Ladakh, India, to know the P-T-fluid evolution of mineral assemblages and the associated tectonic history. The presence of plagioclase, quartz, and biotite inclusions in the coarse-grained poikiloblastic pargasite (amphibole) is indicative of the hydration reaction [Formula omitted], which is consistent with diffusive H.sub.2O-fluxed melting. Phase equilibria calculations are consistent with migmatization at 0.85-1.02 GPa and 640-670 °C in water-saturated conditions (i.e., 0.7 wt% H.sub.2O). The monophase primary and secondary carbonic fluid inclusions present in quartz display eutectic temperatures between - 56.9 and - 56.6 °C, suggesting pure CO.sub.2 composition. The isochores of primary CO.sub.2 inclusions reveal that the post-peak migmatization event took place between 0.59-0.55 GPa and 550-670 °C, which occurred due to density reversal during the re-equilibration. The fluid inclusion microtextures preserved the signature of isothermal decompression, which is well corroborated with mineralogical P-T calculations. Primary inclusions were preserved initially as carbonic-aqueous fluids; however, the H.sub.2O phase diffused out subsequently and dissolved with the melt such that the inclusions became pure carbonic. Fluid infiltration along the Karakorum Shear Zone played a critical role in forming migmatites. The P-T path derived from thermodynamic modeling and fluid inclusion data are consistent with isothermal decompression during exhumation following crustal thickening of the Asian continent (or South Tibaten Crust) between 18 and 15 Ma.