REFINING THE HIMALAYAN RETROGRADE METAMORPHIC PATH: EVIDENCE FROM FLUID INCLUSIONS IN UNDEFORMED QUARTZ VEINS, CENTRAL NEPAL

The Lesser Himalayan metamorphic series and High Himalayan crystalline gneiss complex, central Nepal, host cross-cutting quartz veins containing abundant H₂O+CO₂+NaCl fluid inclusions. The inclusions are inferred to have formed on the post-metamorphic retrograde path.

The fluid inclusions contain variable amounts of CO₂ and salinities are generally low (<10 wt. % NaCl). Most inclusions within a given array or plane homogenize to either the liquid or vapor state. However, a few arrays or planes contain inclusions that homogenize to both the liquid and vapor phase, strongly suggesting the trapping of an immiscible fluid. These inclusions homogenize to liquid between 295 and 306^oC, or to vapor between 310 and 315^oC. The slightly higher vapor homogenization temperatures are expected in the trapping of H₂O-bearing immiscible liquids because of excess H₂O from wetting of inclusion walls. Because these fluids are trapped directly on the H₂O+CO₂+NaCl solvus, precise pressure estimates can be obtained from total homogenization (to liquid) temperatures and known PVT properties of H₂O+CO₂+NaCl fluids (Darling and Bassett, 2002; Am. Min.). Using the equation of state of Duan et al. (2003; GCA), calculated pressures on the solvus range from 936 to 1160 bars. Calculated liquid compositions on the solvus are XH₂O=0.61-0.64, XCO₂=0.36-0.39, and XNaCl=0.0033-0.0056. Assuming no post-entrapment changes, the fluid inclusions must have been trapped at conditions of 295 to 306^oC and 936 to 1160 bars. These PT conditions help further refine the clockwise post-metamorphic retrograde path for this part of the Himalayas and document a relatively high geothermal gradient at the time of trapping.