A fluid inclusion study of blueschist-facies lithologies from the Indus suture zone, Ladakh (India): Implications for the exhumation of the subduction related Sapi-Shergol ophiolitic melange

The best occurrence of blueschist-facies lithologies in Himalaya is that of the Shergol Ophiolitic M lange along the Indus suture zone in Ladakh region of north-western India. These lithologies are characterized by well preserved lawsonite-glaucophane-garnet-quartz assemblages. This paper presents for the first time the results of a detailed fluid inclusion study on these lithologies, in order to understand the fluid P-T evolution and its tectonic implications. The blueschist rocks from Shergol Ophiolitic M lange record metamorphic peak conditions at similar to 19 kbar, 470 degrees C. Several types of fluid inclusions are trapped in quartz and garnet, most of them being two-phase at room temperature. Three types of fluid inclusions have been recognised, basing on microtextures and fluid composition: Type-I are primary two-phase carbonic-aqueous fluid inclusions (V-CO2 - L-H2O); Type-II are two-phase (L-H2O - V-H2O) aqueous fluid inclusions, either primary (Type-IIa) or secondary (Type-IIb); Type-III are re equilibrated fluid inclusions. In the Type-I primary carbonic-aqueous inclusions, H2O is strongly predominant with respect to CO2; the homogenization temperature of CO2 range from -7 to -2 degrees C. The clathrate melting temperature in such inclusions varies in between + 7.1 and + 8.6 degrees C. Type-II two-phase aqueous fluid inclusions show a wide range of salinity, from 7.8-14 wt.% NaCleq (Type-IIa) to 1.65-6.37 wt.% NaCleq (Type-IIb) with accuracy +/- 0.4 wt.% NaCleq. Type-I and Type-IIa primary fluid inclusions are hosted in peak minerals (garnet and quartz included in garnet), therefore they were likely entrapped at, or near to, peak P-T conditions. The dominantly aqueous fluid of both Type-I and Type-IIa inclusions was most likely produced through metamorphic devolatilization reactions occurring in the subducting slab. Despite their primary nature, the isochores of Type-I and Type-IIa inclusions do not intersect the peak metamorphic conditions of the blueschist mineral assemblage, suggesting that these inclusions stretched or re-equilibrated during nearly isothermal decompression from 19 kbar to 3 kbar or less, at T = 290 degrees C. This conclusion is further supported by their large variability in shapes and sizes which range from irregular inclusions('C'/arc shaped, hook shape and satellite type). This decompression stage was followed by nearly isobaric cooling, testified by the occurrence of dendritic networks of decrepitated and 'imploded' fluid inclusions.