Tracing Late-Stage Fluid Sources and Vein Formation within Ophiolitic Mélanges from the Indus Suture Zone, Ladakh Himalaya

Quartz-calcite veins in the Zildat ophiolitic mélange (ZOM) and Shergol ophiolitic mélange (SOM) of the Indus Suture Zone preserve a diversity of fluid activity in the late stages of ophiolitic mélange formation. This article presents fluid-inclusion and isotope geochemistry of these veins to understand their source and evolution in terms of pressure and temperature. The microstructures of quartz and calcite veins indicate deformation temperatures between 200° and 400°C. The δ13C and δ18O values of calcite veins from the ZOM and SOM are within the mixing hyperbolas of marine and primitive-mantle fields in the mixing model. The Sr and Pb isotopic values of calcite veins from the ZOM suggest a mid-ocean ridge basalt (MORB) fluid source of vein formation that was radiogenically enriched by metasomatism in a suprasubduction zone. For the SOM, fluids may have originated from the enriched-mantle (EM) and the depleted-MORB-mantle rocks. It is inferred that the carbonic fluids were derived from ultramafic lithologies and oceanic crust that formed the ophiolitic mélange rocks, which also host these veins. These source rocks have EM and MORB geochemical signatures that are also obtained in the quartz-calcite veins, as characterized by their C-O-Sr-Pb isotopic ratios. The magmatic saline fluid is inferred to have formed in the early stages of vein formation and to have been subsequently diluted, as exemplified by the presence of low-saline secondary aqueous inclusions. The microthermometry fluid pressure-temperature estimation of veins from the studied sections suggests that the maximum depth of emplacement of veining fluid was about 24.5 MPa (corresponding to ∼2.5 km) at 336°C. The vein-forming fluids (calcareous and siliceous) were introduced into the fractures that developed in the host as a result of deformation.