Bimodal stable isotope signatures of Zildat Ophiolitic Mélange, Indus Suture Zone, Himalaya: implications for emplacement of an ophiolitic mélange in a convergent setup

Abstract

Zildat Ophiolitic Melange (ZOM) of the Indus Suture Zone, Himalaya, represents tectonic blocks of the fragmented oceanic metasediments and ophiolite remnants. The ZOM is sandwiched between the Zildat fault adjacent to a gneissic dome known as Tso Morari Crystalline (TMC) and thin sliver of an ophiolite called as the Nidar Ophiolitic Complex. The ZOM contain chaotic low-density lithologies of metamorphosed oceanic sediments and hydrated mantle rocks, in which carbonates are present as mega-clasts ranging from 100 meters to few centimeters in size. In this work, calcite microstructures, fluid inclusion petrography and stable isotope analyses of carbonates were carried out to envisage the emplacement history of the ZOM. Calcite microstructure varies with decreasing temperature and increasing intensity of deformation. Intense shearing is seen at the marginal part of the melange near Zildat fault. These observations are consistent with the melange as a tectonically dismembered block, formed at a plate boundary in convergent setup. The δ18O and δ13C isotope values of carbonates show bimodal nature from deeper (interior) to the shallower (marginal, near the Zildat fault) part of the melange. Carbonate blocks from deeper part of the melange reflect marine isotopic signature with limited fluid–rock interaction, which later on provide a mixing zone of oceanic metasediments and/or hydrated ultramafic rocks. Carbonates at shallower depths of the melange show dominance of syn-deformation hydrous fluids, and this has later been modified by metamorphism of the adjacent TMC gneisses. Above observations reveal that the melange was emplaced over the subducting Indian plate and later on synchronously deformed with the TMC gneissic dome.