Isotopic fingerprinting of fluid circulation at the terminal stage of the Himalayan orogeny: An example from the Himalayan forearc basin, Indus Tsangpo suture zone, Ladakh, India

Abstract

Quartz and calcite veins are omnipresent in the Indus basin sedimentary rocks (IBSRs) of the Indus Tsangpo suture zone (ITSZ) which were formed by the accumulation of sediments derived from both sides of the tectonic plate. These veins appear to have formed just after the sediment diagenesis and were deformed together with the IBSR. The veins were studied for their rare earth elemental and isotopic geochemistry (C–O–Sr–Pb). The light rare earth element/mid rare earth element with an Eu/Eu* ratio of the veins (quartz and calcite) is >1 for the northern side (Tibetan side) and <1 for the southern side, indicating a mantle/magmatic and marine-related fluid source, respectively. The carbon, oxygen and strontium isotopes range from - 14 ‱ to - 1 ‱ VPDB, + 5.7 ‱ to + 24.9 ‱ Vienna standard mean ocean water and 0.7056 to 0.7099, respectively. The Sr–O mixing model points towards the mantle-enriched fluid on the northern side of the IBSR, whereas on the southern side, the fluid was nearly marine. The intermixing of the fluid took place in the middle part of the Indus basin sediments comprising a mixed litho-unit succession. Similar characteristics of vein fluids with the host rocks indicate the derivation of fluids from the associated host IBSR, further substantiated by the lead isotopic systematics.