Depositional environment and tectonic backdrop of meta-carbonates in the Eastern Himalayan ophiolites, India: insights from calcite microstructures, whole-rock elements and stable isotopes

Abstract

Tuting–Tidding Suture Zone (TTSZ) is the eastern extension of the Indus–Tsangpo Suture Zone (ITSZ) and is exposed in the Arunachal Himalaya, India. It comprises rocks of ophiolitic affinity associated with amphibolites, volcano-sedimentary units and meta-carbonates, occurring as a folded sequence of dismembered outcrops. The meta-carbonates are banded and massive in nature and appear devoid of any microfossils. We present whole-rock geochemistry and stable isotopes of these meta-carbonates to explain their depositional as well as post-depositional characteristics. REE patterns (ƩREE = 3.63–47.10), with almost flat to slight enrichment of HREE as compared to LREE [(La/Yb)SN = 0.60–1.37] are comparable with REE patterns of marine carbonates. The δ13CPDB and δ18OPDB values range between 0.33 and 4.29‰ and − 13.90 and − 6.50‰, respectively. Poor correlations between isotopic ratios of δ13CPDB and δ18OPDB, values of whole-rock elemental ratios (Mg/Ca, Fe/Sr, Mn/Sr, Ca/Sr), and extremely low Na/Ca indicate that these rocks were formed in a hypersaline environment with least effect of post-depositional diagenetic processes. Calcite microstructural studies of the TTSZ meta-carbonates suggest that they have undergone deformation up to mylonitic stage, which generally occurs at temperatures above 400 °C. This deformation can be correlated with the metamorphism and deformation event of the ophiolite that occurred during accretion and exhumation of the sequence in the cold subduction zone. Thus, based on the microstructural, geochemical, and isotopic evidences, we propose that the TTSZ meta-carbonates were formed in a shallow marine environment during the late stage of an intra-oceanic subduction and were later deformed along with the rest of the ophiolitic rocks during accretion and exhumation. © 2021, The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.