Detrital chrome spinel evidence for a Neotethyan intra-oceanic island arc collision with India in the Paleocene

Abstract

Models that support a single collision scenario for India and Eurasia are incompatible with the evidence that an intra-oceanic island arc (IOIA) existed within the Neotethyan Ocean. Understanding the spatial and temporal extent of any IOIA is crucial for India-Eurasia collision studies as the entire ocean, including any intra-oceanic features, must have been consumed or emplaced prior to continental collision. Here, we review what is known about the Neotethyan IOIA and report evidence from sedimentary successions in NW India and southern Tibet to constrain when and where it was emplaced. We use detrital mineral geochemistry and supporting provenance and age data to identify the source of the sediments and compare the timing of erosion of IOIA-derived material in both regions.Detrital chrome spinels, extracted from distinct sedimentary horizons in southern Tibet (Sangdanlin) and NW India (Ladakh), exhibit similar average geochemical values (TiO2=0.09 and 0.24%, Cr#=0.66 and 0.68 and Mg#=0.45 and 0.53, respectively) and supra-subduction zone (SSZ), forearc peridotite signatures. Furthermore, they overlap with in-situ chrome spinels reported from the Spongtang Ophiolite in NW India and the Sangsang Ophiolite in southern Tibet. As with many of the ophiolitic remnants that crop out in and adjacent to the Yarlung-Tsangpo and Indus suture zones (YTSZ and ISZ respectively), the Spongtang and Sangsang ophiolites formed in an IOIA setting. Linking the source of the detrital chrome spinels to those analysed from remnant IOIA massifs in the YTSZ and ISZ is strong evidence for the emplacement of the IOIA onto the Indian margin.The timing of the IOIA collision with India is constrained by the depositional ages of the chrome spinel-bearing sediments to the end of the Paleocene (Thanetian) in southern Tibet and the Early Eocene in NW India. This indirectly provides a maximum age constraint of Late Paleocene-Early Eocene for intercontinental collision between India and Eurasia. Additionally, this study highlights the importance of targeting distinct sedimentary horizons in collision zones to find evidence for discrete tectonic events that may be obfuscated by later collisions.