THE PROTEROZOIC-CAMBRIAN CONFIGURATION OF THE NORTH INDIAN MARGIN: INSIGHTS FROM DETRITAL ZIRCONS AND NEODYMIUM ISOTOPES

The Precambrian-Cambrian configuration of the north Indian margin remains a topic of active debate, particularly with regard to the relationship of lithotectonic units of the Himalaya. Due to the lack of macrofossils and material suitable for geochronology in Precambrian strata, analyses of detrital zircons and whole-rock neodymium isotopic compositions (εNd) have become important tools in Himalayan research. Numerous studies have proposed that specific lithotectonic units can be distinguished from each other via distinct isotopic signatures, with a further suggestion that the observed difference in isotopic signatures resulted from an exotic origin of two major Himalayan lithotectonic units. Here we test these hypotheses by comparing detrital zircon age distributions from Proterozoic-Cambrian strata of the Himalaya to those from similar aged strata of the Marwar and Vindhyan Supergroups of the central Indian craton. Our results show that rocks of similar depositional age contain similar populations of detrital zircons in both the Himalayan and cratonic successions, with notable populations of 2.5, 1.9-1.6, 1.1-1.0, 0.8, and 0.5 Ga detrital zircons in nearly all rocks with appropriate depositional ages. All Himalayan detrital zircon aged distributions are present in similar aged strata of the cratonic successions including those proposed to be exotic. In addition, we show that the noted difference in εNd values occurs within a single lithotectonic zone across a ~500 million year regional unconformity, thus variation in εNd is related to depositional age and does not reflect a tectonic boundary. Numerous paleotectonic events have affected the north Indian margin, and this is reflected by a consistent introduction of young zircons into the sedimentary system. Integrated detrital zircon and εNd data can be used for first-order approximations of depositional ages of particular stratigraphic units, however these data cannot be used to characterize Himalayan lithotectonic zones. Furthermore, the observed similarity of detrital zircon age distributions in strata of the north Indian margin likely resulted from shared sources of detritus, which implies the Himalaya and cratonic succession were not isolated from each other during deposition.