Geochemistry and U–Pb Geochronology of Volcaniclastic Rocks From the NW Lesser Himalayan Sequence: Implications for Arc‐Related Tectonic Setting at Northern Proto‐Indian Continental Margin During the Neoproterozoic

ABSTRACT The protracted tectonic history concerning the northern edge of the pre‐Himalayan proto‐Indian plate margin during the Neoproterozoic time has been a long‐standing subject of debate, with various hypotheses enduring over several decades. Although a continental rift to passive margin setting has been proposed for the deposition of Neoproterozoic rocks of the Outer Lesser Himalayan Sequence (OLHS), the similarity in detrital zircon age peaks and Nd isotope values of OLHS rocks with those of the Higher Himalayan Crystalline Sequence (HHCS) led to the proposition of an arc‐related setting. So far, there is no such report of Neoproterozoic arc‐related magmatism from the Lesser Himalayan Sequence. In the present study, an interbedded volcaniclastic horizon has been identified from the basal stratigraphic level of the Nagthat Formation, OLHS. Geochemistry and U–Pb zircon geochronology are used to understand the provenance and tectonic setting of the Nagthat volcaniclastic rock. The angularity of the framework minerals, moderate degree of weathering (avg. Chemical Index of Alteration value 62), and immature nature (Index of Compositional Variability 1–1.1) suggest that a contemporaneous volcanic province was probably the main source. Whole rock geochemical analysis shows high Th/Sc (1.67–3.40, avg. 2.71), La/Sc (4.74–8.21, avg. 7.14), and low Cr/Th ratios (1.78–5.45, avg. 3.02). It supports that the volcaniclastic was supplied from an acid to intermediate source rocks, such as rhyolite and dacite/andesite. U–Pb detrital zircon analysis yielded a major age peak around 775 Ma. However, comparison of the U–Pb zircon age data with published datasets from the Lower (Chandpur Formation) and upper (Nagthat Formation) stratigraphic horizons suggests a timing of volcanism between 800 and 700 Ma. Geochronological and geochemical characteristics of the volcaniclastic rock suggest a signature transitional from passive to active continental margin setting, indicating that at least subduction was ongoing during the Tonian (800–700 Ma) at the northern proto‐Indian plate margin.