Prodigious shift in provenance across Permian-Triassic Boundary at Guryul Ravine Section, Kashmir, Tethys Himalaya, India: Evidences from Sr and Nd isotopes

The Permian-Triassic Boundary (PTB) section (Late Permian Zewan Formation and Early Triassic Khunamuh Formation) at Guryul Ravine (Kashmir, India) preserves potentially important information about the depositional conditions and provenance changes across the PTB. Several palaeobiological, sedimentological and geochemical studies have been conducted on this archetypal section to understand the causes of mass extinction at the PTB, however no integrated chemo-stratigraphic study focusing on provenance and geodynamics has been attempted. In the present study, samples from a 36 m thick interval across the PTB are analysed for major and trace element concentrations, Sr-, Nd- and Corg isotope geochemistry along with TOC (total organic carbon) contents to understand the contemporary environmental and provenance changes. The Late Permian Zewan Formation have relatively low chemical index of alteration (CIA) values (55-77) and low alpha AlCa (0.1-12.6), alpha AlNa (0.7-9.6), alpha AlMg (1.0-6.1) values, implying moderate weathering conditions; whereas the Early Triassic sediments of overlying Khunamuh Formation show higher CIA (69-79) and alpha AlCa (1.4-14.0), alpha AlNa (2.2-11.5), alpha AlMg (2.0-6.4) values indicating more intense chemical weathering conditions. The vertical profiles of delta 13Corg, delta 34Spyr and various redox-indicators suggest euxinic-anoxic condition during the latest Permian, and show a significant change in depositional conditions from anoxic to oxic across the PTB. The significant finding of the present study is the dramatic change of Sr- and Nd isotopic compositions across the PTB, indicating remarkable shift in provenance. The Late Permian samples show much less-radiogenic epsilon Nd(0) (avg. -15.27), radiogenic 87Sr/S6Sr(0) (avg. 0.72409) and older TDM ages (1.35-1.56 Ga) compared to the early Triassic samples (avg. epsilon Nd(0) = -9.49, 87Sr/S6Sr(0) = 0.719282, TDM = 1.71-1.76 Ga). The epsilon Nd(0) and 87Sr/S6Sr(0) composition of Late Permian rocks overlap with the signatures of Tethyan Himalaya and Neoproterozoic-Cambrian Marwar Supergroup of NW India, indicating similar old cratonic sources. Higher epsilon Nd(0), younger TDM ages and lower 87Sr/86Sr(0) in Triassic samples suggest contribution from juvenile sources and Panjal volcanics, Cadimian arc of Cimmeria or AfricanNubian shield could be the likely source terrains.