Volcanic facies as a guide to the palaeodepth and palaeotectonic setting of ancient oceanic crust: the case of the Nidar ophiolite, Ladakh, Indian Trans-Himalaya

Abstract

Ophiolites, found in orogenic belts, are slices of ancient oceanic lithosphere obducted on land during continental collision and ocean closure. They provide valuable insights into submarine volcanological and petrological processes. Palaeotectonic interpretations of ophiolites have heavily depended on geochemical data, despite the considerable submarine alteration and even metamorphism commonly observed in ophiolites. No independent checks on the geochemistry-based inferences are usually provided or sought. Here, we present a hitherto unavailable volcanic facies perspective on the ~ 140 Ma Nidar ophiolite, exposed > 4100 m above sea level in the Ladakh region of the Indian Trans-Himalaya. We describe features of pillow lavas and hyaloclastite forming the oceanic crust and of peperite and silicic volcanic ash layers in the overlying sedimentary cover (mainly radiolarian cherts, dated at 132–112 Ma). The whole volcanosedimentary sequence is inconsistent with a mid-ocean ridge setting. We interpret it as having formed in a shallow (~ 2.5 km) submarine environment, with ongoing explosive silicic eruptions, in an Early Cretaceous, compositionally bimodal, intra-oceanic island arc in the Neo-Tethys Ocean. Geochemical-isotopic data on the Nidar ophiolite have previously been used to argue for an intra-oceanic arc origin. We suggest that a volcanic facies approach to the study of ophiolites can be a valuable guide to their palaeodepths and palaeotectonic settings.