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.

中文翻译：

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火山相作为古洋壳古深度和古构造环境的指南：以 Nidar 蛇绿岩为例，拉达克，印度跨喜马拉雅山

在造山带中发现的蛇绿岩是大陆碰撞和海洋闭合期间在陆地上包裹的古代海洋岩石圈的切片。它们为海底火山学和岩石学过程提供了宝贵的见解。蛇绿岩的古构造解释在很大程度上依赖于地球化学数据，尽管在蛇绿岩中通常观察到相当大的海底蚀变甚至变质作用。通常不提供或寻求对基于地球化学的推论进行独立检查。在这里，我们展示了迄今为止无法获得的火山相透视图，该岩相在印度跨喜马拉雅山的拉达克地区暴露于海拔 4100 m 以上的~140 Ma Nidar 蛇绿岩。我们描述了形成洋壳的枕状熔岩和透明碎屑岩的特征，以及上覆沉积覆盖层中的钙磷矿和硅质火山灰层（主要是放射虫燧石，年代为 132-112 Ma）的特征。整个火山沉积序列与大洋中脊环境不一致。我们将其解释为在新特提斯洋的早白垩世、成分双峰、洋内岛弧中形成于浅（~ 2.5 公里）海底环境中，伴随着持续的硅质爆发性喷发。Nidar蛇绿岩的地球化学同位素数据以前曾被用来论证洋内弧的起源。我们建议采用火山相方法研究蛇绿岩可以成为了解其古深度和古构造环境的宝贵指南。整个火山沉积序列与大洋中脊环境不一致。我们将其解释为在新特提斯洋的早白垩世、成分双峰、洋内岛弧中形成于浅（~ 2.5 公里）海底环境中，伴随着持续的硅质爆发性喷发。Nidar蛇绿岩的地球化学同位素数据以前曾被用来论证洋内弧的起源。我们建议采用火山相方法研究蛇绿岩可以成为了解其古深度和古构造环境的宝贵指南。整个火山沉积序列与大洋中脊环境不一致。我们将其解释为在新特提斯洋的早白垩世、成分双峰、洋内岛弧中形成于浅（~ 2.5 公里）海底环境中，伴随着持续的硅质爆发性喷发。Nidar蛇绿岩的地球化学同位素数据以前曾被用来论证洋内弧的起源。我们建议采用火山相方法研究蛇绿岩可以成为了解其古深度和古构造环境的宝贵指南。Nidar蛇绿岩的地球化学同位素数据以前曾被用来论证洋内弧的起源。我们建议采用火山相方法研究蛇绿岩可以成为了解其古深度和古构造环境的宝贵指南。Nidar蛇绿岩的地球化学同位素数据以前曾被用来论证洋内弧的起源。我们建议采用火山相方法研究蛇绿岩可以成为了解其古深度和古构造环境的宝贵指南。