Paradigm shift in understanding the Yarlung-Tsangpo ophiolites: Insights from comparisons with ultraslow-spreading ocean ridges

Abstract

The Yarlung-Tsangpo ophiolites (YTO) are crucial tectonic windows that preserve coherent remnants of the Neo-Tethyan oceanic lithosphere in the Tibetan Plateau. They are particularly characterized by geological features that are much different from the Penrose model, but closely resemble oceanic lithospheres generated by detachment faults at slow- to ultraslow-spreading ridges. However, geochemical studies on the YTO have widely accepted an origin of suprasubduction zones (SSZ). The seemingly paradoxes between geological and geochemical evidence exemplify the “ophiolite conundrum” that have also been commonly observed in global ophiolites, which thus should be reconciled by any model. The YTO, especially the Xigaze ophiolite in its central segment, exhibits a complete but laterally highly variable oceanic crustal sequences, indicating strong magma focusing during its formation. Notably, the lower oceanic crust is either absent or formed through episodic and intermittent magma supplies, probably representing the low end of the magma budget among ocean ridges over time. Decoupled with the thin crust, the mantle peridotites underneath show significant compositional variations and are highly heterogeneous in osmium isotopes, indicating that they represent mantle residues of a heterogeneous asthenosphere containing recycled ancient mantle domains of variable origins. These findings, combined with the widespread occurrence of detachment faults, collectively support an origin of ultraslow-spreading ridges for the YTO. In this scenario, the ophiolite conundrum of the YTO can be elucidated through alternative models: 1) re-melting of ancient mantle domains within the heterogeneous asthenosphere, and 2) subduction re-initiation that superimposed arc-like magmatism on pre-existing mid-ocean-ridge-basalt-like oceanic crusts. Therefore, we suggest that ancient and modern oceanic lithospheres generated at ultraslow-spreading ridges provide resolutions to the “ophiolite conundrum” and also call upon a paradigm shift in our understanding of global ophiolites from the Penrose model to the Chapman model.