Recycling ancient refractory peridotites causing the crust-mantle decoupling of the Xigaze ophiolite (South Tibet): New constraints from the Buma mantle massif

Thin oceanic crust coexisting with refractory peridotites is increasingly documented along mid-ocean ridges. This crust-mantle decoupling has been attributed to the recycling of ancient refractory peridotites underneath the ridges. However, the extent to which such recycling has influenced crustal accretion in ancient ocean basins remains inadequately explored. The Xigaze ophiolite in southern Tibet, a major fragment of Neo-Tethyan oceanic lithosphere, is characterized by its thin crust juxtaposed with thick mantle sections. This study presents geochemical, highly siderophile element (HSE), and Re-Os isotopic data from the Buma mantle massif in the western Xigaze ophiolite. The Buma peridotites display a wide range of whole-rock and mineral compositions, with Al2O3 contents of 0.5-2.2 wt% and spinel Cr# [=100 x Cr/(Cr + Al)] of 15-60. Rare earth element patterns in whole rocks and pyroxenes suggest that the Buma lherzolites and harzburgites were formed by 5-8 % and 12-19 % melting of a depleted mid-ocean ridge basalt mantle source, respectively. Moreover, these peridotites display uniform HSE patterns, with Ru/Ir and Pt/Pd ratios resembling typical abyssal peridotites, and exhibit mostly unradiogenic Os-187/Os-188 ratios (0.1225-0.1326), yielding Re-depletion ages up to 0.9 Ga. Our results, in combination with published data, suggest that all Xigaze mantle massifs were formed by anhydrous melting beneath the Neo-Tethyan ocean ridge without evidence of sub-arc hydrous melting. The high melting degrees (15 +/- 3 %) recorded in the Xigaze ophiolitic peridotites are inconsistent with the mapped thin crusts (<3 km) if all melting occurred beneath the paleo-ridge. Instead, this crust-mantle decoupling is best explained by recycling of ancient mantle, which appears essential in triggering magma-starved seafloor spreading.