Formation of gabbronorites in the Purang ophiolite (SW Tibet) through melting of hydrothermally altered mantle along a detachment fault

Abstract

The Purang ophiolite tectonically was thrust over an Upper Cretaceous mélange south of the Yarlung-Zangbo Suture Zone (YZSZ) in SW Tibet. It comprises a large ultramafic massif of ~ 800 km 2 , but plutonic and volcanic sections are notably absent. The Purang peridotite massif is dominated by harzburgites with minor clinopyroxene-poor lherzolites, which are occasionally intruded by gabbronoritic and basaltic dykes. They are characterized by low Al 2 O 3 and CaO, which can be modeled by ~ 15–25% degrees of isobaric batch melting at a pressure of 20 kbar. Spinel Cr# values give an estimated ~ 12–19% degrees of fractional melting for the Purang peridotites. Clinopyroxenes in Purang peridotites show variable LREE patterns and their HREE contents could be reproduced by ~ 10–16% degrees of fractional melting from a depleted mantle source. The Purang gabbronorites are olivine-free, with early crystallization of orthopyroxene and clinopyroxene relative to plagioclase. Pyroxenes in gabbronorites have more variable Mg# than those in peridotites. Plagioclases are characterized by very high anorthite contents of 94–99. Mineral compositions suggest that the Purang gabbronorites crystallized from high-silica hydrous melts, which are strongly depleted in incompatible elements. Such hydrous melts could be derived from serpentinized peridotite via melting along a detachment fault at a slow-spreading ridge that was triggered by the upwelling asthenosphere. The detachment fault exhumed the Purang peridotite massif as an oceanic core complex at the seafloor, which was emplaced and preserved in the suture zone. Therefore, we suggest that the Purang ophiolite was formed in the mid-ocean ridge rather than subduction-related settings. • Gabbronorites intrude into the mantle peridotites in Purang ophiolites. • Both orthopyroxene and clinopyroxene crystallized earlier than plagioclase in the Purang gabbronorites. • The gabbronorites represent cumulates of SiO 2 -rich, refractory hydrous melts. • The Purang gabbronorites were formed by melting of serpentinites along a detachement fault. • The Purang ophiolite was exhumed as an oceanic core complex at the mid-ocean ridge.