“Hot” subduction initiation and the origin of the Yarlung-Tsangbo ophiolites, southern Tibet: New insights from ultrahigh temperature metamorphic soles

Abstract

Metamorphic soles within ophiolite mélanges record key information on subduction initiation and evolution of paleo-oceans. Mafic granulite and amphibolite occur in metamorphic soles within the Saga and Bairang ophiolitic mélanges of the Yarlung Tsangbo suture zone, southern Tibet. In fresh mafic granulite, the early prograde assemblage (M1) is preserved as inclusions in the core of garnet. Minerals in matrix and garnet mantle define the peak metamorphic assemblage (M2). Near-isothermal decompression (M3) is recorded by the garnet rim and a symplectic corona. The mineral assemblage of the fresh amphibolite only records one major stage. Many samples were overprinted by strong metasomatism at sub-greenschist-facies conditions (M4). According to geochemical fingerprints, the soles originate from a mid-ocean-ridge environment with strong affinities to the non-metamorphic oceanic crust of the Yarlung Tsangbo ophiolites. P-T calculations of the granulites by pseudosection techniques, Zr-in-rutile and REE-based thermobarometry show ∼690-760°C/9.5-12.5 kbar for the prograde (M1) stage, 900-970°C/14-16 kbar for the peak (M2) stage followed by 902-983°C/9.5-12.6 kbar for the retrograde (M3) stage. From the amphibolite P-T conditions of <700-880°C/7-12.5 kbar were derived. This results in a clockwise trajectory with peak-high pressure to ultrahigh-temperature conditions characterizing a “hot” subduction environment. U-Pb dating of magmatic zircon yields ∼133-127 Ma as protolith ages of the soles, almost coeval with the igneous ages of the unmetamorphosed oceanic crust. U/Pb ages of metamorphic zircon show a similar range within uncertainties (∼131-119 Ma) indicating that initiation of subduction subsequently followed the protolith crystallisation. The metamorphic soles were subducted to a depth of 50 km triggering partial melting in the mantle wedge by dehydration reactions and formation of supra-subduction-zone magma. The mantle wedge became cooler and more buoyant when the subduction zone matured, which caused exhumation of the metamorphic soles and surrounding mantle rocks as well as cooling. A new model for a “hot” subduction initiation close to a mid-ocean ridge near the Asian margin is proposed based on the data from this study and previous studies. The relict mid-ocean-ridge basalts after subduction initiation and the subsequent supra-subduction-zone basalts formed the YTSZ ophiolites during ∼130-120 Ma.