Gabbroic eclogites formed during rapid and cold subduction of the Paleo‐Tethys oceanic lithosphere in the Changning–Menglian orogenic belt, southeastern Tibetan plateau

Abstract

AbstractThe Changning–Menglian orogenic belt (CMOB) in the southeastern Tibetan plateau separates Gondwana‐ from Eurasia‐derived continental blocks and marks the main suture of the Paleo‐Tethys, as evidenced by a variety of oceanic basalt‐derived eclogites. However, it is uncertain whether the belt contains high‐pressure rocks derived from gabbro, which is a key component of oceanic ophiolite. Here, we present a study of newly discovered gabbroic eclogites from the CMOB. These eclogites preserve relic gabbroic crystals (diopside, bytownite/anorthite and ilmenite) that survived metamorphism and occur in the form of inclusions within porphyroblasts. The eclogites have positive εNd(t) values of +1 to +8 and have an affinity to N‐MORB, with positive Eu anomalies and no depletions in high‐field‐strength elements (e.g. Nb, Ta, Zr and Hf). Cumulate gabbros generated in a mid‐ocean ridge setting are possible protoliths for the studied samples. The eclogite facies mineralogy is defined by the assemblage of garnet + omphacite + kyanite + talc + phengite + rutile, which was followed by the post‐kinematic crystallization of winchite and clinozoisite, and a later symplectite assemblage (diopside + sodic plagioclase + calcic amphibole + clinozoisite). Phase equilibrium modelling, average P–T thermobarometry and conventional mineral geothermobarometry constrain the P–T conditions for the peak‐stage and initial post‐peak‐stage metamorphism and symplectite formation to 25.6–27.1 kbar/595–637°C, 15.3–17.9 kbar/563–605°C and 5.5–7.3 kbar/470–500°C, respectively, consistent with a subduction depth of 75–85 km. Metamorphic zircons yielded a Triassic mean U–Pb age of 223.7 ± 2.9 Ma, which is interpreted to record the early‐stage decompressive overprinting. The similar paragenetic sequences, mineral evolution, peak P–T conditions and P–T–t paths for the gabbro‐ and basalt‐derived eclogites in the CMOB indicate that these rocks formed in the Paleo‐Tethys subduction regime. The lack of deformation, and the cold and rapid subduction history, contributed to the local preservation of gabbroic minerals and igneous textures under high‐pressure conditions in the studied rocks. The gabbroic eclogites provide insights into the detailed metamorphic evolution during the burial–exhumation cycle of ophiolites in the Paleo‐Tethyan regime.