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

Abstract

The 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 ε_Nd(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.