Early Cambrian high pressure/low temperature metamorphism in the southeastern Tarim craton in response to circum-Gondwana cold subduction

The circum-Gondwana subduction initiated by the early Cambrian has been suggested to reflect the establishment of the modern plate tectonics.The metamorphic rocks with low thermobaric(T/P)ratios indicative of cold subduction in the present tectonic regime have not been well investigated.To better understand the circum-Gondwana subduction and to test its possible link with the emergence of the modern plate tectonics,this study focused on blueschist-facies metamorphic rocks in the Altyn Tagh of the southeastern Tarim craton.Mineral assemblage and chemistry,phase equilibrium modelling,and quartz-in-garnet Raman elastic geobarometry reveal that the zoisite blueschist and glaucophane(Gln)-bearing quartz schist in northern Altyn Tagh were metamorphosed to lawsonite to epidote blueschistfacies at 520–545℃ and 16–19 kbar.It reflects high-pressure(HP)/low temperature(LT)metamorphism with low T/P ratios of<300℃/GPa and thermal gradients of<10℃/km.These blueschist-facies metamorphic rocks underwent rapid decompression starting at P-T conditions of<495℃ and<9.6 kbar during exhumation.Ar-Ar geochronology records paragonite Ar-Ar plateau ages of 520–506 Ma for the zoisite blueschist samples and phengite Ar-Ar plateau ages of 522–516 Ma for the Gln-bearing quartz schist samples,suggesting that the peak HP/LT metamorphism occurred prior to ca.522 Ma.Based on new results and available data from the major Gondwana blocks,cold subduction was suggested to profoundly operate along circum-Gondwana in the early Cambrian after the amalgamation of Gondwana.The extensive circum-Gondwana subduction represents the earliest global cold subduction in Earth’s history associated with the establishment of the modern plate tectonics,as directly recorded by the studied early Cambrian blueschist-facies metamorphic rocks and a dramatic drop in the mean T/P of metamorphism since the early Paleozoic.