The 40Ar/39Ar and Rb–Sr chronology of the Precambrian Aksu blueschists in western China

Abstract

Three metapelite samples from the Aksu blueschist terrane, Xinjiang, China, were dated by the 40Ar/39Ar method on separated phengite grains, obtaining plateau ages in the range of 741−757Ma. In contrast, the measured Rb and Sr isotope data for the three samples yielded isochron ages ranging from 630Ma to 900Ma, suggesting large heterogeneity in the blueschist protolith and suppression of diffusional exchange owing to the low-temperature metamorphic conditions. Because the protolith of Aksu blueschist is composed of oceanic materials that formed 40Ar-free phengite during HP and UHP metamorphism and the apparent 40Ar/39Ar plateaus ages in this study are similar to previous K–Ar and Rb–Sr ages, the existence of excess argon in these rocks is considered to be insignificant. As a result, the 40Ar/39Ar plateau ages in this study (ca. 750Ma) likely represent the approximate time for peak metamorphism, given the low peak metamorphic temperatures for the Aksu blueschist terrane (300−400°C). This strongly implies that modern style, cold subduction tectonics operated along the margin of the Aksu terrane no later than 750Ma, in Neoproterozoic time.