Low–pressure metamorphism of mafic granulites in the Chinese Altay orogen, NW China: P–T path, U–Pb ages and tectonic implications

Abstract Mafic granulites occur as massive lenses within host paragneisses at Wuqiagou area in the southern Chinese Altay orogen. They comprise a suite of fine–grained and coarse–grained mafic granulites, both characterized by an assemblage of orthopyroxene, clinopyroxene, hornblende, biotite, plagioclase, quartz, Fe–Ti oxides, minor K–feldspar, and accessary apatite and zircon. The peak– and post–peak metamorphism evolution is respectively represented by orthopyroxene–clinopyroxene–bearing assemblages, and the clinopyroxene–plagioclase–bearing or hornblende–biotite–plagioclase–bearing associations. Thermobarometries and pseudosection modelling constrain the peak P–T conditions at ~5.5 kbar and 770–820 °C, with gradients of 40–45 °C/km for the this stage, and at ~3.0 kbar and 630–670 °C for the retrograde evolution, indicative of a possible P–T path with a post–peak cooling and slight decompression process. LA–ICP–MS and SHRIMP zircon U–Pb age dating results of two mafic granulite samples show weighted mean ages of 275 ± 1.6 Ma and 271 ± 3.2 Ma, respectively. These ages were interpreted to represent the timing of the low–pressure granulite–facies metamorphic event in this region, suggesting relatively high thermal conditions in the late Paleozoic. This metamorphism was highly coeval with the deep–derived mafic intrusions and Tarim mantle plume activity (~275 Ma). Accordingly, we concluded that the Permian regional thermal anomaly in this region might be closely related to the Tarim mantle plume activity.