Low-pressure granulite-facies metamorphism in the Southern Chinese Altay orogenic belt, NW China: P-T estimates, U-Pb ages and tectonic implications

The Chinese Altay orogenic belt formed in the Paleozoic is an important part of the Central Asian Orogenic Belt (CAOB), accompanying with remarkable metamorphism. Its tectono-metamorphic evolution history is still a controversial topic. In this contribution, we investigate the petrography, metamorphic peak P-T conditions and zircon geochronology of the newly discovered garnet-absent metapelitic and felsic gneisses at Wuqiagou area in the southern Chinese Altay. They are characterized by an assemblage of orthopyroxene + biotite + plagioclase + quartz + Fe-Ti oxides +/- cordierite +/- K-feldspar, in which most K-feldspar grains have transformed into perthites with thin albite lamellae. The Wuqiagou garnet-absent gneisses experienced pervasive partial melting, as testified by microstructures such as mineral pseudomorphs after melt films or pockets. Phase diagram modelling constrains the peak P-T conditions to 3.5-5.5 kbar and 800-900 degrees C, with possible geothermal gradients of 45-75 degrees C/km, indicative of a prominent low-pressure granulite-facies metamorphic event in the southern Chinese Altay. SIMS zircon U-Pb dating results show a weighted mean age of 255.8 +/- 1.8 Ma. This age is interpreted to represent the timing of this low-pressure granulite facies metamorphism, which is highly coeval with the timing of their hosted mafic granulite lenses (255 Ma) and nearby mantle-derived mafic intrusions (257 Ma). Based on available petrological, geochemical and chronological data, we propose that the Permian low-pressure granulite-facies metamorphic event in the southern Chinese Altay was likely associated with the intrusions of deep-derived mafic magma at a relatively shallow crustal level (12-18 km) in a post-orogenic extensional setting, with a possible link with the Tarim mantle plume activity.