Prolonged metamorphism of garnet-orthoamphibole gneisses from the Fuyun area: New insights into metamorphic evolution of the southern Chinese Altai orogen

Abstract

Garnet–orthoamphibole gneisses (GOG) represent a metamorphic lithology originated from hydrothermally altered metasediments or metabasites, and usually comprise low–variance mineral assemblages and are suitable to recover the P–T trajectory. In this study, we firstly report the occurrence of the GOG in the southern Chinese Altai orogen, NW China. Revealing metamorphic P–T–t path for GOG is crucial for better understanding of the orogenic evolution. Petrographic observations, mineral chemistry and phase equilibrium modeling suggest a three–stage metamorphic evolution: 1) M1 assemblage mainly consists of garnet + hornblende + gedrite + biotite + plagioclase + quartz + sillimanite + rutile ± ilmenite, with P–T conditions of 7.7–9.0 kbar and 700–790 °C; 2) M2 stage is represented by symplectitic intergrowths of orthopyroxene + cordierite + ilmenite and local presence of spinel, with P–T conditions of 5.9–6.5 kbar and 800–880 °C; and 3) M3 stage is characterized by an assemblage of anthophyllite/biotite + plagioclase + quartz + ilmenite + cordierite ± staurolite, with P–T conditions of 5–6 kbar and < 650 °C. Therefore, GOG in this region have undergone a generalized clockwise P–T path. LA–ICP–MS dating results for zircon and rutile indicate that the M1 amphibole–facies metamorphism occurred at ∼320 Ma, and the M2 granulite–facies overprint and the post–peak cooling occurred at 300–250 Ma, with a concordant age of 275 ± 4 Ma. The GOG protoliths are considered as chemically altered igneous rocks that originally formed in the Devonian (386 ± 6 Ma). The protracted metamorphic evolution of GOG (>80 Ma) may have witnessed a long–lasting tectonic evolution from continent–arc collision (i.e. between the Junggar terrane and the Chinese Altai orogen) to post–orogenic extension in the late Paleozoic. The latter episode was likely companied by underplating and intruding of mantle–derived magmas as a result of the slab break–off or the nearby Tarim mantle plume activity.