Metamorphism and anatexis of high-pressure mafic granulites in the Dulan unit, North Qaidam, Northwestern Tibet

Abstract

Metamorphism and anatexis of high-pressure (HP) mafic granulites are effective for revealing partial melting and metamorphic process in continental collisional orogenic belts. HP mafic granulites and associated adakitic leucosomes have been recognized in Dulan area, North Qaidam, however, the metamorphic evolution and the linkage between the metamorphism and anatexis remain ambiguous. Based on the petrographic textures and mineral assemblages, three phases of metamorphic evolution are recognized for the HP mafic granulites: (1) the amphibolite-facies prograde metamorphism, represented by mineral inclusions of plagioclase + amphibole + chlorite + quartz in the large-grained garnet; (2) the HP granulite-facies peak metamorphism, represented by garnet + matrix mineral assemblage: garnet + clinopyroxene ± amphibolite + plagioclase + kyanite + zoisite + quartz + rutile; (3) the retrograde metamorphic stage, represented by the symplectite of the fine-grained clinopyroxene + plagioclase in the large clinopyroxene and garnet grains, and the following intergrowth of light-green amphibolite + plagioclase on garnet rims. Conventional geothermobarometries yield the P-T conditions of 565–605 °C and 9.4–10.8 kbar, 15.2–17.9 kbar and 830–925 °C, and 720–795 °C and 11.8–13.2 kbar for the amphibolite-facies prograde metamorphism, the HP granulite-facies peak metamorphism, and the retrograde metamorphic stage, respectively. The P-T pseudosection shows that the HP mafic granulite experienced a clockwise P-T path with the peak stage at 891–910 °C/17.2 kbar and a partial melting degree of 45–50 %, forming the adakitic melts/TTGs. Zircon U-Pb dating suggests that the HP mafic granulites experienced a magmatic event at 458 ± 5 Ma followed by a metamorphic and anatectic event at 445 ± 2 Ma. The HP mafic granulites are rich in light rare earth elements and large ion lithophile elements (e.g., Ba, Sr, Th, U) and poor in high field strength elements (e.g., Nb, Ta, Ti), showing arc-related geochemical affinities. These arc magmatic rocks were buried to depths exceeding 56 km and heated to ca. 900 °C during collisional orogeny, and experienced HP granulite-facies metamorphism and anatexis. Geochronological and geochemical data demonstrate that the leucosome within the HP mafic granulite in the Dulan area may have been generated by the partial melting of metagabbro under HP granulite-facies conditions in an arc root, which migrated and converged to form the coeval tonalite plutons, with HP mafic granulite and garnet pyroxenite representing the residue. These results will provide key insights into the geotectonic processes of metamorphism and anatexis of arc magmatic rocks within the upper plate of the subduction zone as well as their significance for orogenesis.