Ultrahigh-temperature metamorphism and anticlockwise P-T-t path of Paleozoic granulites from north Qinling-Tongbai orogen, Central China

Mafic and semi-pelitic granulites from the Qinling-Tongbai orogen in central China preserve petrological evidence and mineral paragenesis suggesting four distinct stages of metamorphic evolution. The prograde history (M ₁) is recorded by the occurrence of cordierite, orthopyroxene and biotite inclusions in garnet porphyroblasts of the peak-metamorphic (M ₂) assemblage. Peak-metamorphism was followed by cooling with minor decompression (M ₃), which formed symplectites and coronitic textures. The greenschist facies retrograde metamorphic assemblage (M ₄) is represented by hydrous minerals replacing minerals of the M ₂and M ₃assemblages. We present LA-ICPMS zircon U-Pb data which show ages of 432±4Ma for the peak metamorphism and 403 to 426Ma for the retrograde stage. Microstructural analysis, P-T pseudosections, and mineral isopleths in conjunction with the zircon U-Pb ages define an anticlockwise P-T-t path. The P-T estimates for peak metamorphic conditions of 880-920°C and 8.0-10kbar suggest that these rocks witnessed extreme crustal metamorphism under ultrahigh-temperature conditions. The anticlockwise trajectory reported in this study is comparable with similar P-T paths recorded from subduction-collision settings, and correlate the Tongbai granulites to hot orogens developed within a Paleozoic collisional suture. We propose a ridge subduction and slab window setting to explain the formation of the Tongbai orogen, in a convergent plate setting associated with the northward subduction of the Paleo-Tethyan Qinling Ocean.