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.
- Anticlockwise P-T-t path
- P-T pseudosection
- Qinling-Tongbai orogen
- Ultrahigh-temperature metamorphism
- Zircon U-Pb geochronology