Metamorphic P-T-t path and tectonic implication of garnet pyroxenite from Raobazhai, North Dabie Complex, central-eastern China

P-T-t path plays an important role in revealing the tectonic evolution of metamorphic rocks. The debate on the tectonic environment and emplacement process of mafic-ultramafic rocks in Raobazhai have been controversial for a long time. Four generations of mineral assemblages have been recognized in the garnet pyroxenite from Raobazhai, North Dabie. Thermodynamic computation shows that the metamorphic conditions of the garnet pyroxenite are as follows. 1) Ultra-high pressure metamorphism stage (M1): the exsolution of rutile from garnet, quartz from clinopyroxene and opaque minerals from apatite, indicated that the garnet pyroxene in Raobazhai has experienced ultra-high pressure metamorphism (>= 2.5GPa). 2) High pressure granulite-facies metamorphism stage (M2): the temperature and pressure condition is T=648 similar to 700 degrees C and P=1.47 similar to 1.94GPa, which recorded by the mineral assemblage of garnet (porphyroblast) + clinopyroxene (matrix) + plagioclase (matrix). 3) The medium-pressure granulite-facies metamorphism stage (M3): it is characterized by the inner white-eye socket which is mainly composed of orthopyroxene + plagioclase developed on the periphery of garnet, and the temperature and pressure condition is T=781 similar to 796 degrees C, P=0.92 similar to 0.98GPa. 4) Amphibolite-facies metamorphism stage (M4): it is characterized by the outer white-eye socket developed around garnet (porphyroblast), and its mineral assemblage is amphibole + plagioclase under the condition of T=663 similar to 685 degrees C, P=0.50 similar to 0.58GPa. SIMS U-Pb dating of metamorphic zircons from the Raobazhai garnet pyroxenite yielded three different intercept ages of 817.7Ma, 227.6Ma and 208.1 similar to 202.1Ma, respectively. According to the assemblage of amphibole+plagioclase inclusions in zircons, the age of 208.1 similar to 202.1Ma is linked to the lower limit of the amphibolite-facies metamorphic age, the high pressure granulite-facies metamorphism began in 227.6Ma, and the Neoproterozoic age (817.7Ma) is consistent with the protoliths of metamorphic rocks in the region. Combined with previous studies, the clockwise P-T-t path of Raobazhai garnet pyroxenite was recorded, revealing the complete process of subduction (ultra-high pressure metamorphism)-collision-exhumation (decompression heating process, similar to 227Ma) -uplift (decompression cooling process, 208 similar to 202Ma).

Automated summary

The P-T-t path of the Raobazhai garnet pyroxenite recorded a complete process of subduction (ultra-high pressure metamorphism)-collision-exhumation (decompression heating process, around 227Ma) -uplift (decompression cooling process, around 208-202Ma). Through SIMS U-Pb dating, different intercept ages were obtained, revealing the timing of different metamorphic stages in the area.