Metamorphic P-T-t paths of pelitic granulites of the Taihua metamorphic complex in the Mts. Huashan area and tectonothermal implications for the Palaeoproterozoic Trans-North China Orogen

Metamorphic evolution and geochronology of pelitic granulites were reported for the first time in the Taihua metamorphic complex, Mts. Huashan area, southern segment of the trans-North China Orogen (TNCO). Three generations of metamorphic mineral assemblages are recognized: (1) the prograde metamorphic mineral assemblages (M1) are represented by mineral inclusions within the garnet porphyroblasts; (2) the metamorphic peak assemblage (M2) are the garnet porphyroblasts and minerals in the matrix (biotite + orthopyroxene + plagioclase + quartz + K-feldspar + ilmenite + zircon + magnetite); and (3) the retrograde metamorphic mineral assemblages (M3) are represented by the symplectic assemblages around the garnet porphyroblasts, resulted from decomposition reactions between garnet rims and neighboring minerals in the matrix. Calculated by both conventional thermobarometry and pseudosection modeling in the NCKFMASHT system using the Perple_X technique, the P-T conditions of these three metamorphic stages are constrained to be of 4-5 kbar/520-530 °C for the M1 stage, 6.8-8.6 kbar/730-810 °C for the M2 stage and 4.1-6.4 kbar/570-740 °C for the M3 stage, respectively. The derived clockwise P-T paths imply that the Mts. Huashan terrane involved in the subduction and subsequent collision between the Eastern and Western Blocks of the North China Craton (NCC) along the Palaeoproterozoic Trans-North China Orogen (TNCO). High resolution SIMS U-Pb dating of metamorphic zircons reveals the metamorphic ages of 1.85-1.82 Ga. Combined with geochronological data from the literature, it is concluded that the tectonothermal evolution between the Eastern and Western Blocks started as early as ∼1.97 Ga and lasted as late as 1.80 Ga. Furthermore, the eastern Taihua complex records older metamorphic ages and higher peak metamorphic pressures than those of the western Taihua complex, possibly suggesting an eastward subduction model for the TNCO.