Metamorphism and geochronology of the Luoning metamorphic terrane, southern terminal of the Palaeoproterozoic Trans-North China Orogen, North China Craton

Garnet-bearing amphibolites and metapelites of the Luoning metamorphic terrane, Taihua metamorphic complex, southern terminal of the Palaeoproterozoic Trans-North China Orogen (TNCO), preserve different metamorphic mineral assemblages formed at three metamorphic stages in the Palaeoproterozoic. The prograde mineral assemblages (M1) comprise of hornblende + plagioclase + quartz biotite inclusions preserved in the garnet porphyroblasts in the amphibolites or quartz + biotite + plagioclase inclusions preserved in the garnet porphyroblasts in the metapelites. The metamorphic peak mineral assemblages (M2) are represented by the garnet porphyroblasts as well as the matrix minerals plagioclase + hornblende biotite +/- quartz in the amphibolites or quartz + biotite + plagioclase +/- hornblende in the metapelites, respectively. The retrograde mineral assemblages (M3) are the worm-like, fine-grained white-eye socket symplectitic assemblages formed between the relict garnet porphyroblasts and the matrix minerals, consisting of plagioclase + hornblende +/- biotite intergrowths in the amphibolites or plagioclase + biotite + quartz intergrowths in the metapelites, respectively. Geothermobarometric computation shows that the Luoning terrane records clockwise P-T trajectories including near isothermal decompression (ITD) processes, from 630-790 degrees C/5.3-8.9 kbar through 720-820 degrees C/8.9-10.7 kbar to 690-740 degrees C/7.1-7.7 kbar. SIMS U-Pb dating of zircons separated from the amphibolites and metapelites presents that the age of the protolith of the amphibolites is similar to 2.32 Ga and the metamorphism occurred during 1.97-1.94 Ga, suggesting that the subduction-collision and the subsequent exhumation occurred in the late Palaeoproterozoic. Combined with the literature data of the different discrete terranes within the Trans-North China Orogen, it is further deciphered that the tectono-metamorphic event of the Trans-North China Orogen started as early as 1.97 Ga and lasted as late as 1.80 Ga, suggesting that the tectono-metamorphic event of the orogen is a long and complex history. (C) 2015 Elsevier B.V. All rights reserved.