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

Abstract

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°C/5.3–8.9kbar through 720–820°C/8.9–10.7kbar to 690–740°C/7.1–7.7kbar. SIMS U–Pb dating of zircons separated from the amphibolites and metapelites presents that the age of the protolith of the amphibolites is ∼2.32Ga and the metamorphism occurred during 1.97–1.94Ga, 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 1.97Ga and lasted as late as 1.80Ga, suggesting that the tectono-metamorphic event of the orogen is a long and complex history.