Tectonic evolution of a complex orogenic system: Evidence from the northern Qinling belt, Central China

Abstract

The Qinling orogenic belt preserves the records of a long evolution history of convergence between the North China Craton and the South China Craton. In this study, we present results from new geological, geochemical, zircon U–Pb geochronological and Lu–Hf isotopic investigations on a suite of orthogneisses from the Taihua Group (THG) and Tietonggou Group (TGG). We also present geochronological data on schist and migmatite from the North Qinling belt (NQB) and metasedimentary rocks from the Wuguan unit (WGU) which is exposed along the southern side of the Shangdan suture zone. Two orthogneisses from the THG define several stages of arc magmatism at ∼2.51Ga, ∼2.34Ga, ∼2.28Ga and ∼2.16Ga, followed by metamorphism at 1930±31Ma. Zircons from the TGG trace a tectonothermal event at 1897±15Ma. Geochemical data on biotite gneiss from the THG and the TTG (tonalite–trondhjemite–granodiorite) gneiss from the TGG classify these rocks as dacite and display volcanic arc affinity. Zircon Lu–Hf isotopic results suggest that the parental magma for the protolith of felsic gneisses were derived from Mesoarchean crustal components (TDMC=2766–3067Ma). The amphibole gneiss from the THG classifies as metabasalt and the zircons from this rock show dominantly negative εHf(t) values varying from −10.3 to −4.6 and TDMC range of 3088–3437Ma, suggesting magma derivation by melting of Paleoarchean–Mesoarchean subducted oceanic crust.Zircons in the schist from the NQB show a wide age population in the range of 937–1131Ma (peak at 1035Ma). Zircons from the melanosomes of the migmatite in the NQB define ages between 405 and 379Ma, correlating with the melting event in the NQB during 450–380Ma. The WGU shows age populations of 854–803Ma (with a peak at 829–824Ma), ∼2460Ma, ∼1802Ma and 1180–1000Ma, which are markedly different from that of the NQB and SQB. The ages obtained in our study correlate with the widely reported Grenvillian-aged magmatism in the NQB, and suggest that the NQB might have been a discrete micro-continent during Paleo- and Mesoproterozoic which has been overprinted by Paleozoic tectonic event.