The northern boundary of the Proto-Tethys Ocean: Constraints from structural analysis and U–Pb zircon geochronology of the North Qinling Terrane

Abstract

The North Qinling Terrane, including the Kuanping, Erlangping and Qinling groups, is located in the northern part of the Qinling Orogen. Here we present results from structural analyses of the North Qinling Terrane and its surrounding regions. Three episodes of deformation (D1, D2 and D3) are recognized. The D1 deformation is characterized by NNE-dipping and SSW-dipping penetrative foliations (S1). The S1 foliations are mainly marked by a strong preferred orientation of hornblende crystals, and they were subsequently overprinted by the D2 deformation. The D2 deformation is marked by tight asymmetric folds with NNE-dipping and SSW-dipping axial plane foliations (S2). We propose that the D1 and D2 deformations mainly occurred during ∼440–400Ma after the peak ultra-high pressure metamorphic event (∼520–480Ma). And so these two deformation events could constrain the collision and exhumation processes of the North Qinling Terrane, which probably indicates the closure of the Kuanping Ocean located between the North Qinling Terrane and the North China Block in the early Paleozoic. These two deformations are overprinted by the D3 deformation characterized by ENE-trending upright open folds. Two detrital zircon age spectra for the Kuanping Group with the youngest zircon of ∼640Ma and ∼510Ma, respectively, are different from those recorded for the North China Block, which may reveal the continuous southward subduction of the Kuanping Ocean during the Neoproterozoic to early Paleozoic interval. We propose that the Kuanping Ocean marks the northern boundary of the Proto-Tethys Ocean in the study area.