Early Paleozoic arc-accretion in the northern branch of the Proto-Tethys Ocean: New insights from detrital zircon U-Pb ages and geochemistry of paraschists from the Kuanping Complex, North Qinling Orogenic Belt, China

The Proto-Tethys Ocean existed from the Neoproterozoic through early Paleozoic during the breakup of Rodinia and was closed with the subduction and accretion of numerous East Asian blocks. The North Qinling Orogenic Belt is thought to have formed upon the closure of the Kuanping Ocean between the North China Craton and the Qinling Block. However, the timing of this closure is widely debated between Neoproterozoic and Paleozoic. To date, this controversy remains as a barrier in understanding the ocean-continent configuration and closure process of the Proto-Tethys Ocean. The Kuanping Ocean is best represented by the Kuanping Complex, which is the northernmost unit in the North Qinling Orogenic Belt and consists of a wide range of meta-sedimentary rocks. Here we present a systematic study of the petrology, whole-rock geochemistry and geochronology of paraschists from the complex. The paraschists yield new detrital zircon U[sbnd]Pb ages from 500 Ma to 3584 Ma with two main peaks at 798 Ma and 958 Ma and a less dominant peak at 2460 Ma. Based on our investigations we infer that the protoliths of the paraschists were dominantly wackes and minor litharenites and shales with maximum depositional ages from 578 to 500 Ma, which represents different levels of a sedimentary sequence in an arc-related basin. Their clastic input was sourced mainly from the Qinling Block, with minor addditions from the North China Craton. Combining our results with previous data, we also infer that the Kuanping Complex represents an early Paleozoic accretionary wedge assemblage with existence of late Paleoproterozoic to early Neoproterozoic continental nuclei. Initial intra-oceanic subduction in the Erlangping-Kuanping Ocean occurred at ca. 578 Ma with a northward subduction polarity towards the continental nuclei, resulting in the formation of the Erlangping island arc and the protoliths of meta-sedimentary rocks in the Kuanping Complex in an arc-related basin until ca. 500 Ma. During the accretion of the Erlangping arc and the Kuanping Complex, oceanic crust subducted underneath this continental amalgam at 500–490 Ma. Northward continental subduction of the Qinling Block beneath the consolidated Erlangping-Kuanping arc terrane at 490–460 Ma and their final accretion to the southern margin of the North China Craton at ca. 440 Ma marked the closure of the northern branch of the Proto-Tethys Ocean. However, sedimentation continued in few of the remaining residual oceanic basins until 436 Ma. Our new model suggests that the Erlangping-Kuanping Ocean was part of the northern branch of the Proto-Tethys Ocean and closed in the early Paleozoic with a northward subduction polarity.