Early Paleozoic tectonic evolution of the North Qinling Orogenic Belt in Central China: Insights on continental deep subduction and multiphase exhumation

Abstract

The North Qinling orogenic belt is an important component of Qinling composite orogen in Central China. High-pressure and ultra-high pressure (HP-UHP) rocks occur as lenses or layers surrounded by gneissic rocks in the northern, central, and southern Qinling Complex of the North Qinling belt, considered to have formed through deep continental subduction. The magmatic protoliths of the HP-UHP metamorphic rocks show formation ages (ca. 800Ma), geochemical characteristics, and Pb-Nd isotopic compositions similar to those of the Neoproterozoic igneous rocks of the South Qinling belt. Continental materials of the South Qinling belt were dragged down to mantle depths by the Shangdan oceanic crust subducted and subjected to HP-UHP metamorphism at ca. 500Ma. The Erlangping (and Kuanping) backarc basin formed in response to subduction of the Shangdan ocean and might have developed into a limited small ocean basin at ca. 500Ma.The HP-UHP rocks yielded retrograde metamorphic ages of ca. 470–450Ma and ca. 420–400Ma. These ages are identical to the age of magmatic events in the North Qinling HP-UHP belt at ~500Ma, ~450Ma and ~420Ma, related to deep subduction/collision, slab-breakoff and crustal thinning during post-collisional extension. The dominant ca. 500–400Ma ages of detrital zircons from the Liuling Group of the South Qinling belt match well with those from the three stages of magmatic rocks and HP-UHP rocks in the Qinling Complex. This correlation suggests that the magmatic rocks and HP-UHP metamorphic rocks in the North Qinling belt initially exhumed to the surface, eroded and were then deposited in the Liuling basin in a post-orogenic extensional setting during middle to late Devonian.New evidence suggests that the Qinling Complex is a tectonic complex rather than a uniform stratigraphic unit or a microcontinent as previously believed, and is mainly composed of the exhumed HP-UHP metamorphic rocks, deep subduction- exhumation-related magmatic rocks and the early Neoproterozoic granites together with the host rocks from the over-riding plate at an active continental margin. The early Paleozoic tectonic history of the NQB includes oceanic slab subduction and formation of arc, backarc spreading, continental deep subduction, arc-continent collision, break off, and multi-stage exhumation of the deep subducted slab, as well as extension and thinning and associated erosion and deposition.