Dabieshan UHP Metamorphic Terrane: Sr-Nd-Pb Isotopic Constraint to Pre-metamorphic Subduct ion Polarity

摘要

The Qinling-Dabie orogen in east-central China was produced by Triassic collision between two Precambrian cratons. The traditional view about the pre-collisional plate movement is that the South China Block (SCB = Yangtze craton) underwent northerly subduction beneath the North China Block (NCB = Sino-Korean craton). In this paper, we use the Sr-Nd-Pb isotopic tracer technique to test the validity of this view. In all published models, ultrahigh-pressure and high-pressure metamorphic rocks and subjacent "basement" gneisses of Dabieshan are considered to be the deeply subducted, then exhumed crustal slices of the South China Block. Cretaceous igneous rocks, including granitic batholiths generated by partial melting of the Yangtze middle to lower crust and mafic rocks derived from the subjacent upper mantle, should carry the isotopic signatures of the SCB lithosphere. However, Sr-Nd-Pb isotopic tracer analyses indicate that the Cretaceous granitoids and mafic-ultramafic rocks of Dabieshan have an NCB, isotopic signature; hence the subducted lithosphere has little resemblance to the Yangtze craton. We conclude that an opposite polarity of the Triassic subduction is more likely, namely that the NCB plate was subducted 'southward beneath the SCB plate. A new tectonic model is presented, and we suggest that the plate boundary is located in the southern margin of the Dabieshan.