Qinling gneiss domes and implications for tectonic evolution of the Early Paleozoic Orogen in Central China

摘要

The Qinling arc terrane (QAT) at the southern margin of the North Qinling Early Paleozoic orogen, Central China, is composed of gneiss domes overridden by cold allochthon nappes. The gneiss domes are characterized by migmatitic and plutonic cores of granite or gabbro and overlying upper amphibolite to granulite facies metamorphic sequences with pervasive injection of leucogranitic veins. The gneiss domes display a high temperature domal foliation dipping radially away the core and a down-dipping stretching lineation. Cascading folds occur in the mantling metamorphic rocks. The new data of zircon/monazite U-Pb dating suggest that the gabbroic and granitic plutons formed at 514-476 Ma and 495-441 Ma, respectively, whereas the high-grade metamorphism of the mantling rocks took place at 469-412 Ma, and the leucogranitic veins formed at two main episodes (472-440 Ma and 416-413 Ma). Mica Ar-40/Ar-39 dating yields 402-328 Ma for the cooling ages of the leucogranitic veins and gneissic rocks. The basaltic magma underplated into the QAT lower crust resulted in partial melting, producing granitic magma at the Late Cambrian to Middle Ordovician during northward subduction of the Shangdan Proto-Tethyan oceanic slab. Buoyant upwelling of the migmatitic and plutonic core was promoted by constriction in the lower crust and decompression-extension in the upper crust during the Silurian. Since the closure of the Shangdan Ocean and the collision between the North China Block and South China Block (similar to 400 Ma), the Qinling gneiss domes experienced rapid uplift, and were finally overthrust by the upper allochthon transported from the North Qinling.