Studies On Tectonic Evolution And Dynamic Setting Of The Xiaoqinling Region

The Precambrian tectonic evolution and the destruction of the North China Craton(NCC)have always been the hot topics in the field of geoscience.Recently,It is widely accepted that the collision of the Eastern and Western Blocks along the Trans-North China Orogen(TNCO)at 1.85 Ga,to finally form the coherent basement of the NCC.However,there are still many disputes about the entire tectonic evolution process during Paleoproterozoic period in the TNCO,such as polarity of the subduction,timing of collision and mechanism of post-collisional extension.The Xiaoqinling region in the southern margion of the NCC is an important part of the TNCO,but,there is absent relevant research on Paleoproterozoic tectonic evolution in this area for a long time.Understanding Paleoproterozoic tectonic evolution of the Xiaoqinling region,is significant for improving the tectonic evolution history of the TNCO,as well as further understanding the amalgamation process of the global Columbia supercontinent.Metamorphic core complex(MCC)is the tectonic response for the destruction of the NCC.It can reveal the deep crustal deformation process and the geodynamic setting for the destruction of the NCC.Many studies have been conducteded on the MCCs along the northern margin of the NCC.However,for a long time,there is a significant controversy that whether developed Early Cretaceous MCC in Xiaoqinling region.The mechanisms and timing of formation of the Cretaceous structures in the Xiaoqinling region are also poorly understood.Detailed field investigation and comprehensive analyses show that Taihua complex produced penetrative ductile fabrics and localized ductile shear belts(D1),and it was accompanied by a regional high-grade metamorphism.The D1 structures strike NWW-SEE with the north-dipping attitudes being dominant,and show steep gneissosity,schistosity and mylonitic foliation and gentle mineral elongation lineation.A series of shear sense indicators of macrostructures and micro structures demonstrate a top-to-the-NWW sense of shear for the D1,as a result of NWW-directed thrusting movement.The microstructures suggest deformation temperatures of 600-650?,which correspond to the conditions of exhumation of these orogenic rocks.Zircon U-Pb dating constrains the D1 structural events within 1863-1840 Ma.All the above indicates that D1 structures were the result of shortening deformation during collision.Structural deformation features of D1 and zircon U-Pb dating support the ca.1.85 Ga collision model for the Western and Eastern blocks that involves the SE directed subduction of the Western Block.Subsequently,the TNCO turns into post-collisional extension stage,inducing the D2 structural deformation and intense magmatic activity.The second phase of deformation(D2)produced localized E-W striking normal shear belts.During the normal sense shear movement,massive nearly E-W striking syn-kinematic pegmatite dikes are developed.A progression from ductile to brittle deformation is commonly observed in the deformed dikes,suggesting synchronous exhumation related to isostatic uplift.Both ductile fabrics of normal-sense shear belts and pegmatite dike as well as fault-slip data from the pegmatite dikes indicate that N-S(orogen-parallel)extension was responsible for D2 deformation.A series of zircon U-Pb dating results for deformed and undeformed plutons or dikes in the shear belts and syn-kinematic pegmatite dikes constrain the D2 deformation to the time between 1840 Ma and 1703 Ma,suggesting a duration of at least 137 Myr for the orogen-parallel extension.The protracted orogen-parallel extension,the synchronous isostatic uplift,and the intense magmatism in the TNCO are consistent with a gravitational collapse model in a post-collisional setting.Detailed field investigation and research show that Early Cretaceous structures in the Xiaoqinling region represent a Cordilleran-type MCC.The NW-dipping detachment zone on the northwestern edge of the MCC is a ductile extensional shear zone that is overprinted by a later brittle detachment fault.The footwall was cut by a series of ductile normal sense shear belts and later brittle normal faults that strike predominantly NE-SW.Both the ductile and brittle structures indicate that NW-SE extension was responsible for the development of the Xiaoqinling MCC.Zircon U-Pb and mineral 40Ar/49Ar dating results suggest that the MCC initiated at 138 Ma and lasted until 100 Ma,recording a protracted extensional history.The Xiaoqinling MCC experienced an early phase of crustal-scale normal faulting(138-126 Ma)and later isostatic doming(125-100 Ma),consistent with the "rolling-hinge" model.The Xiaoqinling MCC shows similar features and a similar evolution to other intraplate MCCs in the northern and southeastern parts of the NCC,and shows that the southern part of the NCC was also involved in intense backarc extension and magmatism.Distribution of these intraplate MCCs indicates synchronous backarc extension over a length of around 1800 km.Delamination of a flat oceanic slab during roll-back is consistent with such large-scale,synchronous extension in the overriding plate.