Reappraisal For Protolith Ages And Formation Mechanism Of The Hongzhen Metamorphic Core Complex

The Hongzhen metamorphic core complex is situated in the Yangtze Plate to the east of the Dabie orogenic belt. The metamorphic core complex was considered as an metamorphic core complex of the Cordillera-type produced by Early Cretaceous NE-SW extension, and the Dongling Complex has long been treated as the metamorphosed basement of the Yangtze Plate. However, recent studies suggest NW-SE extension for the Early Cretaceous period awhich is in contradiction with the Early Cretaceous extensional direction indicated by the core complex. Recent dating work also shows that protolith ages of the Dongling Complex are not Paleoproterozoic. It is suggested therefore that further studies are needed for this important extensional structure.We collected 11 samples for zircon dating from the core complex region, including 7 ortho-metamorphic and 2 rocks para-metamorphic samples from the Dongling Complex as well as two lower Sinian and lower Cambrian meta-sandstone samples. The dating results give emplacement ages of 829?812 Ma for the deformed, metamorphosed plutons in the Dongling Complex, protolith ages of 761?754 Ma for the meta-volcanic rocks in the complex, the youngest ages of 787 Ma and 780 Ma for the meta-sediments in the complex as well as 685 Ma and 711 Ma for the lower Sinian and lower Cambrian meta-sandstones. It is demonstrated that protolith ages of the volcanic-sedimentary rocks in the Dongling Complex are Neoproterozoic and they belong to the Yangtze Plate basal cover of the Nanhua period, rather than the metamorphosed basement. The metamorphosed plutons in the complex were emplaced during the Neoproterozoic, rather than the Paleoproterozoic plutons proposed before.Zircon growth rims are common in several dating samples, and their U-Pb ages concentrate at 127 Ma. Their formation temperatures are estimated at ?500? by using the Ti-in-zircon method. The age of 127 Ma suggests a thermal event in the complex that is related to synchronous intrusion of the nearby Hongzhen pluton and affects previously obtained muscovite 40Ar/39Ar ages of 129-125 Ma from the ductile shear zone in the core complex. It is demonstrated therefore that the muscovite 40Ar/39Ar ages cannot be used for representing formation time of the ductile shear zone in the core complex.Two groups of NE-striking normal faults with opposite dips were developed on both sides of the exposed core complex in the Early Cretaceous. The synchronous Huaining and Qianshan rifted basins appeared in the south and west of the region respectively. Fault-slip data indicate that the Early Cretaceous extensional structures were produced by NW-SE extension, rather than the NE-SW extension as proposed previously. It is suggested therefore that an extensional dome, rather than the Cordillera-type metamorphic core complex, formed under the common action of normal faulting along the two groups of faults with opposite dips and exhumation induced by the Hongzhen intrusion during the Early Cretaceous.