Late Proterozoic-Mesozoic Tectonomagmatic Events in the Yingba Area, Western Inner Mongolia, China
The Geological Lab Center, China University of Geosciences, Beijing
Abstract:In this paper, we focused on the structural and geochronological analyses for the Yingba area in western Inner Mongolia, China. At least three stages of Late Proterozoic to Mesozoic tectonomagmatism have been identified. New and previous zircon U-Pb ages have revealed Carboniferous (325~313 Ma) granodiorite and porphyritic granite, Early Permian (291~277 Ma) alkaline granite and purple red medium-fine grained granite and Early Cretaceous (~134~130 Ma) pegmatite and quartz monzonite. The first tectonic event is characterized by NW-SE compression after the Early Permian (290.9±1.8 Ma), which generated the low-angle NW-dipping gneissic foliations in the granodiorite and alkaline granite. The second event is indicative of NW-SE extension, during which a SE dipping low-angle ductile shear zone, which is characterized by the medium-high temperature (450~650 ºC) mylonite, crosscut the gneissic granodiorite and gneissic alkaline granite, and so it apparently postdated the first tectonic event (<290.9±1.8 Ma). The ductile shear zone is crosscut by the ~134 Ma pegmatite veins, indicating that the extension occurred before the Early Cretaceous pegmatite veins. The third tectonic event is represented by NW-dipping moderate-angle brittle normal faults, which resulted from NW-SE extension after the Early Cretaceous and crushed the ~134 Ma pegmatite into cataclasite. The Yingba area has recorded a typical process from the formation of an orogen to intracontinental extension during Late Proterozoic to Mesozoic.