Exhumation of plutons controlled by boundary faults: Insights from the kinematics, microfabric and geochronology of the Taibai shear zone, Qinling Orogen, China

The Taibai granitic plutons lie between the Taibai ductile shear zone to the north and the Shangdan suture to the south. The deformation mechanism of the ductile shearing is crucial to understanding the exhumation processes of the multiple plutons after the Late Mesozoic period. Geological investigations, microstructures, and kinematic vorticity calculations (RGN and Wallis method) indicate that the Taibai shear zone deformed in response to pure shear-dominated (54–65%) transpression and top-to-NW shear sense as a result of NE-SW oblique contractional tectonics. The quartz crystallographic preferred orientations of the prism <a> slip system, the grain boundary migration, and sub-grain rotation dynamic recrystallization of quartz—combined with the plagioclase–hornblende thermometer—constrain the main deformation temperatures to a range of 400–650 °C, which suggests amphibolite to greenschist facies conditions. In addition, it is extremely likely that the mylonites experienced late-stage, lower temperature deformation as demonstrated by the sporadic bulging recrystallization, the quartz basal <a> slip system, and the two-feldspar geothermometer calculation. The samples collected from the weakly deformed mylonitic granite pluton and the undeformed quartz-feldspathic dike that intruded into the mylonites yield zircon U–Pb ages of 129 ± 1 Ma and 115 ±1 Ma, respectively. This information, with the lower intercept ages of ca. 120 Ma obtained from the mylonite samples, suggests that the ductile shearing probably occurred from ca. 129 Ma to 115 Ma. Combined with the regional geological data, these findings suggest that the Taibai shear zone and the Shangdan suture accommodated the oblique upward extrusion of the Taibai plutons during the Early Cretaceous time.