Fabrics and geochronology of the Taibai ductile shear zone: Implications for tectonic evolution of the Qinling Orogenic Belt, central China

Abstract

The Taibai ductile shear zone affecting the northern margin of the Taibai granitic pluton which emplaced in the North Qinling Orogenic Belt, is crucial to understand the amalgamation and post-collision history between the North China Block and the South China Block. The shear sense indicators and the kinematic vorticity numbers (0.42–0.59) suggest that the Taibai shear zone is an elongating general shear zone involving both sinistral shear and pure shear components. The quartz lattice preferred orientations mainly indicate the activation of prism 〈a〉 slip system combined with rhomb 〈a〉 slip to prism 〈a〉 slip system. The dynamic recrystallization of quartz is accommodated by sub-grain rotation and bulging recrystallization while plagioclase mainly displays bulging recrystallization. These characteristics suggest that the mylonites have experienced ductile shear deformation under greenschist facies conditions at temperatures of ∼400–550 °C. Zircons from granitic mylonite yield upper and lower intercept ages of 425 ± 4 Ma and 144 ± 1 Ma, respectively, suggesting the protolith of the mylonite crystallized at ca. 425 Ma and was involved into a hydrothermal activity at ca. 144 Ma. Muscovite 40Ar/39Ar dating of the mylonite sample yield a plateau age of 346 ± 5 Ma and an inverse isochron age of 344 ± 4 Ma, constraining the ductile shear deformation occurring during the Early Carboniferous time. We suggest that the Taibai shear zone was formed by NE-SW oblique compression with top-to-the-NW shear sense during the Early Carboniferous time, following the N-S shortening caused by collision between the North China Block and South China Block along the Shangdan suture at ca. 400 Ma.