ZHANGBALING METAMORPHIC BELT: STYLE AND TIMING OF COLLISION BETWEEN NORTH AND SOUTH CHINA

The Zhangbaling metamorphic belt is located east of the Tan-Lu fault and represents the only metamorphic terrane that connects the Dabie Shan and Su-Lu UHP orogens in southeastern China. Mapping reveals that this ~N-S belt is composed of a subhorizontal midcrustal shear zone, and a ductile-brittle transition zone emerging from beneath a foreland fold-thrust belt. The subhorizontal shear zone consists of mylonitic felsic volcanics (Zhangbaling Group) overlain by Sinian (Late Proterozoic) schists and metatillite deformed under greenschist- and blueschist-facies conditions. Lineation changes gradually from NE in the south to N and NW in the north, with top-to-north sense of shear. Strain/vorticity analysis of rigid clasts in mylonitic felsic metavolcanics suggest combined pure and simple shear (Wk~0.8). Quartz c-axes fabric patterns imply basal < a > slip system under relatively low temperature. Ar-Ar dating of phengitic white micas show ages of 225-236 Ma interpreted as fabric-formation ages. A layer of strongly elongated pebbles in metatillite marks the contact between the Zhangbaling metavolcanics and the overlying units. Metamorphic grade and finite strain decrease abruptly upward as the Sinian metatillite changes gradually to siltstone and fine-grained sandstone. A subhorizontal east-dipping detachment zone, marked by ductilely deformed Sinian limestone, separates the ductile midcrust from the overlying foreland fold-thrust system that shows a SE vergence. These new data from structural analysis and geochronology suggest that the Zhangbaling belt is part of the collision system between North and South China. We propose two alternative hypotheses: (1) this belt developed as part of the Dabie-Shan/Su-Lu system and was later rotated along the Tan-Lu fault; the belt-parallel lineation pattern in the ductile crust is best explained by an initial arcuate shape of the orogen; and (2) this belt formed in a transform zone that connected the Dabie Shan and Su-Lu "head-on" collision zones. According to this second hypothesis, the Zhangbaling belt is a subhorizontal "attachment zone" developed between rheologically distinct layers of lithosphere; deformation was controlled by the boundary conditions associated with a continental transform system.