Ca. 835–823 Ma doming extensional tectonics in the west Jiangnan accretionary orogenic belt, South China: Implication for a slab roll-back event

Abstract

The Neoproterozoic Jiangnan accretionary orogenic belt recorded the accretion and collision of the Yangtze and Cathaysia blocks to form a stablized South China Block, but related geometry and kinematics is poorly constrained, leading to largely varied tectonic models. Here, we present detailed field investigation and kinematic analysis of the plutonic-metamorphic complexes in the Yuanbaoshan and Sanfang areas of the west Jiangnan orogenic belt, which enables identification of extensional granite-cored domes. In the dome margins, down-dipping lineations display a radial pattern and dome dominated foliations are extensively developed. The shearing structures within the plutonic-metamorphic complexes display extensional shearing surrounding the Yuanbaoshan and Sanfang granitic dome cores. Gneissic granites and massive ones from both the Yuanbaoshan and Sanfang plutons yield comparable crystallization ages of ca. 835–823 Ma that are within age errors of each other, as are the sheared recrystallized asymmetric quartz veins and mylonites dated at 831 Ma. Overall ages of the deformed Sibao Group and the undeformed overlying Danzhou Group, along with those of the granite plutons and mylonites, suggest formation of the granite-cored domes at ca. 835–823 Ma, coeval to the timing of emplacement of the granitic plutons. Locally, top-to-the-E thrusting structures are also observed in the west Yuanbaoshan and Sanfang areas and are inferred as at ca. 860–835 Ma, coinciding well with E- or SE- directed structures developed elsewhere in the Jiangnan orogenic belt, but in contrast with doming extensional shearing structures. Therefore, overall geometry and kinematics in the west Jiangnan belt indicate development of granitic dome related extensional ductile shearing deformation dated at ca. 835–823 Ma and a possible top-to-the-E compressional ductile thrusting deformation within 860–835 Ma. Given the previously inferred regional geology observations, along with age and chemical data across the Jiangnan orogenic belt, the dominant extensional shearing deformation in the region argue for a slab roll-back event within an accretionary belt, typical of domes-and-basins structures formed in accretionary convergent continental margin. The top-to-the-E thrusting is here interpreted as corresponding to compressional regime generated by the west directed subduction of oceanic crust beneath the northern Guangxi continental margin arc in the west Jiangnan orogenic belt.