The construction mechanism of the Neoproterozoic S-type Sanfang-Yuanbaoshan granitic plutons in the Jiangnan Orogenic Belt, South China: Insights from geological observation, geochronology, AMS and Bouguer gravity modeling

The Neoproterozoic S-type granitic plutons, distributed along the suture zone of the Yangtze and Cathaysia blocks, play an important role in the tectonic evolution of the Jiangnan Orogeny. However, few studies had focused on the scientific issues on the physical construction process of these plutons, therefore, we carried out multidisciplinary approaches on the Sanfang and Yuanbaoshan plutons in the western Jiangan Orogenic Belt to figure out the construction mechanism of these plutons. Zircon Hf and U-Pb dating indicates that the magma, originated from the crustal partial melting, was crystallized at ca. 830 Ma. Field investigation shows that the plutons are dome shaped, and the inclinations of bedding/foliation of the country rocks, the Sibao group, gradually decreases away from the contact zone with pluton boundaries. A top-to-the-E kinematic sense was observed in the eastern margin of the Yuanbaoshan pluton. Both the textural analysis and AMS measurement reveal primary magmatic and secondary general top-to-the-W/NW post-solidus fabrics. The Bouguer gravity modeling reveals that the feeder zone probably locates in the southern part of each pluton and these two plutons are connected at depth. According to the primary magmatic fabric patterns, deep geometric shape as well as structural features of the country rocks, we propose that crustal partial melting magma probably initially intruded into the pre-existing mechanically weak (fold/fault) zone of the Sibao group, and meanwhile laterally and upward accreted to progressively construct the dome-shape plutons. The intrusion of ca. 850-820 Ma S-type granites in the Jiangnan region possibly marked the latest stage of the Jiangnan Orogeny. However, the timing and mechanism of the post-solidus deformation are not well constrained, the proposed interpretations remain hypothetical yet.