Neoproterozoic subduction-accretionary tectonics of the South Qinling Belt, China

Abstract

The Neoproterozoic magmatism in the South Qinling Belt and northern margin of the South China Block is important key to understanding the tectonic evolutionary history of the South China Block, and to reconstruct the tectonics of the supercontinent Rodinia. The Xiaomoling magmatic complex in the South Qinling Belt consists mainly of basalts in the Mogouxia area, gabbros and diorites in the Chishuigou, Lengshuigou and Qianyouhe areas. The basalts in the Mogouxia are characterized by depletion of LREEs and LILEs, and non- differentiation of HFSEs, suggesting their deviation from a slightly depleted MORB mantle source. Taken together with geochemical and geological data, the basalt in the Mogouxia area is interpreted to have derived from a subduction-related back-arc basin setting. The gabbros from both Chishuigou and Lengshuigou sections display clear LREE, Th, and LILEs enrichment, and depletion of Nb, Ta and Ti, and highly differentiation in HFSEs, which are similar to that of a typical Island Arc Basalts. The diorites from both Chishuigou and Qianyouhe areas display strong LREE enrichment, depletion of Nb, Ta, P and Ti, enrichment of LILEs, and flat distribution pattern of HFSEs in the primitive mantle-normalized trace element pattern. According to their geochemistry, both gabbros and diorites from different localities of the Xiaomoling magmatic complex were mostly likely formed in an island-arc tectonic setting. The zircons concentrated from the gabbro and diorite yield LA-ICPMS U-Pb ages of 753±4Ma and 843±5Ma, respectively, representing their formation ages. Integrated all the available geological, geochemical and geochronological data, we propose that the Xiaomoling magmatic complex represents the northernmost of the accretionary belt to the north of the South China Block, while the Hannan-Huangling Neoproterozoic intrusions mark the southernmost of the accretionary belt. Therefore, the most of southern Qinling represents a wide subduction and accretionary tectonic belt in Neoproterozoic times. Our new geochronology and previously published reliable ages estimate a long-lived subduction during ca. 850–700Ma at the present South Qinling area, to the north of the South China Block.