Geochemical characteristics, zircon U-Pb age and metallogenic tectonic background of the Mesozoic acidic volcanic rocks from Xianqiao area in North Dabie Mountain

The structural-magmatic evolution of East Qinling-North Dabie Mountains is a popular issue.The eruption time of the volcanic rocks in the belt still has a greater dispute for a long time, which is still unclear during rock formation and diagenetic tectonic environment.Based on the studies of geochemistry, zircon LA-ICP-MS U-Pb dating of dacite in Xianqiao bentonite deposit in the western segment of North Dabie Mountains, it is shown that acidic volcanic rocks from Xianqiao area display high contents of K₂O(3.96%~5.01%), are enriched in K₂O+Na₂O(7.97%~8.96%)and weak peraluminous, with A/CNK values of 1.0~1.15.The volcanic rocks should belong to high-K calc-alkaline to shoshonite series.The rocks are enriched LREE and depleted HREE, with weak negative Eu anomaly, strongly depleted in the high field strength elements Nb and Ta and high Nb / Ta ratio, and has high(La/Yb)_N(25.68~28.25)and Sr/Y(34.92~41.42)ratios, which has obvious geochemical characteristics of adakite.Zircon LA-ICP-MS U-Pb dating result shows that the Xianqiao ore-forming rocks(dacite)was erupted at 133±1 Ma, consistent with the age of volcanic rocks of Chenpeng Formation in Shuangqiao bentonite deposit and Huangchengshan silver deposit.This research reveals that the Mesozoic acidic volcanic rocks from Xianqiao area with the characteristics of adakitic rocks were formed by partial melting of the thickened lower crust, which were formed at post-collisional tectonic setting in Mesozoic.The large-scale magma activity resulting from lower crust delamination and thinning of the lithosphere has not yet occurred in the Xianqiao area of North Dabie Mountains at about 133 Ma.