| Yanshanian tectonic-magmatic event is one of the most significant geological events in neighboring sections of Zhejiang, Jiangxi, and Anhui provinces (the WZG region). This magmatic event is not only huge, but also related to the polymetallic mineralization. Based on the detailed field work, geochronology, geochemistry and oxygen fugacity characteristics, companying with tracing the regional faults and their framework, we try to identify and reveal the temporal and spatial distribution, evolution trend, material source and regional geological background of different period igneous rocks; analyze magmatic oxygen fugacity characteristics of the Yanshanian igneous rocks; discuss the characteristics of magma oxygen fugacity Under different tectonic settings during different periods; attempt to provide evidence for ascertaining the genetic relationship between magmatic rocks and tectonic setting of the study area; provide evidence to rebuild the Yanshanian tectonic evolution history and explore potential metal mineral resources in the study area.According to systematic field investigation and detailed information collection, there are two-episodic igneous rocks with entirely different features:the one formed in the180-140±5Ma is mainly composed of granodiorite, Quartz diorite and monzonitic granite; whereas the other formed in the140±5-120Ma is mainly composed of granite porphyry, alkali-feldspar granite, alkali granite, diabase and large-scale bedded output of basalt and rhyolite. Geochemistry and isotope geochemistry analysis also confirmed that the different characteristics of two-episodic igneous rocks:the former have low SiO2and high Al2O3content, no negative Eu anomalies, but are depleted of Nb, Ta and Ti, and genetic types of rocks belongs to I-type or S-type igneous rocks, related to northwestward subduction of oceanic plate during Early Yanshanian; Whereas the latter have high SiO2and low Al2O3content, strong negative anomalies in Eu, Ba, Sr, P, and Ti, and genetic types of rocks belongs to highly fractionated granites or aluminous A-type granites, formed in a within-plate extensional setting. The content of trace elements in zircon showed that igneous rocks formed during180-140±5Ma generally have higher Ce4+/Ce3+value, namely the magma have high oxygen fugacity; whereas igneous rocks formed during140±5-120Ma generally have lower Ce4+/Ce3+value, indicating the magma with low oxygen fugacity. Based on the analysis of metallogenic elements for the Yanshanian igneous rocks, we found that the early Yanshanian igneous rocks are enriched in Cu, Au, Mo, Ni elements, whereas the late Yanshanian igneous rocks are depleted of these elements. Thus, the high oxygen fugacity contributes to the formation of Cu, Au, and Mo deposits. Combining previous research on the oxygen fugacity, we think that the magma of high oxygen fugacity is often closely related to the subduction of oceanic slab.Regional tectonic analysis confirmed that the study area has undergone strong extrusion during Middle-Late Jurassic, formed large-scale thrust nappes; after the early cretaceous, a series of graben basins formed in extension environment. The evidences including igneous tectonic assemblage, sedimentary formation, metamorphism and geochemistry supports the view that the tectonic regimes transform from the extrusion to the extension at140±5Ma. |
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