Protoliths reconstruction and formation conditions of basement metamorphic rocks in the Xiangshan uranium orefield, Central Jiangxi

Abstract: Located in the suture zone of the Cathaysia and Yangtze tectonic belt and lies on the southern side of Jiangshao fault, the Xiangshan uranium orefield is the largest volcanic uranium orefield and is also the earliest discovered volcanic uranium orefield in China. Its basement is a set of ancient metamorphic rock series. Micro palaeophyte fossils and previous studies indicate that the basement metamorphic rocks formed in the Qingbaikou period of Neoproterozoic. This paper mainly discusses the composition of metamorphic rock and analyzes its formation environment. Most metamorphic samples are characterized by intermediate SiO2 content, high K2O/Na2O and Fe2O3T+MgO, and obviously low CaO content. Analytical results of all samples show larger total ΣREE values and abundant LREE, as well as depletion of Eu, implying that the rock samples originated from the continental crust. The overall compositions of various sample patterns are very close to each other in the REE distribution diagram, and REE patterns of all samples are of right-oblique type, similar to patterns of the post-Archean shale and the upper crust. From the Simonen and Nielv diagram, the authors infer that the original rocks of Shenshan Formation were mainly composed of complex mineral sandstones and feldspathic sandstone, the original rocks of Kuli Formation consisted mainly of quartzite sandstone and feldspathic sandstone, and the original rocks of Shangshi Formation comprised mainly feldspathic sandstone, with a little siliceous marl and shale sandstone. Combined with residual structural features analysis, it is inferred that these metamorphic clastic sedimentary rocks formed in a shallow shore - shelf sedimentary environment. Th/Sc ratios of all samples vary in a small range (0.5-1.1) , while Zr/Sc ratios vary quite considerably (8-24), revealing the complex composition of the source region of the metamorphic rocks and the probable existence of several sedimentary cycles. In Th-Sc-Zr/10 and La-Th-Sc diagram, most of the data fall into the continental island arc area, indicating that this set of rocks might have been deposited in the basins near the continental island arc. Hf-La/Th diagrams indicate that the provenance of the original rock mostly came from ancient strata of uplift and erosion, and acidic arc source region only possessed a small proportion. It is thus obvious that the magmatic activity during its deposition was not obvious, but the source region was accompanied by tectonic uplift. Combined with geochemical characteristics, it is held that the protoliths of basement metamorphic rocks in the Xiangshan uranium orefield were formed in the residual basin of an extension environment, resulting from the post-collision between Cathaysia and Yangzi Blocks.