Basement components of the Xiangshan-Yuhuashan area, South China: defining the boundary between the Yangtze and Cathaysia blocks

The exposed metamorphic rocks and xenolithic metamorphic rocks in the Xiangshan-Yuhuashan (XSYHS) area in central Jiangxi Province, South China, were studied in order to reveal the compositions of the basement rocks and their tectonic implications. Bulk geochemistry and zircon U-Pb-Hf isotopic compositions suggest that the basement metamorphic rocks in the XSYHS area are mainly composed of Neoproterozoic (after 822-683Ma) meta-sedimentary rocks. The exposed meta-sedimentary rocks show higher K₂O/Na₂O and Al₂O₃/(Na₂O + CaO) and lower Eu/Eu^* than those occurring as xenoliths in the Yuhuashan volcanic-intrusive complex, indicating relatively strong weathering and leaching in the provenance area. Geochemical data suggest that the detritus of the exposed metasedimentary rocks was derived mainly from felsic to intermediate igneous rocks, similar to Neoproterozoic sedimentary rocks in the southern Yangtze Block, whereas the xenolithic metasedimentary rocks contain more recycled material, similar to those in the Cathaysia Block. These xenoliths from the mid-lower crust also have significantly high U contents. Consequently they probably are an important source for uranium in the XSYHS volcanic rocks.

U-Pb ages of detrital zircons show that the protoliths of the xenoliths are mainly composed of Neoproterozoic, Neoarchean and Paleoproterozoic detritus, similar to the components of the Wuyi terrane in the eastern Cathaysia Block. In contrast, the protoliths of the exposed metasedimentary rocks consist mainly of Neoproterozoic detritus with minor Neoarchean and Paleoproterozoic fragments, similar to those in the southern Yangtze Block. Both geochemistry and zircon U-Pb-Hf isotope compositions suggest that the deep basement metamorphic rocks in the XSYHS area belong to the Wuyi terrane, whereas the surface metamorphic rocks are derived from the southern Yangtze Block. Therefore, the real boundary between the Yangtze and Cathaysia blocks lies north of the study area. The exposed Yangtze-related metamorphic rocks were overthrust southward from the southern Yangtze Block, and onto the Cathaysia basement, during the Indosinian to Yanshanian Orogeny.