Genesis of the Late Carboniferous Daling rapakivi granites at the southeastern end of the Xiao Hinggan Mountains

Abstract：Rapakivi-textured feldspars occur in Late Carboniferous weakly gneissic, medium-grained porphyaceous monzogranite in the Daling area between Hegang and Yichun cities at the southeastern end of the Xiao Hinggan Mountains. Most of them are idiomorphic and broad sheet-like or broad-prismatic in shape, and consist of an alkali feldspar core and a plagioclase mantle. Some are ovoid or globoid in shape without a plagioclase mantle and about 1.5-3.5 cm in size. Their petrographical features are comparable to those of the typical rapakivi granite. In addition, dark-colored microgranular dioritic enclaves are generally developed, and they have close relationship with rapakivi K-feldspars. The enclaves have typical textures of magma and acicular apatites and contain K-feldspar and quartz megacrysts of host rocks. The enclaves are mainly rounded in shape, showing pronounced plastic rheological characters. They usually have a clear-cut contact relationship with the host rocks and sometimes transition or foggy contacts could also be seen. The above-mentioned evidence fully shows that the enclaves are MME type enclaves. On the basis of the geological characteristics of the pluton, rapakivi-textured K-feldspar phenocrysts, dark-colored microgranular dioritic enclaves and petrochemistry and geochemistry, the Daling rapakivi granites were the product of magma mixing and formed in an orogenic environment, and their age and tectonic setting are different from those of typical rapakivi granites.