Geochemical Characteristics and Tectonic Significance of Rapakivi Granites in Yingfeng

Yingfeng rapakivi granite, outcropping in the north of the Qaidam basin orogenic belt between the Qaidam plate and South Qilian, is one of the Properozoic rapakivi granites found in China. This research shows that Yingfeng rapakivi granite is a typical Proterozoic rapakivi granite that has rapakivi texture and features of A-type granite, and belongs to A₁ subtype, and the magma assemblage has two-apex characteristics. Rapakivi texture is mainly a polymer composed of several K-feldspar porphyritic crystals, with one plagioclase nuclear in it. The surface of the porphyritic crystal is kaolinized unevenly, its stripped structure is obvious and has orderliness. The matrix is composed of fine-particulate quartz and has obvious recrystalization and orientation textures. Its petrochemistry is characterized by high K, A/NKC < 1, A/NK > 1, belonging to quasi-aluminous rock. It is rich in minor elements Ba, U, Th, Ce, Hf, Sm, depleted in Sr, Ta, Nb, Zr, Y, Rb/Sr ((0.17)-0.6), and its Rb/Ba (0.03-0.24) ratio is very low. The differentiation of the rock is not high. The ∑REE, Ce, Zr contents are high, and the content of Ga can reach over 25×10^-6, far higher than that of other granite types. Its Eu ((0.75) ×10^-6-4.3×10^-6) is depleted slightly, belonging to LREE enrichment type. Through the analysis of the geochemistry of minor elements and REE, Yingfeng rapakivi granite is a kind of magmatism produced in the plate. It is the "dry" magma that is formed by the granulite, which is influenced by the uprush of the mantle of base intrusion or delamination, and then melted and then differentiated, and invaded in the post-collision area at the regional extentional tectonic environment. At the same time, with the decrease of temperature, Na-plagioclase is separated from the K-feldspar and transferred to the brim of the K-feldspar to form the ring porphyritic A₁ granite.