Geochemical characteristics of weakly metamorphosed orthogneisses outside the Wulian boundary fault along the northwestern margin of the Sulu UHP metamorphic belt and their protolith petrogenesis

Orthogneisses in the Wulian area are mainly syenogranitic gneiss and less commonly quartz syenitic gneiss, showing a mildly alkaline trend. The rocks are characterized by low CaO and MgO contents, and high Na2O contents, belonging to the peraluminous series though the rocks have relatively alkaline contents. These orthogneisses are also characterized by relatively strong fractionation between LREE and HREE, a strong negative Eu anomaly and particular strong enrichment in large-ion lithophile elements (Rb, Ba, Th, U and K) and depletion in high-field-strength elements (Ti, Zr, Hf, Nb and Ta), as well as pronounced negative Ba and P anomalies. The general geochemical characteristics of the rocks are comparable to those of granitic gneisses in the Sulu orogenic belt in some degrees but notably different from those of Precambrian orthogneisses in the North China craton. It is inferred that the protolith of the orthogneisses was derived by partial melting of K-rich metabasaltic sources of the continental lower crust in the Neoproterozoic and that the parental magmas mainly evolved by fractional crystallization of feldspar (oligoclase and K-feldspar), accompanied by fractional crystallization of small amounts of amphibole, biotite and accessory zircon, apatite and allanite.