Genesis and Geological Significance of Waijushan Neo-archean Pluton in Songshan Area

Songshan is the typical area in China, where the neo-archean granitic rocks were well exposed. The Waijushan gneissic trondhjemite is mainly distributed in Waijushan, which shows grayish-white colour, with medium-grained granitic texture, gneissic and banded structures. The Waijushan gneissic trondhjemite has high concentrations of Na₂O (4.08%~5.30%) and SiO₂ (average 65%), and poor in Fe and Mg, with high ratio of Na₂O/K₂O (1.5~3.9) and the content of Al₂O₃ of 14.14%~17.44% (average of 15.33%, larger than 15%), showing the feature of TTG gneiss with peraluminous. It has the similar geochemical characteristics with the high Si Cenozoic adakite, for example, the ΣREE is 121×10^-6~267×10^-6, the HREE is strongly depleted, and the LREE and HREE are strongly differentiation, with weak negative Eu anomalies, high content of Sr, high ratio of Sr/Y, and low contents of Cr, Yb and Y; It is also similar to island arc volcanics on the relational features among Nb, Y, Rb, Th, La and Yb in the rocks. The geochemical characteristics of the Waijushan pluton shows that it was formed by the partial melting of aqueous basaltic oceanic crust under the high pressure environment during subduction process.