Origin and evolution of ultrapotassic intermediate magma: The Songxian syenite massif, Central China

Abstract

A series of magmatic syenites containing extremely high K2O contents (up to 15 wt%, mean of 12 wt%) have been identified in the Songxian region of Qinling Orogenic Belt (Central China). An integrated investigation, combining mineral composition, whole-rock elemental and SrNd isotopic geochemistry and zircon U–Pb geochronology, was conducted to track the origin and evolution of the ultrapotassic intermediate magma. The Songxian syenites mainly consist of alkali-feldspar syenite and aegirine-augite syenite, which outcrop as five independent stocks or dykes in the field. Zircon U–Pb dating of the Songxian syenites yields emplacement ages of 233–237 Ma. Alkali feldspar is the dominant mineral for both rock types, while aegirine-augite, aegirine and subordinate riebeckite are characteristic of the aegirine-augite syenite. Both the alkali-feldspar syenite and the aegirine-augite syenite belong to a weakly peraluminous to peralkaline series, and are characterized by extreme enrichment in K and large ion lithophile elements (LILEs; e.g., Rb, Ba, Sr) and prominent negative anomalies of high field strength elements (HFSEs; e.g., Nb, Ta). In comparison to the alkali-feldspar syenites, the aegirine-augite syenites have higher CaO and Na2O contents and Mg# values, but lower Al2O3 and K2O contents. All the syenites show relatively homogeneous and enriched initial SrNd isotopic compositions with initial 87Sr/86Sr = 0.7069–0.7090 and εNd (t) = −14.5 to −22.0, similar to the regional mafic rocks and indicative of an enriched lithospheric mantle source. The composition of clinopyroxene records a prolonged fractional crystallization process of the syenitic magma, which has a key role in promoting K enrichment in the residual melt. The Songxian syenites and the other analogous Early Mesozoic alkaline rocks and carbonatites in the southern margin of North China Craton were generated in a post-collisional extensional regime and represented a regional geodynamic transition from convergence to extension.