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

A series of magmatic syenites containing extremely high K2O contents (up to 15 wet, 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 Sr-Nd 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 (ULEs; 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 Sr-Nd isotopic compositions with initial Sr-87/Sr-86 = 0.7069-0.7090 and epsilon Nd (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. (C) 2020 Elsevier B.V. All rights reserved.