Acta Petrologica et Mineralogica

Phonolite represents the final evolution product of the sodic alkaline magma in the continental volcanic province, and revealing its genetic mechanism is of great significance for understanding the magmatic evolution process of the continental volcanic province. A systematic mineralogical, petrological and geochemical study of the phonolite exposed in the Tuoyun Mesozoic-Cenozoic volcanic basin in the Southwestern Tianshan Mountains, China, has been carried out to reveal the magma series and magmatic evolution process of Tuoyun volcanic rocks. Tuoyun phonolite is characterized by the presence of alkali feldspar, clinopyroxene, fayalite and amphibole phenocrysts, and nepheline can be seen in the groundmass. In terms of geochemical composition, Tuoyun phonolite is characterized by high Na₂O (7.9%~8.6%) and K₂O (5.2%~6.3%). The total rare earth element ranges from 408×10^-6 to 470×10^-6, and the chondrite-normalized REE patterns have obvious Eu anomaly. In the primitive mantle-normalized trace element spider diagram, the samples show negative anomalies of Ba, Sr, P, Ti and positive anomalies of Nb, Ta, Zr and Hf. The depleted Nd isotopic composition shows that phonolite is genetically related to the associated basanite. Based on the MELTS thermodynamic simulation, the phonolite could be produced by the basanite through fractional crystallization of alkali feldspar(36%), clinopyroxene (21%), spinel (10%), fayalite (6%) and amphibole (5%). Further EC-AFC modeling exhibits that the phonolites has suffered crust contamination to some degrees(10%) during the evolution. The study shows that the Tuoyun volcanic rocks belong to the peralkaline series of basanite-phonotephrite-phonolite, and the phonolite represents the final evolution product of the peralkaline rock series in Tuoyun volcanic basin.

Southwestern Tianshan; Tuoyun basin; phonolite; fayalite; magma evolution; AFC process