The ultra-potassic rocks on the southwestern margin of the Ordos basin and their tectonic significance

Abstract：Ultrapotassic dykes on the southwestern margin of the Ordos basin display massive structure and porphyritic texture. According to their mineral association, these rocks are mainly composed of aegirine augite ivernite and pseudoleucite porphyry, whereas the phenocrysts contain leucite, melanite, aegirine augite and some feldspar. These rocks have high K2O, Al2O3, CaO and low SiO2, MgO, TiO2 and P2O5 content. In the spider diagrams, they are enriched in K, Rb, Sr. Ba, Pb and depleted in Nb, Ta, Zr, Hf. Zircon U-Pb dating reveals that they have crystallization ages of (110.8±0.98)Ma and(107.6±0.92)Ma, respectively, belonging to Early-Cretaceous. It is held that these ultrapotassic rocks resulted from the lithosphere thinning process on the western margin of the Ordos basin.