Estimating the parental magma composition and temperature of the Xiaohaizi cumulate-bearing ultramafic rock: Implication for magma evolution of the Tarim large igneous province, northwestern China

The ultramafic dikes in the Tarim large igneous province (Tarim LIP), exposed in the Xiaohaizi area in the northwestern Tarim Basin of northwestern China, have porphyritic textures, and the olivine and clinopyroxene are as the major phenocryst phases. The groundmass therein consists of clinopyroxene, plagioclase and Fe-Ti oxides, with the cryptocrystalline texture. The olivine phenocrysts in one typical ultramafic dike have Fo (Mg/(Mg+Fe)) numbers ranging from 73 to 85, which are not in equilibrium with the olivine (Mg# of 89) from the host rock crystalized. Combined with microscope observation, both the olivine and clinopyroxene phenocrysts as well as some Fe-Ti oxides in the ultramafic rock are accounted as cumulates. The liquid (parental magma) composition of SiO₂ of 45.00 wt.%–48.82 wt.%, MgO of 9.93 wt.%–18.56 wt.%, FeO of 5.85 wt.%–14.17 wt.%, CaO of 7.54 wt.%–11.52 wt.%, Al₂O₃ of 8.70 wt.%–11.62 wt.% and TiO₂ of 0.00 wt.%–3.43 wt.% in the Xiaohaizi ultramafic rock was estimated by mass balance, and the results show a reasonable liquid proportion in the cumulate-bearing ultramafic dike (ca. 45%–60% in the whole rock). The estimated parental magma composition corresponds to a melting temperature of 1 300–1 550 ºC, which is equal or higher than those of a normal asthenosphere mantle, supporting the involvement of a mantle plume. Combined with other previous studies, an evolution model for the formation processes of the Xiaohaizi ultramafic dike of the Tarim LIP is proposed.