Magmatic evolution of the Wajilitag mafic-ultramafic complex in the Tarim Basin, NW China

The Wajilitag mafic-ultramafic complex, as an important lithological unit of the Permian Tarim Large Igneous Province (TLIP), consists of the layered mafic-ultramafic intrusion and ultramafic cryptoexplosive brecciated dykes. The former consists mainly of olivine pyroxenite, pyroxenite and gabbro with layered structures, which has genetic link with a large scale V-bearing ilmenite-titanomagnetite series mineralization. Olivine from the intrusion has low Fo values of 67-76 mol% and magmas in equilibrium with olivine have high FeO/MgO ratios ranging of 1.1-1.6. Meanwhile, ilmenite exsolution lamellae are present in clinopyroxene. The Wajilitag magmas therefore are considered to be enriched in iron and titanium, which may account for the large amounts of Fe-Ti oxides hosted therein. The presence of ilmenite exsolution lamellae in magnetite suggests that the layered mafic-ultramafic intrusion and the massive Fe-Ti ore deposits hosted therein formed in a slowly cooling magmatic crystallization process from a deep magma chamber. The ultramafic cryptoexplosive brecciated dykes, on the other hand, have similar lithological and geochemical characters with the kimberlite, comprising xenoliths of ultramafic rocks from the layered intrusion. They have steeply right-dipping REE patterns (ΣREE = 964.1-1298.6 ppm) and show humped distribution patterns on the primitive mantle normalized spidergram. Although the xenoliths and matrix in the ultramafic cryptoexplosive brecciated dykes are quite different on the trace element and REE characteristics, they have almost the same PGE patterns, indicating that they were probably derived from the same mantle source. Based on other geophysical and geological data, we suggest that the Wajilitag mafic-ultramafic complex was generated from the depleted mantle at the boundary between the lithospheric and asthenospheric mantle (~ 150 km depth) by low degree partial melting. It is a product of mantle plume activity, similar to the coeval continental flood basalts in the TLIP.