Postcollisional mantle-derived magmatism, underplating and implications for basement of the Junggar Basin

The late Paleozoic postcollisional granitoids, mafic-ultramafic complexes, and volcanic rocks are extensively distributed around the Junggar Basin; th ey are generally characterized by positive epsilon(Nd)(t) values, implying that the magmas were mantle-derived and contaminated with crustal materials to some extents. The emplacement of mantle-derived magmas and their differ entiates in the upper crust is the expression of deep geological processes at shallow level, while much more mantle-derived magmas were underplated in the lower crust and the region near the crust-mantle boundary, being compo nent part of basement of the Junggar Basin. The postcollisional mafic-ultra mafic complexes would not be generated by re-melting of residual oceanic cr ust, which was considered as the basement of the Junggar Basin, unless very high degrees of partial melting occurred. Even if old continental crust ha d been present before collision, it would have been strongly modified by th e mantle-derived magma underplating. This interpretation is compatible with the existing geophysical data.