Petrological, geochemical and geochronological constraints on the origin of the Xiadong Ural–Alaskan type complex in NW China and tectonic implication for the evolution of southern Central Asian Orogenic Belt

Abstract

The Xiadong mafic–ultramafic complex is located in the Central Tianshan Terrane, the southern part of the Central Asian Orogenic Belt. It is composed of dunite, hornblende clinopyroxenite, hornblendite and hornblende gabbro. Several dykes of hornblendite, hornblende gabbro and gabbroic diorite cut the main body. These rocks are characterized by adcumulate textures. The dunites contain high-Fo (92.3–96.6) olivines with rare orthopyroxene and plagioclase. Hornblende, chromite, and magnetite are common phases in all of the rock types. Geochemically, the rocks of the Xiadong complex display high MgO contents and extremely low trace element abundances. Although the rocks of the main body show higher MgO contents and lower trace element abundance compared to the dykes, both are characterized by flat REE patterns and enrichment of LILE relative to HSFE. These petrological, mineralogical and geochemical features indicate that the Xiadong complex is a typical Ural–Alaskan type complex. The cross-cutting and intrusive relationships of the rock units and their distinct geochemical features suggest that the main body represents a different pulse from the intrusive dyke. The positive whole-rock εNd(t) values, together with variable (87Sr/86Sr)i ratios, show a depleted mantle source metasomatized by subduction-related material. Zircon U–Pb dating on one hornblende gabbro from the main body of the complex yields a U–Pb 206Pb/238U age of 479 Ma, and zircons from three samples of the dykes yield ages of 477, 379 and 313 Ma. All the zircons have positive εHf(t) and slightly higher δ18O than normal mantle, and their Hf–O isotopic values are correlated to their ages. The older samples are comparable to those from the Permian mafic–ultramafic complexes in the Beishan Terrane, which were derived from a mantle source metasomatized by subduction of the South Tianshan ocean, while the younger samples are similar to those from the Paleozoic complexes in the Eastern Tianshan, which record the subduction features of the Junggar ocean. These results suggest that the Central Tianshan was a continental arc from at least the Ordovician to the Carboniferous. The lithospheric mantle beneath the Central Tianshan was depleted in Sr–Nd–Hf isotopic compositions and was metasomatized by melts and/or fluids from the subduction of the South Tianshan and Junggar oceans in Paleozoic. The formation of the Xiadong Ural–Alaskan type complex was the product of arc magmatism via high-degree melting of the lithospheric mantle beneath the southern Central Asian Orogenic Belt.