Keketuohai mafic–ultramafic complex in the Chinese Altai, NW China: Petrogenesis and geodynamic significance

Abstract

Devonian magmatism was very intensive in the tectonic evolutionary history of the Chinese Altai, a key part of the Central Asian Orogenic Belt (CAOB). The Devonian Keketuohai mafic–ultramafic complex in the Chinese Altai is a zoned intrusion consisting of dunite, olivine gabbro, hornblende gabbro and pyroxene diorite. The pyroxene diorite gives a zircon U–Pb age of 409±5Ma. Variations in mineral assemblage and chemical composition suggest that the petrogenesis of the Keketuohai Complex was chiefly governed by fractional crystallization from a common magma chamber. Low SiO2, K2O and Na2O contents, negative covariations between P2O5, TiO2 and Mg# value suggest insignificant crustal assimilation/contamination. Thus the positive εNd(t) values (0 to +2.7) and slight enrichments in light rare earth elements (e.g., La/YbN=0.98–3.64) suggest that their parental magma was possibly produced by partial melting of the lithospheric mantle. Model calculation suggests that their parental magma was high-Mg (Mg#=66) tholeiitic basaltic melt. The Keketuohai intrusion was coeval with diverse magmatism, high temperature metamorphism and hydrothermal mineralization, which support a previously proposed model that ridge subduction most likely played an important role in the tectonic evolution of the Chinese Altai.