Geochemical and geochronological studies of the Alegedayi Ophiolitic Complex and its implication for the evolution of the Chinese Altai

The Alegedayi Ophiolitic Complex (AOC) was discovered in the northwestern part of Altai, Xinjiang, China. Strips of mafic rocks including gabbro, diabase, pillow basalt and pyroxenite, all deformed, mostly underwent low-grade metamorphism, are intercalated with marine-facies sedimentary strata consisting of shale, siltstone and chert. SHRIMP zircon dating of a metagabbro sample gave an age of 439±17 Ma. In terms of whole-rock geochemistry, the AOC is composed of three distinct groups. Whereas the majority has transitional or enriched mid-ocean ridge basalt (T-/E-MORB) affinity, varieties with oceanic island basalt (OIB) and supra-subduction zone (SSZ) affinities were also identified. Our data show that the petrogenesis of this mafic complex involved interaction among depleted mantle, enriched component, recycled sediments, slab-derived fluid, and metasomatized mantle wedge. Taking into account the coeval adakites and high-Mg rocks to the southwest and a slightly older arc to the northeast, we suggest that the AOC reflects an intra-arc spreading event that occurred in response to an early Silurian ridge subduction which had profound effects on the tectonic evolution of the Chinese Altai.