Geochemistry of the ~Â 430-Ma Jingbulake mafic-ultramafic intrusion in Western Xinjiang, NW China: Implications for subduction related magmatism in the South Tianshan orogenic belt

The pyroxene diorite has a SHRIMP zircon U-Pb isotope age of 431 ± 6 Ma. All the rocks have nearly flat chondrite-normalized rare earth element patterns and show negative Nb-Ta and P-Ti anomalies on mantle-normalized trace element diagrams. They have positive εNd values (+ 1.6 to + 4.0), and low (87Sr/86Sr)i ratios (0.7039-0.7048), indicative of a depleted mantle source. The rocks all have low Th/Nb ratios, narrow isotopic variations, and slightly negative Zr-Hf anomalies, suggesting insignificant crustal contamination. They are enriched in large ion lithophile elements, such as Cs, Rb, Ba, Pb and Sr, and have constant but radiogenic Pb isotopic ratios of 206Pb/204Pb (18.159-18.250), 207Pb/204Pb (15.599-15.633) and 208Pb/204Pb (37.978-38.250), consistent with the involvement of subduction-related fluids and sediments in the source region. The Jingbulake intrusion, together with other mafic intrusions in the belt, suggests subduction of the South Tianshan oceanic lithosphere beneath the Yili-Central Tianshan block. The subduction-related fluids and sediments not only metasomatized the mantle wedge above the subduction zone, but also triggered the melting of the mantle due to the addition of water.