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

摘要

The South Tianshan orogenic belt, the southwestern extension of the Central Asian orogenic belt, separates the Yili-Central Tianshan block to the north from the Tarim block to the south. This belt is marked by a Proterozoic metamorphic complex, ophiolites, granitoids, mafic intrusions and younger high-pressure and low-temperature metamorphic rocks. The Jingbulake mafic intrusion in this belt is a zoned body composed of pyroxene diorite, olivine gabbro and wehrlite. Pyroxenite locally intrudes all of these rocks but is most abundant along the boundaries between pyroxene diorite and olivine gabbro. Both the olivine gabbro and wehrlite contain disseminated sulfide mineralization, and a sulfide rich orebody intrudes the pyroxenite. 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 (~(87)Sr/~(86)Sr); 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 ~(206)Pb/~(204)Pb (18.159-18.250),~(207)Pb/~(204)Pb (15.599-15.633) and ~(208)Pb/~(204)Pb (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.