Abstract:

Geochemistry, Rb-Sr and Sm-Nd isotopic compositions of the volcanic rocks, from the Qi’eshan Group and Wutongwozi Formation in the eastern Tianshan plate suture zone have been studied in this paper. The basalt from the Qi’eshan Group is highly enriched in light rare earth elements (LREEs) and large ion lithophile elements (LILEs) such as Sr, Ba and U, and depleted in high field strength elements (Nb, Ti and Zr), with variable Eu anomalies (varying from significant negative Eu anomalies to minor positive Eu anomalies) in the chondrite-normalized REE patterns. It is charactered by  isotopic signatures of depleted mantle source, with ε_Nd(t)=5.61-6.99, and I_Sr(330 Ma)=0.703 97-0.704 26. All these geochemical characteristics suggest that the basalt in Qi’eshan Group probably was derived from the lithospheric mantle wedge previously metasomatized by fluids/melts released from the down-going paleo-Asian oceanic slab in the Paleozoic period. The basalt from the Wutongwozi Formation is characterized by enrichment of LREEs and LILEs, without obvious negative anomalies of high field strength elements. High and positive  Nd values (>+9) and relatively low ISr ratios (0.704 71-0.705 33) suggest that the basalt in the Wutongwozi Formation was derived from partial melting of the depleted asthenospheric mantle. In the Zr-Zr/Y discrimination diagram, samples from the Qi’eshan Group are mainly plotted in the field of island arc basalt (IAB), while, those from the Wutongwozi Formation in the field of within plate basalt (WPB). It’s suggested that the Qi’eshan Group formed in an island arc setting owing to the southward subduction of the Junggar oceanic slab, while the Wutongwozi Formation formed in a rifting setting that was developed upon the extensional back-arc basin.

Key words: basalt, trace elements, Sr-Nd isotopes, eastern Tianshan Mts