An Ordovician intra-oceanic subduction system influenced by ridge subduction in the West Junggar, Northwest China

• Published in: International geology review 2014-01, Vol.56 (2), p.206-223
• Main Authors: Shen, Ping, Pan, Hongdi, Xiao, Wenjiao, Shen, Yuanchao
• Format: Article
• Language: English
• Subjects: Sharburti Mountains; Sr- Nd isotopes; Nb-enriched basalts; volcanic rocks; West Junggar; Xinjiang
• ISSN: 0020-6814; 1938-2839
• DOI: 10.1080/00206814.2013.839096

We report elemental and Nd-Sr isotopic data for three types of Ordovician volcanic and gabbroic rocks from the Sharburti Mountains in the West Junggar (Xinjiang), Northwest China. Gabbros and Type I lavas occur in the Early Ordovician Hongguleleng ophiolite whereas Type II and III lavas are parts of the Middle Ordovician Bulukeqi Group. Gabbros and Type I lavas are tholeiites with a depleted light rare earth element (LREE) and mid-oceanic ridge basalt (MORB)-like signature with a crystallization sequence of plagioclase-clinopyroxene, suggesting formation at a mid-oceanic ridge. Type II lavas are Nb-enriched basalts (NEBs, Nb = 14-15 ppm), which have E-MORB-like REE patterns and Nb/Yb and Th/Yb ratios. They come from mantle metasomatized by slab melts. Type III lavas are further divided into two sub-types: (1) Type IIIa is tholeiitic to calc-alkaline basalts and andesites, with REE patterns that are flat or slightly LREE enriched, and with a negative Nb anomaly and Th/Yb enrichment, indicating that they were generated above a subduction zone; (2) Type IIIb is calc-alkaline basalts and andesites, which are strongly enriched in LREE with a marked negative Nb anomaly and Th/Yb enrichment, suggesting generation in a normal island-arc setting. The initial 87 Sr/ 86 Sr ratios of Type III lavas range from 0.70443 to 0.70532 and ϵ Ndt ranges from +1.5 to +4.5, suggesting that these melts were derived from mantle wedge significantly modified by subducted material (enriched mantle I (EMI)) above a subduction zone. Contemporary tholeiitic to calc-alkaline basalt-andesite and NEB association suggest that the NEBs erupted during development of the tholeiitic to calc-alkaline arc. We propose a model of intra-oceanic subduction influenced by ridge subduction for the Ordovician tectono-magmatic evolution of the northern West Junggar.