Development of a complex arc–back-arc basin system within the South Tianshan Ocean: Insights from the Wuwamen ophiolitic peridotites

Ophiolites within the South Tianshan orogenic belt directly record the nature and tectonic evolution of the South Tianshan Ocean (STO), which is key for understanding the accretionary history of the Central Asian Orogenic Belt. Here we present petrological and Os isotopic data for the Wuwamen mantle peridotites from the South Tianshan orogenic belt, which provide new insights into the tectonic origin of this ophiolite and evolution of the STO. The lherzolites have low spinel Cr# (0.1–0.2), similar to those of fertile oceanic or continental peridotites that have undergone low degrees of melting. However, the extremely high Na2O (1.5–1.8 wt%) and Al2O3 (5.5–6.2 wt%) contents of the clinopyroxenes are atypical of oceanic peridotites, and resemble those of subcontinental peridotites. The harzburgites have more refractory compositions, suggesting higher degrees of melt extraction. Both the lherzolites and harzburgites have low initial 187Os/188Os ratios (< 0.121), which reflect Proterozoic melt depletion and a long-term lithospheric evolution. These data confirm an ancient subcontinental lithospheric mantle origin for the Wuwamen peridotites. The replacive dunites have moderate spinel Cr# (0.3–0.4), reduced Os and enriched 187Os/188Os (0.142–0.160), indicative of reaction between host lherzolites and percolating mid-ocean ridge basalt (MORB)-type melts with radiogenic Os isotopic signatures, presumably in a subduction-related setting. This and the associated back-arc basin basalt (BABB)–type mafic crust suggest the Wuwamen ophiolite formed in a continental margin back-arc setting. A comparison with other ophiolites in the South Tianshan orogenic belt further indicates that a complex arc–back-arc system existed along the southern margin of the Central Tianshan, which was associated with northward subduction within the STO.