Geochemical characteristics of the Lower Carboniferous volcanic rocks of the Wuluate Formation in the Western Kunlun Mountains and their geological significance

Abstract: The Wuluate Formation of Lower Carboniferous is distributed in the northern part of Western Kunlun Mountains. Associated with thick carbonate?rock?and thin? marble, it is a set of volcano-sedimentary rocks mainly?composed of grayish green?pillow basalts and grayish?white rhyolite. Based on the study of the chemistry of lavas, the authors hold that the major element values of the basalts are similar to the average values of tholeiite: SiO2 values vary between 52.10% and 54.42%， (Na2O/K2O)>1，TiO2(0.50%~2.03%)，and Mg# values are in the range of 52~70. The ΣREE values of the samples range from 14.46×10-6 to 91.83×10-6，whereas (La/Yb)N values vary between 0.46 and 1.69，respectively. All these characteristics are extremely similar to things of the E-BABB and different from features of OIB and N-MORB. The basalts show depletion of Rb, suggesting that the lower crust contamination existed in the magma processing. Rhyolite belongs to both calc-alkaline and high-K calc-alkaline volcanic rock series. Concentrations of REE are higher than those of basalts, the (La/Yb)N radios are between 1.30 and 11.99, and (La/Sm)N radios are from 1.15 to 4.07. All these features are similar to features of the lower crust. Different REE patterns and trace element features between the two rock types and the absence of trend from the basalts to the rhyolite imply that the rhyolite and basalt had different sources and origins. Trace element diagrams and regional geological characteristics indicate that the Wuluate Formation formed in an extension environment of the back-arc pull-apart condition. The study of the genetic characteristics of rocks suggest that the basalts resulted from 5% –10% partial melting of spinel lherzolite in the depleted mantle, and were contaminated by the lower crust later. The heat carried by the mantle material led to the partial melting of the lower crust, and then the rhyolite was produced. Comprehensive studies show that the northern part of Western Kunlun Mountains was in a stretch tension environment (back–arc extensional environment) in the early Carboniferous, and the Wuluate Formation was the magmatic product of the tectonic event.