The volcanic rocks in the Harlik orogenic belt of eastern Tianshan, Xinjiang are combined with a set of acid to basic rocks (rhyolite, dacite, andesite, basalt) intercalated with pyroclastic rocks(dacite tuff). U-Pb dating of zircons from the samples of rhyolite by LA-ICP-MS yields weighed mean 206Pb/238U ages of (468.8±9.1) Ma, which indicates that these volcanic rocks formed in Middle Ordovician. Geochemical analysis results show that the SiO2 contents are from 49.12% to 78.24%; TiO2 contents are from 0.12% to 1.00%; Al2O3 contents are from 11.31% to 20.86%; A/CNK ratios are from 0.80 to 1.31(with an average of 0.99); and Rittmann indexes (σ) are from 0.19 to 3.86(with an average of 1.29); Mg# values are from 8.87 to 49.29(with an average of 31.20). The rocks are rich in large ion lithophile elements(LILEs, e.g., Rb, Ba) and light rare earth elements(LREEs,e.g., La, Ce) and depleted in high field-strength elements(HFSEs, e.g., Nb, Ta, Ti). According to the relevant diagrams, the ratios about the trace elements and rare earth elements, and xenocrystic zircon, it indicates that there are typical arc volcanic rocks in supper-subduction zone, and that the magma intruded the continental crust. Moreover, based on the analysis of regional geological data, during the Early Paleozoic (Middle Ordovician to Early Silurian), we can claim that there is a large island arc evolution related to the Caledonian tectonic magmatism, which tectonic background is the oceanic crust subduction of island arc environment, and its formation may be related to east Junggar Kelamaili ocean of the south subduction.
