LA-ICP-MS zircon U-Pb dating and geochemical characteristics of Donggou ophiolites in Zamashi area of northern Qilian Mountain

Donggou ophiolite occurs in Zamashi area along the eastern segment of northern Qilian orogenic belt. The ophiolite is mainly composed of pyroxene peridotite, gabbro and mafic volcanic rocks, making up a relatively complete ophiolite unit. Single-grain zircon LA-ICP-MS U-Pb isotopic dating of mafic volcanic rocks yielded a weighted average age of 499.3±6.2 Ma, which represents the ophiolite formation age, equivalent to Late Cambrian. According to geochemical studies, the ophiolite mafic volcanic rocks are of tholeiitic series, the chondrite-normalized REE patterns are nearly of flat-type; the (La/Yb)N ratios are between 0.97 and 1.26; the trace element distribution patterns basically show flat curves with the exception of a few large-ion lithophile elements (Ba, Rb, U, K); Nb, Ta, Zr, Hf have not experienced loss, suggesting geochemical characteristics of mid-ocean ridge basalt (N-MORB); in Zr-Zr/Y and Ti/100-Zr-3Y tectonic discrimination diagrams, all the samples fall into the MORB area formed in mid-ocean ridge. A comparative study of the region shows that the ophiolite and Yushigou as well as Cuancigou ophiolite jointly constitute the spreading ridge-type ocean ophiolite.