Geochronology and geochemistry of the Middle Proterozoic Aoyougou ophiolite in the North Qilian Mountains, northwestern China

The Aoyougou ophiolite lies in an early Palaeozoic orogenic belt of the wes tern North Qilian Mountains, near the Aoyougou valley in Gansu Province, no rthwestern China. It consists of serpentinite, a cumulate sequence of gabbr o and diorite, pillow and massive lavas, diabase and chert. Ages of 1840 +/ -2 Ma, 1783 +/-2 Ma and 1784 +/-2 Ma on three zircons from diabase, indicat e an early Middle Proterozoic age. The diabases and basalts show light rare -earth element enrichment and have relatively high TiO2 contents, character istic of ocean island basalts. All of the lavas have low MgO, Cr, Ni conten ts and Mg numbers indicating a more evolved character. They are believed to have been derived from a more mafic parental magma by fractionation of oli vine, Cr-spinel and minor plagioclase. Based on the lava geochemistry and r egional geology, the Aoyougou ophiolite was probably believed to have forme d at a spreading centre in a small marginal basin. Subduction of the newly formed oceanic lithosphere in the Middle Proterozoic produced a trench-are- basin system, which is preserved in the North Qilian Mountains.