ID-TIMS zircon U-Pb age of Yulonggou intrusive rocks in southern Qilian Mountain and its geological significance

The Riyueshan-Hualong basic-ultrabasic rock belt has a number of well-developed intrusive bodies composed mainly of amphibole gabbro, gabbro, norite, pyroxenite, augite peridotite and peridotite，which are related to copper-nickel mineralization. This paper reports some achievements made in evaluation of magmatic type copper-nickel ore resources potential in Hualong County, Qinghai Province, on the basis of field survey and investigation of Yulonggou intrusive body. ID-TIMS U-Pb dating of zircon from the Yulonggou intrusive body yielded an age of 442.7Ma±1.6 Ma. Petrologic and geochemical characteristics and regional comparative investigation show that Yulonggou intrusive body is a ferruginous series ultrabasic-basic complex characterized by good magmatic differentiation and high abundance of useful elements such as Ni, Cu and Co, therefore favorable for mineralization. 440 Ma was in the period when subduction orogeny was converted into collision orogeny in the whole Qilian Mountain and then into the orogenic stage with the closure of Laji Mountain small ocean basin, thus forming Yulonggou basic-ultrabasic intrusive body in a post-collision extensional environment. A comparison of information with other intrusive rocks in Riyueshan-Hualong basic-ultrabasic rock belt reveals that all the intrusive rocks might have been formed in the same period, characterized by specific dynamic mechanism and evolution process. This extensional rifting event had a very close relationship with mineralization, so the research has very important practical significance.