Zircon LA-MC-ICPMS U-Pb dating of tuff from mesoproterozoic Gaoyuzhuang Formation in Jixian County of North China and its geological significance

Abstract

A 15 cm thick tuff intercalation in dolomite was recently discovered in the lower horizon of the third member (the Zhangjiayu Subformaiton) of the Gaoyuzhuang Formation in the Mesoproterozoic Jixian System at the Dongshuichang Village, Jixian County. A zircon U-Pb age of (1577±12) Ma for the tuff was obtained by means of LA-MC-ICPMS method, consequently for the first time providing precise and reliable chronological constraint on the depositional age of the Gaoyuzhuang Formation in the Jixian area and a new correlation marker for Mesoproterozoic strata in the whole Yanshan area of the northern North China Craton. At the same time, abundant macro-alga fossils represented by Parachuaria glabra Sun, the same as those discovered previously in the Mesoproterozoic Standard Section in the Dahongyu Valley, were also found in the dolomite beds directly overlying the newly-discovered tuff beds. Therefore, the newly-obtained tuff age can further provide constraint on the living time of the macro-alga Parachuaria glabra Sun in the Gaoyuzhuang Formation. Combined with the zircon U-Pb age (1560±5) Ma obtained previously for the tuff bed in the top horizon of the third member of the Gaoyuzhuang Formation in Yanqing County, northwestern Beijing, it can be concluded that the depositional age of the third member of the Gaoyuzhuang Formation is in the time span of 1580~1560 Ma, and the boundary between the Gaoyuzhuang Formation and the underlying Dahongyu Formation can be quite confidently placed at 1600 Ma that is equivalent to the lower boundary of the International Mesoproterozoic Calymmian System.