摘要:
最近在天津蓟县东水厂村附近中元古代高于庄组三段(即张家峪亚组)下部白云岩中发现了一层15 cm厚的凝灰岩夹层,测得其锆石LA-MC-ICPMS U-Pb年龄为(1577±12) Ma。这一进展不仅使得蓟县地区高于庄组沉积时代第一次获得了精确可靠的年代学约束,也为华北克拉通北缘整个燕山地区中元古界的精确对比提供了新的标尺。与此同时,由于在其紧邻的上覆层位还发现了与标准剖面早年所发现的以光面拟丘阿尔藻(Parachuaria glabra Sun)为代表的同类的宏观藻类,因此本测年结果实际也同时标定了这一宏观藻类群的生存时代。结合笔者等早先获得的北京延庆地区高于庄组三段上部凝灰岩的锆石 U-Pb 年龄((1560±5) Ma)可以确认,整个燕山地区高于庄组三段主体的沉积时限应为1580~1560 Ma,同时可以更为精确地将高于庄组与下伏大红峪组的界线年龄限定为1600 Ma左右,相当于目前通用的国际中元古界盖层系(Calymmian System)的底界。
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 byParachuaria 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-algaParachuaria 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.
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