北京密云元古宙常州沟组之下环斑花岗岩古风化壳岩石的发现及其碎屑锆石年龄

Discovery of the paleo-weathered mantle of the rapakivi granite covered by the Proterozoic Changzhougou Formation in the Miyun area,Beijing and their detrital zircon dating

在北京密云地区,最近发现环斑花岗岩(脉)上发育有古风化壳,并被长城系常州沟组砂岩覆盖。风化壳物质的组成主要为来自环斑花岗岩的原地风化残留物和粗碎屑岩,采用SHRIMP和LA-ICP-MS方法,分别获得环斑花岗岩古风化壳碎屑岩的碎屑锆石U-Pb年龄值为(1682±20)Ma、(1708±6)Ma等,与相邻的密云环斑花岗岩年龄相同。这套古风化壳碎屑岩的存在和测年结果显示,长城系常州沟组的底界年龄应小于1682Ma。

Recently,the Proterozoic rapakivi granites（dike） with the paleo-weathered mantle-clastic rocks covered directly by the sandstones of the Changzhougou Formation of Changchengian System are discovered in the Miyun County,Beijing.The detrital components of the rocks embrace the in-situ weathered mantle saprolites and coarse-grained clastic rocks from rapakivi granites.Through SHRIMP and LA-ICP-MS dating methods,we have obtained the U-Pb dating ages of 1682±20 Ma and 1708±6 Ma from the detrital zircons sampled in the rocks;those age data are the same with the nearby Miyun rapakivi granite.The existence of this suite of weathered saprolites and clastic rocks,as well as the analysis of the dating results,have shown that the age of the base of the Changzhougou Formation should be younger than 1682 Ma.