小兴安岭“晚古生代”地层的时代与物源：地质与碎屑锆石 U-Pb 年代学证据

本文对出露于小兴安岭的"晚古生代"红山组和黑龙宫组进行了碎屑锆石LA-ICP-MS U-Pb定年,旨在准确限定红山组和黑龙宫组的沉积时限,并揭示其物源组成。样品中大多数锆石呈自形—半自形,显示典型的岩浆生长环带或条痕状吸收,暗示其岩浆成因。研究结果显示,采自红山组标准剖面泥板岩中的碎屑锆石42个分析点产生以下年龄峰值:747、807、849、903、956、1 167和1 811 Ma,表明红山组沉积于747 Ma之后;采自伊春地区黑龙宫组泥板岩中的碎屑锆石97个分析点产生以下年龄峰值:700(发生Pb丢失)、805、902、1 764、2 446和2 467Ma,确定黑龙宫组沉积于805Ma之后。近年来在该地区"晚古生代"地层中碎屑锆石的定年结果显示普遍存在561 Ma年龄,鉴于红山组和黑龙宫组中缺乏上述锆石年龄组合,认为研究区的红山组和黑龙宫组的形成时代分别为747~561 Ma和805~561 Ma,时代置于新元古代。基于两组碎屑锆石的年龄频数和区域地质年代学资料的对比分析,两个地层单元中出现大量新元古代岩浆锆石,证明研究区可能存在新元古代岩浆事件,岩浆产物为两组地层提供物源;而中—古元古代碎屑锆石的存在,同时暗示该区沉积时地表或地表浅部应存在更为古老的前寒武纪残片。

Age and Provenance of the Late Paleozoic Strata in Lesser Xing ’an Range:Evidence from Field Geology and Detrital Zircon U Pb Ages

This paper reports the LA ICP MS U Pb dating results of detrital zircons from Hongshan and Heilonggong Formations in Lesser Xing ’an Range in order to constrain their formation age and provenance .Most of the detrital zircons are euhedral‐subhedral in shape and display striped absorption or oscillatory zoning in CL images ,implying their magmatic origin .The dating results of the magmatic zircon indicate that 42 detrital zircons from clayslate stones in Hongshan Formation yield age populations of 747 Ma ,807 Ma ,849 Ma ,903 Ma ,956 Ma ,1167 Ma ,and 1811 Ma ,implying that the sedimentation of Hongshan Formation could take place after 747 Ma .97 detrital zircons from clayslate stone in Heilonggong Formation from Yichun yield the age populations of 700 Ma (Pb losing occurred) , 805 Ma , 902 Ma , 1764 Ma , 2446 Ma , and 2467 Ma;which suggests that the sedimentation of Heilonggong Formation could take place after 805 Ma because of the Pb losing .Combined with the detrital zircon dating results (wide occurrence of 561 Ma age population) from the Late Paleozoic strata in the study and the adjacent areas ,we conclude that the formation ages of Hongshan and Heilonggong formations are between 747 and 561 Ma and between 805 and 561 Ma , respectively ; i .e ., Neoproterozoic rather than Late Paleozoic as believed previously .Based on the comparison between the age populations of the detrital zircons from the two formations and the geochronological data in the study area ,the sediments of the two formations were mainly sourced from the Neoproterozoic stones . Furthermore ,the Neoproterozoic magmatic zircons have been identified in this study ,to be the evidence of the Neoproterozoic magmatic events in the area .Meanwhile ,the presence of detrital zircons with Meso‐Paleoproterozoic ages reveals the existence of ancient Precambrian remnants at or near the surface .