内蒙古西拉木伦河两岸志留-泥盆系碎屑锆石年龄及其构造意义

为了通过碎屑岩的物源对比讨论古亚洲洋的闭合过程,笔者选择西拉木伦河北侧林西双井子地区和南侧奈曼旗下石碑地区的志留纪地层进行碎屑岩锆石U-Pb年代学研究。北侧样品LX0831-11为粉砂质板岩,采于西拉木伦河北岸上志留统杏树洼组。碎屑锆石年龄分为三组:385~531Ma(N=52)、872~1097Ma(N=11)、1344~1901Ma(N=11),碎屑锆石的最小年龄限定地层沉积下限为中-晚泥盆世。南侧样品130417-06为石英岩屑砂岩,采于奈曼旗下石碑组顶部砂岩中。碎屑锆石年龄分为四组:370~523Ma(N=34)、884~1481Ma(N=21)、1573~1900Ma(N=6)、2369~2588Ma(N=8),碎屑锆石的最小年龄限定地层沉积下限为晚泥盆世。这两个分别来自西拉木伦河南、北两侧原志留纪样品,显示一致的志留-泥盆纪及晚元古代碎屑锆石年龄谱,表明两者从泥盆纪开始即具有相同的沉积物源;而代表兴蒙造山带的元古代碎屑锆石在奈曼旗地区的出现,说明泥盆纪以来兴蒙造山带的剥蚀物已到达华北板块北缘。因此,本次碎屑锆石年代学研究暗示华北板块与其北部松辽地块在中-晚泥盆世之前已经完成拼合过程,即此时两者间已不存在古亚洲洋。

Detrital zircon ages of the Silurian-Devonian clastic rocks in south and north banks of Xar Moron River, Inner Mongolia

In order to determine closure time of the paleo-Asian Ocean at the northern margin of North China Craton, sedimentary and detrital zircon U-Pb geochronological studies of the Upper Silurian strata on northern and southern margins of the Xar Moron River have been carried out. Sample LX0831-11 was collected from silty slate of the Upper Silurian Xingshuwa Formation in Shuangjing area (Linxi) of the northern margin of Xar Moron River, which is mainly composed of lithic fragment (50%), quartz (30%) and feldspar (15%) with a few muscovite (5%). Seventy five zircons were selected randomly for LA-ICP-MS U-Pb dating and all of them are concordant. Based on the zircon CL images and U-Pb age concordia diagrams, the zircon can be divided into three groups: 385~531Ma (N=52) with a peak age of 412Ma, 872~1097Ma (N=11), 1344~1901Ma (N=11). The youngest zircon age (385±8Ma) constrains a maximum sedimentary age of the Middle-Late Devonian. The first group may be related to the denudation of the Early Paleozoic Airgin Sum-Sonidzuoqi-Xilinhot magmatic belt. The second group and the third group show provenance from the Proterozoic basement of the Xing'an-Mongolian Orogenic Belt. Sample 130417-06 was collected from sandstone on the top of Upper Silurian Xiashibei Formation in Xiashibei area (Naiman Banner), which located at the southern margin of Xar Moron River. The Xiashibei Formation consists mainly of limestone, sandstone and slate, and is rich in shallow marine fossils, e.g. corals, bryozoans and stromatoporoids in its middle part, representing a stable continental margin of the North China Craton. The sample is mainly composed of lithic fragment (65%), quartz (30%) and feldspar (5%). Seventy five zircons are selected randomly for U-Pb dating and 69 of them are concordant. Based on the zircon CL images and U-Pb age concordia diagrams, these ages can be divided into four groups: 370~523Ma (N=34) with a peak age of 412Ma, 884~1481Ma (N=21), 1573~1900Ma (N=6) and 2369~2588Ma (N=8). For the first age group, the 500~400Ma zircons may be related to the Early Paleozoic Tugurige-Damaoqi-Ondor Sum-Xianghuang Banner-Jiefangyingzi arc magmatic belt. While 370~400Ma zircons may come from the Early-Middle Devonian alkaline rocks and mafic intrusion along the northern margin of the North China Craton. The second group corresponds to the Precambrian basement of the Xing'an-Mongolian Orogenic Belt. The third and fourth groups display the similar distribution as the North China Craton, which indicates the North China Craton was a probable source area for the two groups. The Xar Moron River is considered as the Late Paleozoic suture line between the North China Craton and blocks to the north. However, the two samples collected from the both banks of the river show similar detrital zircon age distribution from the Middle Devonian to Mesoproterozoic, which indicates the same provenance for these two samples after the Middle Devonian. On the other hand, because the age distribution of the Songliao Block is characterized by the Newproterozoic to Mesoproterozoic zircons, their occurrence in the north margin of North China Craton suggests that the two blocks were near each other and the Paleo Asian Ocean between them has been closed during the Middle to Late Devonian.