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本文主要研究扬子地台北缘的陕西西乡县穆家沟地区志留系底界界限年代.此区古生代沉积有下寒武统、中上奥陶统、下志留统及二叠系一套滨海浅相地层,化石丰富,可与国内外对比.区内以造陆性垂直升降运动为主,除奥陶系与志留系整合接触外,其余为平行不整合接触.未发现有同期岩浆活动,岩石基本未变质.下志留统龙马溪组厚10m,含Glyptograptus persculptus,Parakidograptus acuminatus及Orthograptus vosiculosus三个笔石带.主要为砂质页岩、页岩,常夹有硅质岩条带.上与崔家沟组(S_1~1)页岩(厚82m)、下与观音桥组(O_3~3)砂岩(厚0.25m)呈过渡关系.主要物源来自北部的汉南古陆.对龙马溪组页岩进行全岩及分粒级对比研究,选取6个样品的不同粒级共8个点测试Rb-Sr等时线年龄t=438Ma,~(87)Sr/~(86)Sr初始比为0.7146,推荐此值为志留纪底界年龄.
This paper mainly studies the age boundaries of the Silurian boundary in the Mujiagou area, Xixiang County, Shaanxi Province, in the northern margin of the Yangtze platform.The Paleozoic sediments of this area include the Lower Cambrian, Middle-Upper Ordovician, Lower Silurian and Permian Coastal shallow facies, rich in fossils, can be compared with at home and abroad.The area is mainly caused by vertical continental movement, except for the Ordovician and Silurian integration contact, the rest are parallel unconformity contact.Not found in the same period of magma In the Lower Silurian Longmaxi Formation, there are three peninstrips, including Glyptograptus persculptus, Parakidograptus acuminatus and Orthograptus vosiculosus, which are mainly composed of sandy shale and shale with siliceous bands Upper and Cuijiagou (S_1 ~ 1) shale (82 m thick), and the Guanyinqiao Formation (O_3 ~ 3) sandstone (0.25 m thick) have a transitional relationship with the main source of Hannan ancient land in the north. The whole rock and fractionation of Longmaxi shale were studied. The Rb-Sr isochron age (t = 438Ma, ~ (87) Sr / ~ (86) The initial Sr ratio is 0.7146, which is recommended as the Silurian bottom boundary age.