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林山岩群是华北克拉通中部造山带南缘前寒武纪结晶基底的一部分,它对于探讨中部造山带碰撞时代及其在克拉通南缘的分布具有重要意义.选择林山岩群主体的变基性岩(斜长角闪片岩)为主要研究对象,利用锆石U-Pb定年和岩石地球化学方法,获得的结果表明斜长角闪片岩样品锆石多呈长柱状,且外形不规则,具弱分带、斑杂状分带和扇形分带结构,其中少量锆石保留有早期原岩残留的岩浆核.锆石核部具岩浆振荡环带特征,大部分Th/U比值范围为0.10~9.24,且外形不规则,边部具溶蚀结构;绝大部分锆石具弱分带、斑杂状分带和扇形分带结构,以及低Th/U比值范围为0~0.09,个别比值为0.12,显示为典型的变质锆石特征.LA-ICPMS锆石U-Pb定年结果表明该斜长角闪片岩的核部残留加权平均年龄为2 528±43Ma(MSWD=1.5,n=10),经历变质作用的加权平均时限为2 474±22Ma(MSWD=1.3,n=21).全岩地球化学结果显示林山岩群变基性岩贫硅(47.70%~50.77%,平均值为48.90%)和低钾(0.56%~1.78%,平均值为1.13%),轻、重稀土分异较弱,亏损微量元素Nb、Th、Ti、Zr等高场强元素,且具有低的微量元素Zr含量(<130×10-6)和Zr/Y比值(<4).这些特征表明林山岩群形成于俯冲作用背景下的岛弧环境.因此,上述证据共同反映了华北克拉通东、西陆块碰撞产物的中部造山带经历了新太古代晚期-古元古代早期的俯冲碰撞造山作用.
The Linshan Group is a part of the Precambrian crystalline basement in the southern margin of the central orogenic belt in the central part of the North China Craton and is of great significance in exploring the collision age of the central orogenic belt and its distribution in the southern margin of the craton. The results show that the zircons of the plagioclase amphibolite are mostly long-columnar and irregular in shape with Weak zoning, zebra zonation and fan-shaped zonation, among which a small amount of zircon retained the magma nuclei of early protolith.The zircon nuclei have the characteristics of magma oscillatory zone, and the majority of Th / U ratios range from 0.10 ~ 9.24 with an irregular shape and a dissolution structure at the edges. Most of zircons have weak zoning, patchy zonation and fan-shaped zonation, and the low Th / U ratios range from 0 to 0.09 with a ratio of 0.12 , Showing typical metamorphic zircon characteristics. The U-Pb dating of zircons by LA-ICPMS shows that the residual weighted mean age of the plagioclase amphibolite is 2 528 ± 43Ma (MSWD = 1.5, n = 10) The weighted average time of metamorphism is 2 474 ± 22Ma (MSWD = 1.3, n = 21). The whole rock geochemistry The results show that the Linyuan Group metamorphic rocks are poor in silicon (47.70% ~ 50.77%, average 48.90%) and low potassium (0.56% ~ 1.78%, with an average of 1.13% Depleted trace elements such as Nb, Th, Ti and Zr, with low trace element Zr content (<130 × 10-6) and Zr / Y ratio (<4) Therefore, the above evidence together reflects that the central orogenic belt of the collision products of the eastern and western continental blocks of North China craton experienced the subduction collision orogeny in the late Neo-Archean-Early Paleoproterozoic.