山西云中山地区“界河口群”重新厘定

Redefinition of jiehekou group in Yunzhongshan area,Shanxi Province

云中山位于五台地区和吕梁地区的过渡部位,对研究五台杂岩和吕梁杂岩的构造关系具有重要的地质意义。云中山地区变质基底由一套石英岩、黑云变粒岩、大理岩和斜长角闪岩等变质表壳岩和新太古代花岗片麻岩组成,前人将这套变质表壳岩建造称为“界河口群”,但是从野外观察来看,这套变质建造的岩石组合与吕梁山区界河口群有很大的差异,应重新厘定。本次工作在野外调查的基础上,根据岩石组合的不同,将云中山变质表壳岩划分为两部分:Ar3Y(Y代表云中山变质表壳岩)呈残留体保存在新太古代花岗片麻岩中,主要岩石类型包括:黑云变粒岩、石英岩、角闪变粒岩、斜长角闪岩等,其中花岗片麻岩的年代学研究表明该区界河口群的形成年龄老于~2.5 Ga;Pt1qz(qz代表石英岩建造)与新太古代花岗片麻岩呈韧性剪切构造接触,部分建造被古元古代晚期侵入体分隔,主要岩石组合为一套石英岩、磁铁变粒岩、大理岩、斜长角闪岩组合,年代学研究显示,这套岩石的沉积时限应在1.94~2.14 Ga。对两套岩石中变质火山岩的地球化学研究表明,Pt1qz形成于伸展背景下,有可能是被动大陆边缘的构造环境的产物,Ar3Y则形成于俯冲消减的岛弧构造背景下。

Yunzhongshan lies between Wutai area and Lvliang area,which plays an important role in studying tectonic relationship between Wutai complex and Lvliang complex.The metamorphic basement in Yunzhongshan is mainly composed of Neoarchaean granitic gneisses and a suit of metamorphosed supracrustal rocks which was called“Jiehekou Group”consisting of quartzite,Bi-leptynite,marble and amphibolite.However,according to field investigation,there are many differences between these metamorphosed supracrustal rocks and the typical Jiehekou Group in Lvliang area.Based on the rock association in this field investigation,these metamorphosed supracrustal rocks are divided into two parts.Ar3Y(Y represents Yunzhongshan metamorphosed supracrustal rocks)resided in the Neoarchaean granitic gneisses contains Bi-leptynite,quartzite,Hb-leptynite,amphibolite and so on.Chronological study of the granitic gneisses shows that the forming age of Ar3Y is older than~2.5 Ga;Pt1qz(qz represents quartzite)has a ductile shearing contact with the Neoarchaean granitic gneisses,part of which is intruded by late Paleoproterozoic intrusions aging of~1.94 Ga.Pt1qz consists largely of quartzite,magnetite leptynite,marble and amphibolite.The research of geochronology implies the formation age of Pt1qz is 1.94~2.14Ga.The geochemistry of the metavolcanics rocks in two suits of metamorphosed supracrustal rocks respectively suggest that Pt1qz was formed in the background of spreading tectonics with the products of passive continental margin and Ar3Y was developed in an island arc environment of subduction zone.