摘要
位于北京市东北部的云蒙山岩浆杂岩体由十余个侵入体组成 ,它们普遍遭受了强烈韧性变形改造 ,与太古宇杂岩和变形变质的中 上元古代沉积岩共同卷入了云蒙山背形的构造系统中。野外调查和对岩石薄片的观察表明 ,岩浆杂岩体中各侵入体均具有变形变质特征 ,主要造岩矿物间具有明显的交代关系。虽然各侵入体在形成时代上存在明显的先后关系 ,但它们具有的片麻状构造是共同的 ,并且与围岩的变形是统一而协调的。这表明云蒙山岩浆杂岩体的片麻理不是岩体侵位时形成的 ,而是在岩体冷凝固结后受到构造变形作用改造的结果。即杂岩体侵位与其发育的片麻理构造所反映的变形事件在时间上存在明显差距。根据同位素年代学资料 ,岩浆侵位发生在燕山中期 ;但其与围岩一起卷入云蒙山背形构造系统的变形事件发生得较晚 ,可能形成于燕山晚期。云蒙山杂岩体中长英质侵入体的矿物学特征表明其在侵位后曾经历麻粒岩相区域变质作用 ,而在燕山晚期随变质核杂岩下盘的抬升又经历了角闪岩相退变质作用。
Yunmengshan Magmatic Complex, which is located in the northeastern part of Beijing Municipality, is composed of more than ten plutons. All of them had been suffered intensive ductile deformation, and were involved into Yunmengshan antiform system with its surrounding Archaean metamorphic complex and meta-sedimentary rocks of mid-and upper Proterozoic strata. Field and thin section observations show that all plutons in Yunmengshan Magmatic Complex have metamorphic as well as deformation features. There are metasomatic relations among main rock-forming minerals. Although there are clear intrusive relations among plutons in the Complex, all of these plutons have uniform gneissic structure; meanwhile, the gneissic structure is coordinate with the wall-rock foliation. This fact means that the gneissose of Yunmengshan Magmatic Complex did not form in the plutons' intrusion period, but was the result of structural deformation event occurred after the solid stage of plutons; therefore, there was significant time interval between pluton intrusion and deformation event that is exhibited by uniform gneissic structure of Yunmengshan Complex. According to isotopic geochronology, intrusion of Yunmengshan plutons happened in Middle Yanshanian. However, the deformation event that involved plutons and its wall rock into Yunmengshan antiform, occurred much later, possibly in Late Yanshanian. The mineralogical features of felsic plutons in Yunmengshan Complex suggest that they might experience granulite grade regional metamorphism after intrusion, and then amphibolite retrograde metamorphism during the uplift of footwall of a Late Yanshanian metamorphic core complex. Yunmengshan Magmatic Complex represents a lithological assemblage of new-formed middle crust of Yanshan orogenic belt.
出处
《北京地质》
2004年第3期1-11,共11页
Beijing Geology
基金
国家自然科学基金 (4 0 10 4 0 0 3
4 0 376 0 13)
中国科学院王宽诚博士后奖励基金
中国博士后科学基金资助
关键词
侵位
构造变形
区域变质作用
燕山晚期
岩浆杂岩体
Yunmengshan Magmatic Complex
intrusion
structural deformation
regional metamorphism
Yunmengshan antiform
Middle Yanshanian
Late Yanshanian
Beijing
