吕梁西榆皮地区古元古代麻粒岩相变质作用及其构造意义

发布时间：2018-04-03 08:49

  本文选题：麻粒岩　切入点：锆石年代学　出处：《西北大学》2015年硕士论文

【摘要】：华北克拉通中部造山带的中部西缘地区,出露大量古元古代片麻状花岗岩、变质表壳岩及少量变质基性岩脉,被认为是一套经不同程度变质而成的杂岩而被称之为“吕梁杂岩”。前人已对该区一些深变质岩的变质时代和期次有一定研究,但对其变质作用和演化过程的研究还较薄弱,作为中部造山带的重要组成部分和经历多期变质作用改造的深变质岩,其变质年代及演化历史的研究无疑将为华北克拉通早期构造演化过程提供重要的依据。本文通过对吕梁杂岩东南部西榆皮地区的片麻岩及基性麻粒岩的野外特征、岩相学特征、矿物成分分析及同位素年代学等方面的研究,初步确定了片麻岩及基性麻粒岩所经历的变质作用期次、峰期P-T条件及变质P-T轨迹,进而对该区的构造演化做了初步探讨。区内主要发育石榴石夕线石黑云二长片麻岩及基性麻粒岩组合,片麻岩中常伴有深熔长英质脉体,基性麻粒岩呈侵入脉体状发育于石榴石夕线石黑云二长片麻岩中。岩相学观察显示,石榴石夕线石黑云二长片麻岩的主要矿物组合为：石榴子石+夕线石+黑云母+钾长石+斜长石+石英；基性麻粒岩的主要矿物组合为：石榴子石+斜方辉石+单斜辉石+角闪石+斜长石,均显示出麻粒岩相的矿物组合特征,代表了吕梁地区一套深变质岩石组合。石榴石夕线石黑云二长片麻岩中的石榴子石富Fe、Mg,亏损Mn、Ca,且核部成分较均一,边部成分变化较大,显示出退变环带特征；基质黑云母富Mg,但包裹于石榴石中的黑云母与石榴子石边缘的黑云母Mg、Fe成分差异很大,证明在退变质过程中两者发生了Fe-Mg交换；基性麻粒岩中的石榴子石富Fe、Ca,相对亏损Mg、Mn,同样发育退变环带；斜方辉石主要为富Mg、贫A1的紫苏辉石；单斜辉石主要为富Ca,贫Fe、Mg的次透辉石。计算获得了石榴石夕线石黑云二长片麻岩的峰期温压条件为：T=～700℃,P=～6kbar,退变质阶段的温压条件为：T=～530℃,P=～2kbar;基性麻粒岩的峰期温压条件为：T=497℃,P=8.7kbar;由此获得一顺时针的变质P-T演化轨迹。片麻岩中的深熔花岗岩脉的锆石U-Pb年代学研究揭示,该深熔作用的时代为1981Ma,从而限定该片麻岩最大变质年龄为1981 Ma。基性麻粒岩中的锆石定年,分别获得1945±20Ma和1928±25Ma两期年龄,1945Ma解释为峰期变质年龄,1928Ma则代表了麻粒岩抬升过程退变质年龄。结合前人获得的片麻岩中独居石年代学结果,认为西榆皮地区的峰期变质年龄介于1947～1942Ma之间,退变质可能发生在1928Ma及其后期的1919～1857Ma期间。因此认为,吕梁地区同中部带其它地区一样,在1950Ma也经历了陆-陆碰撞陆壳增厚,并致使该区经历了麻粒岩相的变质作用,此后在1930Ma陆壳抬升减薄,从而叠加了角闪岩相的退变质作用,这一过程在吕梁西榆皮地区的片麻岩和基性麻粒岩中均有很好的记录。
[Abstract]:In the central western margin of the central orogenic belt of the North China Craton, a large number of ancient Gu Yuan granitic granites, metamorphic epidermal rocks and a small amount of metamorphic basic vein are exposed.Lv Liang complex is considered to be a set of metamorphic complex rocks.The metamorphic ages and stages of some deep metamorphic rocks in this area have been studied, but the study on their metamorphism and evolution process is still weak. As an important part of the central orogenic belt and the deep metamorphic rocks undergoing multi-stage metamorphism transformation, the metamorphic rocks have been studied.The study of metamorphic age and evolution history will undoubtedly provide an important basis for the early tectonic evolution of the North China Craton.In this paper, the field characteristics, petrographic characteristics, mineral composition analysis and isotopic chronology of gneiss and basic granulite in Xiyupi area of southeastern Lv Liang complex are studied.The metamorphic stages, peak P-T conditions and metamorphic P-T loci of gneiss and basic granulite are preliminarily determined, and the tectonic evolution of the area is discussed.The gneiss and basic granulite assemblage are mainly developed in this area. The gneiss is often accompanied by deep melt feldspar vein, and the basic granulite is developed in the garnet siliite black cloud gneiss as an intrusive vein.The petrographic observation shows that the main mineral assemblages of garnet sillimanite biotite biotite feldspar plagioclase quartz are: pomegranate sillimanite biotite feldspar plagioclase quartz;The main mineral assemblages of basic granulites are pomegranate clinopyroxene clinopyroxene hornblende plagioclase which all show the mineral assemblage characteristics of granulite facies and represent a set of deep metamorphic rock assemblages in Lv Liang area.The pomegranate Shi Fu FeMgO in garnet silimanite black cloud gneiss is depleted in MnCa-Ca.The composition of the nuclear part is more uniform, and the composition of the edge part changes greatly, showing the characteristics of the degenerative ring zone.The matrix biotite is rich in Mg-rich, but the composition of biotite in garnet is very different from that in the edge of garnet, which indicates that Fe-Mg exchange occurred during retrograde metamorphism.In the basic granulite, the pomegranate Shi Fu Feancai, which is relatively deficient in MgPe mn, also developed a degenerative ring zone; the clinopyroxene is mainly Mg-rich and A1-poor perilla pyroxene, while the clinopyroxene is mainly Ca-rich and Fe-mg-poor subdiopyroxene.璁＄畻鑾峰緱浜嗙煶姒寸煶澶曠嚎鐭抽粦浜戜簩闀跨墖楹诲博鐨勫嘲鏈熸俯鍘嬫潯浠朵负:T=~700鈩，

本文编号：1704485