大别造山带混合岩特征及成因

发布时间：2018-04-19 11:51

本文选题：混合岩 + 岩石类型　；参考：《合肥工业大学》2017年硕士论文

【摘要】：在大陆深俯冲和碰撞形成造山带过程中,地壳普遍存在的深熔作用可影响造山带内地壳的热力学和流变学性质,进而可能会导致造山带最终垮塌。造山带中的混合岩主要由深熔作用形成,所以混合岩研究对于理解部分熔融、变质演化和造山过程的相互关系具有重要意义。大别造山带的北大别构造单元广泛发育混合岩。前人对北大别混合岩的命名和分类比较混乱,缺乏系统的观察和分类;对于北大别混合岩的成因机制各个学者观点不一,且对于混合岩形成的确切时代依然存有争议。选择罗田和岳西穹隆中混合岩开展了野外观察、岩相学、矿物化学、岩石化学和锆石LA-ICP-MS U-Pb定年等系统工作,发现北大别混合岩主要分为叠层状和膨胀结构中等深熔混合岩以及眼球状和析离体状高度深熔混合岩两种类型。叠层状浅色体、浅色脉体和花岗岩显著亏损Fe_2O_3、MgO、MnO、TiO_2和CaO,高度深熔浅色体则稍微富集这些元素。叠层状浅色体的全岩主量元素组成分为花岗质和英云闪长质,浅色脉体为花岗质。高度深熔浅色体的全岩主量元素组成与片麻岩相似,为花岗闪长质。浅色体和花岗岩表现为LREE以及不相容元素Ba、Th和U的富集,而Nb和Ti显著亏损。叠层状浅色体具有强烈的Eu正异常,花岗质浅色脉体和花岗岩则表现为Eu负异常,高度深熔浅色体无明显Eu异常。这些都指示高度深熔浅色体、叠层状浅色体和花岗质浅色脉体具有成因联系,分别结晶于不同演化程度的熔体。锆石U-Pb同位素定年结果表明,北大别混合岩的原岩主要为来自新元古代的岩石、另有少量的古元古代地壳。混合岩化作用从145 Ma持续到124 Ma。斜长石-角闪石温压计估算结果显示,混合岩形成的温压条件为723-768°C和3.7-5.2 kbar,对应于中上地壳环境。混合岩的成因机制以长英质片麻岩水饱和条件下的富水熔融为主。其反应为:黑云母+石英+斜长石+水=角闪石+斜长石(残留)+花岗质熔体。少数混合岩的成因机制为角闪片麻岩中角闪石的脱水熔融。其反应为:角闪石+斜长石+石英=单斜辉石+富水熔体。
[Abstract]:During the process of continental deep subduction and collision forming orogenic belt, the deep melting in the crust can affect the thermodynamic and rheological properties of the crust of the orogenic belt, which may lead to the eventual collapse of the orogenic belt.The migmatite in the orogenic belt is mainly formed by deep melting, so the study of migmatite is of great significance in understanding the relationship between partial melting, metamorphic evolution and orogenic process.Migmatite is widely developed in the Beidabie tectonic unit of the Dabie orogenic belt.Previous scholars have confused the naming and classification of Peking University migmatite, lack of systematic observation and classification, different scholars have different views on the genetic mechanism of Peking University migmatite, and there are still disputes about the exact age of the formation of migmatite.Luotian and Yuexi dome migmatists were selected to carry out systematic work such as field observation, petrography, mineral chemistry, petrochemistry and zircon LA-ICP-MS U-Pb dating.It is found that the Beidabie migmatite is mainly divided into two types: laminated migmatite and expandable structure medium deep melt migmatite and eyeball and exfoliated high deep melt migmatite.Laminated light-colored bodies, light-colored veins and granites are significantly depleted of Fes _ 2O _ 3MgO _ 2O _ (mn) O _ (2) TiO _ 2 and CaO _ 2, and are slightly enriched in these elements by the highly deep-melt light-colored bodies.The main element compositions of the laminated light-colored bodies are granitic and diorite, while the light-colored veins are granitic.The main element composition of the whole rock is similar to that of gneiss and is granodiorite.The light-colored bodies and granites are characterized by enrichment of LREE and incompatible elements Ba-Th and U, while NB and Ti are significantly depleted.There are strong positive EU anomalies in laminated light-colored bodies, negative EU anomalies in granitic light-colored veins and granites, and no obvious EU anomalies in highly submersible light-colored bodies.All of these indicate that there is a genetic relationship between the laminated light-colored bodies and granitic light-colored veins, and they are crystallized in melts with different degrees of evolution.The results of zircon U-Pb isotopic dating show that the primary rocks of the Beidabie migmatite are mainly from Neoproterozoic rocks and a small amount of Paleoproterozoic crust.The migmatization lasted from 145 Ma to 124 Ma.The temperature and pressure conditions of the mixed rocks are 723-768 掳C and 3.7-5.2 kbar. which correspond to the middle and upper crustal environment.The genetic mechanism of migmatite is water-rich melting of felsic gneiss under water saturation.The reaction was: biotite quartz plagioclase water = amphibole plagioclase (residual) granitic melt.The genetic mechanism of a few mixed rocks is dehydration and melting of amphibole in hornblende gneiss.The reaction is: amphibole plagioclase quartz = clinopyroxene rich water melt.
【学位授予单位】：合肥工业大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：P588.36

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