华北克拉通东南缘中生代岩浆作用：对克拉通破坏启动机制和岩浆过程中镁同位素行为的制约

发布时间：2018-03-08 22:28

本文选题：岩浆作用　切入点：郯庐断裂带　出处：《中国科学技术大学》2016年博士论文　论文类型：学位论文

【摘要】：岩浆作用是地球内部各圈层之间能量及物质交换的重要途径和媒介,在陆壳形成、生长和演化中具有重要作用。同时,作为来自深部地球的样品,岩浆岩可以作为探视地球深部结构、组成以及了解深部过程的天然探针。岩浆岩成分、时代分布、空间展布特征及其演化可以为大地构造变迁、深部岩浆房形成和演化等提供重要的信息。此外,岩浆过程中的同位素(分馏)行为是稳定同位素地球化学的基础问题,这关系到稳定同位素作为地质过程的示踪剂等基础应用。镁同位素是新近发展起来的新型同位素地质示踪剂,已经在示踪沉积物循环以及碳循环上得到了一定的应用。但是,目前对于高硅岩浆体系(如花岗岩)的镁同位素体系了解很有限,对花岗质岩浆过程中的镁同位素行为也尚不清楚。基于上述的认识,本博士论文工作主要包括两个方面的内容：1)利用中酸性岩浆岩这一类“深部探针”来窥探华北克拉通破坏的深部过程及启动机制；2)对酸性岩浆(主要是花岗岩)开展镁同位素地球化学研究,尝试理解镁同位素在地壳岩浆活动中的同位素行为。本博士论文第一个研究内容是郯庐断裂带两侧分布的中生代岩浆岩进行了元素地球化学、地质年代学和氧同位素的分析调查,来限定和理解克拉通岩石圈减薄、郯庐断裂带的大规模走滑活动和大量的岩浆作用间的联系和深部过程。针对该区域内的中生代岩浆岩进行了分析结果显示沿郯庐断裂走向分布的高镁埃达克岩的侵入年龄都集中在在130-134Ma之问,这种时间和空间分布上的特征表明这些高镁埃达克质岩的形成和郯庐断裂在中生代的大规模走滑(156-162Ma)活动具有明显的成因上的联系。此外,苏鲁造山带内的低镁埃达克质岩(120-127Ma)整体上形成于稍晚于高镁埃达克质岩形成的时代,这一结果同样稍晚于大别造山带低镁埃达克质岩的形成时代(129-143Ma)。综合上述结果,显示郯庐断裂带中生代的切穿岩石圈的走滑活动可能是华北克拉通东南缘的岩石圈减薄的启动机制：断裂带大规模的走滑可能引发了断裂附近加厚下地壳局部破碎而形成一些比重比较大的下地壳碎片,这些碎片沉入较轻的软流圈和橄榄岩反应而显著提高熔体的Mg#而形成年龄稍老的高镁埃达克质岩；另外,断裂的活动也同时使苏鲁造山带下方的下地壳的重力不稳定的程度增加,进而引发苏鲁造山带的大规模的去山根过程,形成低镁埃达克质岩和一些普通花岗岩。此外,对此地区中生代岩浆岩的镁同位素的初步研究表明镁同位素可能不是示踪“拆离的下地壳物质与地幔相互作用进而形成高镁达克质岩岩”这一地质过程的有效手段,从而无法通过镁同位素有效区分高、低镁埃达克质岩。而低镁埃达克质岩均一的镁同位素组成也折射本地区下地壳的镁同位素组成相对均一,并接近地幔值。极少数普通花岗岩的镁同位素偏重,表明极少部分地区存在小规模的沉积物循环加入。本博士论文另一个研究内容是系统分析蚌埠隆起区晚侏罗纪岩浆作用的代表岩体—荆山花岗岩(其岩浆物质来源为深俯冲过程中楔入华北东南缘的华南片麻岩的部分熔融)的各类主要岩性的岩石样品(主体花岗岩、细晶岩及残留体)以及单矿物(石榴子石和黑云母)的主-微量元素化学及镁同位素组成,解析了此岩体形成和演化过程可能发生的镁同位素分馏,进而对陆壳深俯冲过程中的镁同位素循环及陆壳部分熔融过程中的镁同位素分馏提供了启示和限定。荆山花岗岩代表性的淡色花岗岩样品和相关的残留体的全岩镁同位素数据表明,荆山花岗岩具有非常特殊的轻镁同位素特征。综合该岩体的氧同位素特征,其源区应具有典型低氧-高镁的同位素特征。结合周边各主要岩石类型的氧-镁同位素特征及二元混合模拟计算,只有在其岩浆源区加入一定比例的轻镁-重氧的碳酸岩的混熔才有可能形成具有该岩体的低氧高镁的同位素特征。这一结果揭示了俯冲带及周边地区碳酸岩可能进入深部地球岩浆循环,对了解地球深部碳循环过程具有重要的意义。对单矿物的调查结果表明,荆山花岗岩中黑云母与石榴子石间普遍存在大于1‰的镁同位素分馏。这一分馏对于理解部分熔融过程中可能发生的同位素分馏具有重要的启示作用。转熔石榴子石是壳内部分熔融的重要富镁矿物,对其的镁同位素组成及共生富镁矿物的调查可以直接对壳内部分熔融过程中的镁同位素分馏进行直接限定。我们的结果表明,转熔石榴子石和黑云母间存在着显著的分馏,随着石榴子石对镁体系的控制能力的提高(如更多的黑云母分解转换为石榴子石或者石榴子石中镁含量的提高),部分熔融反应过程中产生的熔体的镁同位素会显著的向重镁同位素方向发生分馏。这一推论表明在地壳部分熔融过程(特别是普遍具有转熔石榴子石参与部分熔融的S型花岗岩的形成)很可能会发生显著的同位素分馏,对解释大陆地壳的镁同位素特征和演化具有重要的意义。
[Abstract]:Magmatic activity is between the various spheres of the earth's internal energy and material exchange in an important way and the media, formed in the continental crust, which plays an important role in the growth and evolution. At the same time, as the sample from the deep earth, magmatite can visit as deep earth structure, natural probe as well as the understanding of the deep process of magmatite. Composition, age distribution, distribution characteristics and evolution of space for tectonic evolution, deep magma chamber formation and evolution provide important information. In addition, in the process of magma (isotope fractionation) behavior is a basic problem of stable isotope geochemistry, which relates to the stable isotope tracers as geological processes and other basic applications. Is a new type of isotopic tracer magnesium isotope newly developed, has been in circulation as well as the carbon cycle tracing of sediments has been used. However, in order for high silicon The magmatic system (such as granite) magnesium isotopic system understanding is limited, the magnesium isotope behavior of granitic magma in the process is not clear. Based on the above understanding, this dissertation mainly includes two aspects: 1) deep process and mechanism to start to see the destruction of the North China Craton using acidic magma this kind of "deep rock probe"; 2) of acidic magma (mainly granite) to carry out research on magnesium isotope geochemistry, try to understand the isotope behavior in the earth's crust in magmatic activity. The magnesium isotope doctoral thesis first research content is the Mesozoic magmatite of Tanlu fault zone on both sides of the distribution of elements in the earth chemical analysis, investigation of geochronology and isotope, to define and understand the cratonic lithosphere, the Tanlu fault zone of large-scale strike slip movement and a large number of magmatism between contact and deep Part of the process are analyzed. Aiming at the Mesozoic magmatite region shows the intrusive age along Tanlu fault strike high Mg adakitic rocks distribution are concentrated in the 130-134Ma in question, characteristics of the time and space distribution showed that these high Mg adakitic rocks and the formation of the Tanlu fault in Mesozoic large-scale strike slip (156-162Ma) activity has obvious genetic relationship. In addition, the low Mg adakitic rocks in the Su Lu Orogen (120-127Ma) on the whole formed in the later formation of high Mg adakitic rocks of the era, the results are also slightly later than the low Mg adakitic rocks in the Dabie orogenic belt forming era (129-143Ma). The above results, show the strike slip movement of the Mesozoic lithosphere in cutting of the Tanlu fault zone may be the southeast margin of the North China Craton lithosphere of the starting mechanism of thin: large-scale strike slip fault zone may lead The fault in the vicinity of thickened lower crust fracture formed some relatively large proportion of the lower crustal fragments, these fragments into the asthenosphere and peridotite reaction lighter and significantly improve the melt Mg# and the formation of the old age a little high Mg adakitic rock; in addition, the activity of the fault at the same time the Su Lu orogenic belt below the lower crust of gravitational instability degree increases, causing the Su Lu orogenic belt scale to root process, the formation of low Mg adakitic rocks and some common granite. In addition, this area of the Mesozoic magmatite magnesium isotope preliminary research shows that magnesium isotopic tracer may not be removed from the "crust and mantle interaction the role of effective means and the formation of the geological process of high rock rock" Mei Da Ke, which can not effectively distinguish by magnesium isotopes, low Mg adakitic rocks. While the low Mg adakitic rocks The region also reflected magnesium isotopic composition of lower crust is relatively uniform composition of magnesium isotope uniform, and close to the mantle. A handful of magnesium isotope value of common granite emphasis, indicate the existence of small scale sediment circulation with very few parts of this dissertation. Another research content is the system analysis of Bengbu uplift in late Jurassic magmatism on behalf of the rock - Jingshan granite (the magma origin is wedged into the deep subduction process of the southeastern margin of North Southern China gneiss partial melting) all kinds of main lithology rock samples (the main granite aplite and residue) and minerals (garnet and biotite) of the main trace elements and magnesium isotopic composition, isotopic fractionation analysis of the magnesium rock formation and evolution may occur, and then melting of continental subduction in the process of magnesium isotope cycle and continental Department Provide inspiration and limit the magnesium isotopic fractionation process. Jingshan granite representative granite samples and related residues of the whole rock mg isotopic data suggest that Jingshan granite has light magnesium isotopic characteristics of very special. The comprehensive oxygen isotopic characteristics of the rock mass and isotopic characteristics of typical hypoxia - high magnesium should be the area surrounding the source. Combined with the main rock types of the oxygen isotope characteristics of magnesium and two yuan hybrid simulation, only the light magnesium carbonate rock - added at the magma source region of a certain proportion of the heavy oxygen mixing can be formed with the isotopic characteristics of the rock mass of high magnesium. The hypoxia results revealed carbonate rocks in subduction zones and the surrounding areas may enter the earth deep magmatic cycle, has important significance for understanding the earth's deep carbon cycle process. According to survey results of single minerals, Jingshan biotite granite Common magnesium isotopic fractionation is greater than 1% of the existing parent and garnet. This fractionation has important implications for understanding possible isotopic fractionation during partial melting. The peritectic garnet is an important part of magnesium rich mineral melting within the crust, the magnesium isotopic composition and magnesium rich mineral intergrowth the investigation can be directly on the magnesium isotopic fractionation during partial melting of the crust are directly limited. Our results show that peritectic garnet and biotite have significant fractionation, with the ability to control the garnet system of magnesium increased (such as more biotite garnet or decomposed into the increase of the content of magnesium garnet), magnesium isotopic melt generated partial melting during the reaction will be to the direction of significant heavy magnesium isotopic fractionation occurs. This shows that in the process of partial melting of the crust. (special It is possible that significant isotopic fractionation is likely to take place in the formation of S granites, which is generally involved in the melting of garnet and partial melting. It is of great significance to explain the characteristics and evolution of magnesium isotopes in the continental crust.

【学位授予单位】：中国科学技术大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：P588.11;P597

【参考文献】