造山带地壳的深熔作用：北大别混合岩地球化学研究

发布时间：2017-12-28 03:32

本文关键词：造山带地壳的深熔作用：北大别混合岩地球化学研究　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】：造山带的演化和部分熔融关系密切。大别-苏鲁造山带是典型的大陆碰撞型造山带,在其演化的不同阶段发生了深熔作用,是研究造山带地壳深熔作用理想地区。本学位论文对作为深熔作用产物的北大别混合岩进行了系统的岩石学和地球化学研究,对混合岩的形成机制、部分熔融性质、深熔作用时限、熔体演化、矿物响应等进行了详细讨论,为造山带地壳在折返和碰撞后阶段的深熔作用提供了有力证据,丰富了造山带化学地球动力学。对北大别混合岩进行了岩石学、矿物学和地球化学研究,结果表明混合岩是由水致部分熔融形成。北大别混合岩以发育大颗粒嵌晶状角闪石为特征,角闪石自形发育,且颗粒边界晶面发育,指示其为转熔矿物。角闪石中含有黑云母、石英和斜长石包裹体,角闪石-斜长石温压计结果说明混合岩形成的温压条件为：700-751℃和3.5-5.2kbar,该温压条件显著低于黑云母脱水熔融的固相线,表明发生部分熔融的反应为：黑云母+石英+斜长石+流体=转熔角闪石+熔体。混合岩中淡色体含量显著高于基底片麻岩中含水矿物分解所能产生的熔体量,也指示了北大别混合岩是造山带地壳水致部分熔融形成。含水的转熔矿物(角闪石)、低于含水矿物固相线的温压条件和高的熔体含量是区分水致部分熔融和脱水部分熔融的标志。混合岩中淡色体具有明显不同的地球化学和矿物学组成,指示熔体形成后经历了复杂的演化过程。淡色体具有英云闪长质到花岗质的组成,相较于中间体,具有高的Si02,Na2O,Sr,和Ba,低的CaO,FeO,MgO,Ti02和MnO2.部分淡色体表现为高La/YbN比值,低的HREE含量,无或弱Eu异常,是北大别花岗质片麻岩部分熔融后,转熔角闪石残留源区形成的熔体：大多数部分熔融的初始熔体经历了岩浆演化过程,形成了具有高HREE和低La/YbN比值的淡色体,是这是富含HREE的转熔矿物角闪石夹带进入熔体的结果,部分淡色体具有显著Eu正异常和低REE含量的特征,表明在结晶分异过程中,发生了长石的堆晶。对北大别混合岩的年代学和同位素地球化学进行了研究,混合岩具有显著不同的氧同位素组成和原岩年龄,既有新元古代的遭受过高温热液蚀变的低δ18O值的岩石,也有来自太古代晚期到古元古代早期的高δ18O值古老地壳经历了碰撞后的混合岩化作用。新元古代年龄和低的锆石δ180值是华南板块的独有特征,表明北大别混合岩原岩来自于华南板块。进入片麻岩中引起水致部分熔融的流体可能来源于和具有不同的氧同位素组成的原岩具有相似组成的岩石在深部的脱水,因此流体的氧同位素组成继承自源区的不均一性。北大别混合岩的Sr-Nd同位素总体和基底花岗质正片麻岩类似,表明混合岩的形成机制为北大别片麻岩的原地部分熔融。混合岩化的年代为早白垩世,和早期埃达克岩的年龄一致,是造山带地壳在不同层位深熔作用的产物。部分熔融所需热源来自于构造体制由挤压到伸展的转换时升高的热流值,说明大别造山带开始伸展的时间不晚于150Ma。对北大别混合岩中多相生长的锆石进行了原位的锆石U-Pb定年和微量元素分析,识别出原岩锆石,变质锆石和两阶段的深熔锆石,揭示了混合岩的复杂演化历程。原岩锆石具有高的Th/U比值(0.1)和中等的U含量,陡峭的稀土配分型式,中等到强烈的Eu负异常和显著的Ce正异常,和典型的岩浆锆石特征致,锆石U-Pb定年给出新元古代的年龄,代表了混合岩原岩为来自新元古代的岩浆岩。变质锆石具有平坦的重稀土配分型式(Yb/Gd=1.7～7.2),低的HREE、Na+Ta、Y、Th、U含量,低的Th/U(0.1)比值,轻微的Eu负异常到正异常(0.85～1.44),指示变质锆石为榴辉岩相变质锆石,混合岩经历了三叠纪(211～230Ma)的超高压榴辉岩相变质。深熔锆石的研究表明存在两期的部分熔融,第一期深熔发生在晚三叠-早侏罗纪(188～206Ma),锆石具有高的HREE、Na+Ta、Y、U含量,低的Th/U比值(0.01～0.12),极其陡峭的MREE-HREE配分型式(Yb/Gd=79～738)和显著的Eu负异常,这些微量元素特征说明锆石结晶于深熔熔体。锆石Ti温度为698～868℃,达到了实验得到的多硅白云母脱水熔融的温度,表明超高压含水矿物多硅白云母在热折返阶段发生脱水部分熔融；第二期深熔发生在碰撞后早白垩纪(116～141Ma),锆石具有中等的U含量,变化的Th/U比值(0.03～1.04),陡峭的稀土配分型式(Yb/Gd=14～80)和显著的Eu负异常(0.14～0.78),锆石Ti含量温度计计算给出的温度为699～821℃,高于含水条件下花岗岩固相线和黑云母脱水熔融的固相线,表明造山带地壳的经历了水致部分熔融和脱水部分熔融。早期深熔作用和深俯冲陆壳从地幔深度的热折返阶段相关,晚期深熔和碰撞后造山带从挤压到伸展构造转换相关。
[Abstract]:The evolution of the orogenic belt is closely related to the partial melting of the orogenic belt. The Dabie Su Lu orogenic belt is a typical continental collision orogenic belt. Deep melting has occurred in different stages of its evolution, and it is an ideal area for studying the crustal deep penetration in orogenic belt. In this thesis as the anatexis product in North Dabie mixed rock of petrological and chemical research on the earth system, the mixed rock formation mechanism, partial melting properties, anatexis, time evolution of mineral melt response are discussed in detail, anatexis of the orogenic belt crust in the exhumation and after collision the stage provides strong evidence, enrich the geochemical dynamics of orogenic belt. The petrology, mineralogy and geochemistry of the North Dabie mixed rocks have been studied. The results show that the mixed rocks are formed by partial melting of water. North Dabie mixed rock by the development of large particles of poikilitic amphibole is characterized, amphibole euhedral growth, and grain boundary plane development, indicating its turn molten mineral. Contain biotite, quartz and plagioclase inclusions of hornblende, plagioclase hornblende thermobarometer showed mixed rock formation temperature and pressure conditions were: 700-751 C and 3.5-5.2kbar, the solidus temperature and pressure conditions was significantly lower than that of biotite dehydration melting, partial melting reactions of biotite + plagioclase + quartz + = peritectic fluid amphibole + melt. The content of the pale body in the mixed rocks is significantly higher than that in the gneiss, and also indicates that the Dabie mixed rocks are formed by partial melting of the crust in the orogenic belt. The water bearing melt minerals (hornblende), temperature and pressure conditions below the solidus of water bearing minerals and high melt content are the marks to distinguish partial melting of water from partial melting and dehydration. The light color bodies in the mixed rocks have distinct geochemical and mineralogical composition, indicating a complex evolution process after the formation of the melts. Light body with tonalitic to form granitic, compared to the intermediate, with high Si02, Na2O, Sr, and Ba, low CaO, FeO, MgO, Ti02 and MnO2. part of Leucosome showed high La/YbN ratio and low HREE content, no or weak Eu anomaly, is North Dabie granite quality of gneiss after partial melting, melting to hornblende residual melt source region is formed: most part of initial melting through the process of magmatic evolution, forming a pale body has high HREE and low La/YbN ratio, this is HREE rich peritectic amphibole mineral entrained into the melt, part of body with light a significant feature of positive Eu anomalies and low REE content, showed that the crystallization differentiation process, the feldspar cumulates. On the North Dabie mixed rock geochronology and isotope geochemistry of mixed rocks have different oxygen isotopic composition and protolith age, both Neoproterozoic high temperature hydrothermal alteration by low 18O values of rock, but also from the late Archean to Paleoproterozoic high Delta 18O early generation the value of migmatization of ancient crust experienced after the collision. The Neoproterozoic zircon age and low delta 180 value is the unique characteristics of the Southern China plate, show that the North Dabie protolith mixed from Southern China plate. The fluid causing partial melting of gneiss may originate from deep dehydration of rocks with similar composition of oxygen isotopic composition of Yuan Yan, so the oxygen isotopic composition of fluids inherits from the heterogeneity of source area. North Dabie mixed rock Sr-Nd isotopic overall and basement granitic orthogneiss indicate similar formation mechanism of mixed rock for the North Dabie gneiss in partial melting. In the early Cretaceous migmatization, and early adakites in the same age, is a product of crustal anatexis in different layers. The heat source needed for partial melting comes from the increase of heat flow value when the tectonic system is transformed from extrusion to extension, indicating that the Dabie orogenic belt began to extend at a later time than 150Ma. Zircon U-Pb dating and trace element analysis of zircons in multiphase growth in the North Dabie mixed rock indicate that zircons, metamorphic zircons and two stages of deep melting zircon are identified, revealing the complex evolution of the mixed rocks. The protolith zircon has high Th/U ratios (0.1) and medium U content of REE in steep sub type, moderate to strong negative Eu anomalies and positive anomaly in Ce, and the typical characteristics of magmatic zircon, zircon U-Pb dating are Neoproterozoic age, on behalf of the protolith is from magma mixing Neoproterozoic rocks. Metamorphic zircon has flat REE pattern (Yb/Gd=1.7 ~ 7.2), HREE, Na+Ta, Y, Th, low U content, low Th/U ratio (0.1), slight negative Eu anomaly to positive anomalies (0.85 ~ 1.44), indicating the metamorphic zircon as eclogite facies metamorphic zircon, mixed rock experience the Triassic period (211 ~ 230Ma) of the ultrahigh pressure eclogite facies metamorphism. Study on deep penetration of zircon indicate that there are two stages of partial melting, the first phase of deep penetration occurred in Late Triassic early Jurassic three (188 ~ 206Ma), HREE, Na+Ta, Y with zircon, high U content, low Th/U ratio (0.01 ~ 0.12), extremely steep MREE-HREE pattern (Yb/Gd=79 ~ 738) and significant negative Eu anomaly, the characteristics of these trace elements in zircon crystallization of anatectic melts. Zircon Ti temperature is 698 to 868 DEG C, reached the phengite dehydration melting experiments on the temperature, the Biao Mingchao high hydrous minerals phengite occurred dehydration partial melting in the heat phase second reentry stage; deep penetration in the impact after the early Cretaceous (116 ~ 141Ma), zircon has moderate U contents, Th/U the change of the ratio of (0.03 ~ 1.04), the steep REE types (Yb/Gd=14 ~ 80) and significant negative Eu anomalies (0.14 ~ 0.78), the content of Ti presented the zircon thermometer temperature is 699 ~ 821 degrees higher than the solidus solidus, granite and biotite dehydration melting of water under the condition that the crust orogenic belt has experienced partial melting and water
【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】：P584;P588.36

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