【作者】 潘世语；

【导师】 迟效国；

【作者基本信息】 吉林大学 ， 地球化学， 2012， 硕士

【摘要】 内蒙古地区晚石炭世本巴图组火山岩出露较差，查明区内本巴图组火山岩的岩石组合、地球化学特征等，对研究区域晚古生代古亚洲洋的构造演化提供重要证据。论文选择苏尼特右旗地区晚石炭世本巴图组火山岩作为研究对象，主要解决其岩石组合特征、地球化学属性的问题，通过对火山岩的岩石学、地球化学数据的分析，解决本巴图组火山岩的岩浆源区特征及成因过程，阐明火山岩形成的大地构造背景。结合区域早-晚石炭世岩浆弧、贺根山蛇绿岩以及地层学等方面资料，探讨晚古生代古亚洲洋构造演化过程。1.本巴图组火山岩的时空分布规律本巴图组火山岩主要出露于内蒙古苏尼特右旗东北部地区，其岩性以玄武安山岩为主，含少量英安岩和安山岩。通过对本巴图组火山岩的安山岩样品（ZQ50-112）进行锆石U-Pb同位素LA-ICP-MS技术测年，测年结果显示为300.9±1.6Ma，形成于晚石炭世。2.苏尼特右旗地区本巴图组火山岩的岩石组合与地球化学特征本巴图组火山岩以玄武安山岩为主，含少量英安岩和安山岩，属于低钾拉斑玄武岩系列。火山岩的SiO₂含量介于46.58%⁶4.84%，去LOI之后，重新计算SiO₂含量变化于52.30%⁶8.35%之间，具有相对较高的Al₂O₃（12.44%¹5.09%）、TiO₂（0.61%².52%）和Na₂O（2.82%⁵.48%），且相对贫镁Mg#=16⁵3，贫钾（0.08%⁰.47%）的特征；随着SiO₂含量的不断增加主量元素FeOT、MgO、TiO₂、CaO、P₂O₅、Al₂O₃、K₂O含量明显呈降低的趋势；而Na₂O的含量则表现出增高的趋势。本巴图组火山岩中从玄武安山岩、安山岩到英安岩具有相似的稀土元素特征：稀土元素总量（∑REE=46.48μg/g²03.04μg/g），具有中等富集的特征，δEu值变化于0.68⁰.90。在稀土元素球迷粒陨石标准化配分曲线上，本巴图组火山岩呈平坦型，具有轻、重稀土元素分馏不明显及轻微到弱的铕异常的特征。在微量元素原始地幔（Primitive mantle）标准化蛛网图中，本巴图组火山岩普遍具有富集Th、U、La、Ce、Zr、Hf等不相容元素，相对亏损高场强元素Nb、Ta、Ti等的特征。在稀土元素球粒陨石标准化配分曲线和原始地幔标准化微量元素蛛网图上，本巴图组火山岩均显示了与圣基茨岛弧火山岩相似的特征。结合本巴图组火山岩的球粒陨石标准化配分曲线从玄武安山岩、安山岩到英安岩呈近平行上移的特征和Harker图解中主量元素随SiO2含量变化的特征，认为本巴图组火山岩的成分变化是同源岩浆分离结晶作用的结果。3.本巴图组火山岩的源区性质及其岩浆演化特征本巴图组火山岩中未发现有幔源包裹体的存在，其Mg#、Sc、Co、Ni值均小于地幔原始岩浆的成分范围，说明形成本巴图组火山岩的岩浆并非是幔源的原始岩浆。本巴图组火山岩的岩石学、地球化学特征，与俯冲带发育的正常的岛弧火山岩和埃达克岩的对比表明，本巴图组火山岩的初始岩浆起源于受俯冲洋壳脱水流体交代的上覆地幔楔的部分熔融；在岩浆演化的过程中，经历了以斜长石等矿物为主的分离结晶作用。4．本巴图组火山岩的构造背景苏尼特右旗本巴图组火山岩的岩石组合、岩石系列特征类似于岛弧火山岩。火山岩的稀土元素球粒陨石标准化配分曲线与圣基茨典型的岛弧火山岩相似；微量元素原始地幔标准化蛛网图显示了相对富集Th、U、La、Ce、Zr、Hf等不相容元素，相对亏损高场强元素Nb、Ta、Ti等的特征，亦与圣基茨岛弧火山岩相近。在玄武岩的La-Y-Nb构造环境判别图解中，火山岩样品均落入到岛弧火山岩区内。在La/Nb-La/Ti图解中，通过对典型的构造环境下产出的火山岩进行对比，显示了同圣基茨岛弧火山岩相似的成分特征，进一步说明本巴图组火山岩形成于岛弧环境。5．区域石炭纪-早二叠世主要岩浆活动区域内，早石炭世末-晚石炭世初在苏左旗-西乌旗一带发育活动大陆边缘岩浆弧；晚石炭世末-早二叠世初形成南侧本巴图组火山岩；早二叠世早期形成北侧贺根山蛇绿岩。根据上述区域石炭纪-早二叠世火山岩和花岗质侵入岩以及贺根山蛇绿岩的地球化学成因特征、时空演化规律，结合区域地层学分析，取得以下基本认识：（1）研究区本巴图组火山岩岩石组合为玄武安山岩、安山岩和少量英安岩，地球化学性质属于低钾拉斑玄武岩系列，形成于晚石炭世（300.9±1.6Ma）。（2）火山岩富集大离子亲石元素，相对亏损Nb、Ta、Ti等高场强元素，具有与圣基茨岛弧火山岩相似的地球化学性质。（3）提出古亚洲洋在石炭纪期间，沿佳蒙地块南缘存在向北俯冲作用，俯冲板片在晚石炭世发生后撤作用，形成石炭纪岩浆弧自北向南迁移，并伴随北部贺根山弧后蛇绿岩的形成，认为古亚洲洋最终沿西拉木伦河一线闭合，陆-陆碰撞时间可能为晚二叠世-早三叠世。更多还原

【Abstract】 IntrodcuctionThe Late Carboniferous volcanic rocks of Benbatu Formation are poorly outcroppedin Inner Mongolia. The research of rock associations and geochemical characteristics ofthe volcanic rocks will provide an important evidence for the tectonic evolution ofPaleo-Asian Ocean in Late Paleozoic.In this paper，the volcanic rocks which outcrop in Sonid Youqi area are the mainresearch object. Fist，the rock associations and geochemical characteristics of the volcanicrocks are solved. Then，by the detailed analysis of the petrologic and geochemical data ofthe volcanic rocks，the characteristics of magma source and genetic process are clearlyillustrated. And，the background of the volcanic rocks will be cleared. Finally，combinedwith the early-late Carboniferous magmatic arc，Heggenshan ophiolite and the stratigraphymaterial，the tectonic evolution of Paleo-Asian Ocean in Late Paleozoic will beelucidated.1． The spatial and temporal distribution of the volcanic rocks of BenbatuFormationThe volcanic rocks of Benbatu Formation mostly outcrop in the northeast part ofSonid Youqi，Inner Mongolia. The volcanic rocks are mainly composed of basalticandesite and a few dacite and andesite. The U-Pb isotopic LA-ICP-MS dating of theandesite（ZQ50-112）shows an age of300.9±1.6Ma. So，the volcanic rocks are formed inLate Carboniferous. 2． The rock associations and geochemical characteristics of the volcanic rocks ofBenbatu Formation in Sonid Youqi areaThe volcanic rocks are mainly composed of basaltic andesite and a few dacite andandesite which belong to low-K tholeiite series.The contents of SiO₂of the volcanic rocks are between52.30%⁶8.35%（Remove theLOI）. They arc characterized by relatively high content of Al₂O₃（12.44%¹5.09%）、TiO2（0.61%².52%）and Na₂O（2.82%⁵.48%），but relatively low content of MgO（Mg#=16⁵3） and K₂O（0.08%⁰.47%）. With the increase of the content of SiO₂，the content ofFeOT、MgO、TiO2、CaO、P2O5、Al₂O₃、K₂O presents a trend of decrease，but the contentof Na₂O shows a trend of increase.The rare earth elements of Benbatu volcanic rocks have similar characteristics，whichare the similar total rare earth element contents （∑REE=46.48μg/g²03.04μg/g） and thesimilar anomalies of Eu （δEu=0.68⁰.90）. TiO₂the chondrite-normalized REE diagram，therocks are characterized by LREE and HREE without significant fractionation and slight orweak depletion of Eu. TiO₂the Primitive mantle-normalized spider diagram，the rocks areenriched in incompatible elements （Such as Th、U、La、Ce、Zr、Hf），but depleted ofhigh filed strength elements（HFSE，such as Nb、Ta、Ti）. The characteristics both on thechondrite-normalized REE diagram and on the Primitive mantle-normalized spiderdiagram are similar to the St.Kitts island arc volcanic rocks. Combined with thecharacteristics of REE and the major elements，the contents change of Benbatu volcanicrocks is considered to be the result of the comagmatic fractional crystallization.3. Nature of magmatic source and Magmatic evolution of Benbatu volcanic rocksCompared with the geochemical characteristics of the mantle-derived primarymagma，the magma of the volcanic rocks of Benbatu Formation are not primary magma，which is not only because the rocks are lack of the mantle-derived inclusions，but alsohave the low Mg#values and low contents of Sc，Co，and Ni.Compared with the petrologic and geochemical characteristics of the ordinaryvolcanic rocks and adakite which both develop in subduction zones，the volcanic rocks are considered to be originated from partial melting of the overlying mantle wedge which ismetasomatized by the fluid which is dehydrated from subducted oceanic crust. And，thefractional crystallization mainly relies on plagioclase in the process of magma evolution.4.The tectonic setting of the volcanic rocksThe association of basaltic andesite，andesite and dacite of the Late Carboniferousvolcanic rocks of Benbatu formation in Sonid Youqi is similar to the island arc volcanicrocks. They have the similar geochemical characteristics with the St. Kitts island arcvolcanic rocks both on the chondrite-normalized REE diagram and on the Primitivemantle-normalized spider diagram. TiO₂the La-Y-Nb diagram，all of the samples fall intothe island arc volcanic rocks area. TiO₂the La/Nb-La/Ti diagram，the rocks have a similarcomposition of the St. Kitts island arc volcanic rocks，which suggest the rocks wereformed in an island-arc environment.5. Regional main magmatic activity from Carboniferous to Early PermianRegionally，An active continental margin magmatic arc which was formed at the lateEarly Carboniferous-the early Late Carboniferous lies from Sonid Zuoqi to Xiwuqi. In thesouth of the magmatic arc，there is the existence of the volcanic rocks of Benbatuformation which were formed at the late Late Carboniferous-early Early Permian，and inthe north of the magmatic arc，there is the existence of the Hegenshan ophiolite which wasformed at early Early Permian.Finally，combined with the regional stratigraphic data，a few basic understandings areobtained：（1）The volcanic rocks of Benbatu formation in Sonid Youqi area，Inner Mongolia，mainly consist of basaltic-andesite， andesite and a little dacite. The geochemicalcharacteristics of volcanic rocks generally belong to the tholeiitic series. Zircon U-Pbisotopic dating revealed that the andesite was formed at300.9±1.6Ma.（2） They are characterized by enrichment in LILE but depletion in HFSE（Nb、Ta、Ti），which is similar to the St. Kitts island arc volcanic rocks.（3）The Paleo-Asian Ocean has subducted northward along the south edge of the Jiamushi-Mongolia block since Early Carboniferous. In Late Carboniferous， thesubduction slab became drop back，which made the Carboniferous magmatic arc movefrom north to south and the formation of Hegenshan ophiolite. So，the Paleo-Asian Oceanis considered to close along the Xar Moron River，and the continental collision couldhappen at Late Permian-Early Triassic.更多还原