【作者】 胡波；

【导师】 裴先治；

【作者基本信息】 长安大学 ， 构造地质学， 2005， 硕士

【摘要】 本文以地球化学研究为主,结合区域地质学、构造地质学、岩石大地构造学、同位素年代学及岩相学等方面的分析对甘肃天水地区祁连—西秦岭造山带接合部位清水—张家川早古生代变质基性火山岩和中酸性火山岩进行系统研究,确定其形成时代,识别其成因和大地构造环境,探讨火山岩在区域上的归属,进而阐明研究区的古板块构造演化历史,解决前人研究尚存争论的问题,提出新的看法,同时为探讨祁连—西秦岭构造交接部位的交接关系及形成与演化乃至中国大陆造山带的基本特征及大陆动力学探索提供重要基础科学资料。 通过对清水—张家川早古生代变质火山岩的研究,取得以下主要新进展和初步认识: (1) 清水—张家川早古生代变质火山岩主要由南部的早奥陶世红土堡变质基性火山岩和北部的奥陶纪陈家河群变质中酸性火山岩组成,二者为构造接触关系。 (2) 红土堡变质基性火山岩以灰绿色变质玄武岩为主夹少量灰白色—浅灰色硅质岩,有少量同时代同成分的辉绿岩墙顺层侵入其中。变基性火山岩属于低钾拉斑玄武岩系列;稀土元素分布形式呈LREE弱富集型,具较高的∑REE,无明显Eu异常或弱的正异常,有明显Ce负异常;弱富集大离子亲石元素(K、Rb、Sr、Ba),富集Th,贫Cr、Ni,具Nb、Ta和zr负异常,La/Nb比值大于1,无明显Ti负异常;微量元素和Sr——Nd—Pb同位素特征表明基性岩浆来源于受俯冲流体交代的富集地幔楔,并遭到陆壳物质的混染。地质和地球化学特征及其构造环境判别显示基性火山岩可能形成于汇聚板块边缘的弧后板内裂陷—小洋盆构造环境。 (3) 陈家河群变质中酸性火山岩以变质流纹岩和英安岩为主。变中酸性火山岩属于低钾—钾质钙碱性系列;稀土元素分布型式呈LREE富集型,具较高的∑REE,HREE较平坦,有明显Eu负异常;富集大离子亲石元素(K、Rb、Ba),强烈富集Th,亏损P、Ti、Cr等高场强元素,具Nb、Ta负异常;Sr—Nd—Pb同位素特征表明岩浆来源于受俯冲流体交代的富集地幔和地壳物质部分熔融的混合源区。地质和地球化学特征及其构造环境判别显示中酸性火山岩可能形成于陆缘火山弧构造环境。 (4) 结合区域构造演化,祁连造山带东端地区在早古生代板块构造演化阶段于奥陶纪早期伸展拉张,形成具有弧后裂陷—小洋盆性质的红土堡基性火山岩带,奥陶纪中晚期该洋盆向北俯冲形成以陈家河群为代表的陆缘弧型中酸性火山岩。地质和地球化学特征的分析基本探明了清水—张家川变质基性和中酸性火山岩的大地构造背景及其所在的杨家寺—红土堡—凤阁岭变质基性火山岩和胡毛—新城变质中酸性火山—沉积岩带在早古生代的构造演化过程。 (5) 通过与秦岭造山带早古生代草滩沟群变质火山岩和鹦鸽嘴蛇绿混杂岩及北祁连造山带奥陶纪白银厂矿田变质火山岩的对比研究表明,清水—张家川变质火山岩系在北秦岭未出现延伸,不属于二郎坪变质火山岩带的一部分,不是秦岭商丹带的西延;也不属于北祁连加里东构造带南带变质火山岩系的一部分。这些对比结果从火山岩研究角度为探讨祁连—西秦岭造山带构造结合部位在早古生代的交接构造关系提供了必要的证据。 (6) 研究区所在的祁连构造带与北秦岭构造带在早古生代时的接合关系是北秦岭向西延伸变化为由微地块分割的多个有限洋盆和小洋盆系统,在早古生代从洋盆打开并发育有不同类型蛇绿岩或裂谷火山岩等到发生沿多个俯冲带向北的俯冲,形成多个火山岩浆岛弧构造带直至志留纪俯冲碰撞造山。由红土堡基性火山岩系和陈家河群火山—沉积岩系等组成的早古生代杨家寺—红土堡—凤阁岭弧后裂陷小洋盆和胡毛—新城中酸性火山岛弧带是其中之一。这些初步认识为进一步探讨祁连—西秦岭造山带构造交接部位的形成与演化乃至中国大陆造山带的基本特征及大陆动力学探索提供了重要的基础科学资料。更多还原

【Abstract】 This dissertation mainly studies Qingshui-Zhangjiachuan early Palaeozoic metabasic and meta-intermediate-acidic volcanic rocks in Tianshui, Gansu in geochemistry combining with regional geology, structure, tectonics, isotopic geochronology and petrography etc., to discover their ages, cause of formation and tectonic settings, and to discuss their regional adscription. Further it illustrates the evolving history of paleo-plate in the study area, solves the questions which remain in argument in former studies, and puts forward new ideas. Further to this, it also provides important basic scientific data for discussing the joining relationship, forming and evolvement of the tectonic joint of Qilian and Western Qinling orogenic belts and the basic characteristics of continental orogenic belts, and exploring the continental dynamics in China.The following new progresses and knowledge have been procured by the studies of Qingshui-Zhangjiachuan early Palaeozoic meta-volcanic rocks:(1) Qingshui-Zhangjiachuan meta-volcanic rocks are mainly composed of south early Ordovician Epoch Hongtubao metabasic volcanic rocks and north Ordovician Period Chenjiahe Group meta-intermediate-acidic volcanic rocks. They are structural contact relationship.(2) The Hongtubao metabasic volcanic rocks are mainly composed of grayish green metabasalt, intercalated by a few offwhite-light-grey sillcalite, and intruded along the bedding by some contemporaneous diabase dikes of the same composition. The metabasic volcanic rocks are of tholeiitic series. Their chondrite-normalized REE patterns are of weakly LREE-enriched type, with relatively high ∑REE. There is no pronounced Eu anomaly or there are only weak positive anomalies; there is a pronounced negative Ce anomaly. Their MORB-normalized trace element spidergram shows that the rocks are slightly enriched in large-ion lithophile elements (K, Rb, Sr, Ba), enriched in Th and depleted in Cr and Ni. Nb and Ta show negative anomaly, La/Nb>1, and Ti anomaly is not pronounced. Trace element and Sr-Nd-Pb isotope characteristics show that the basic magma roots in enriched mantle wedge metasomatized by fluid in subduction belt, hybridized by continental crust material. The geological and geochemical characteristics show that the metabasic rocks formed in a back-arc rift-small ocean basin tectonic environment at convergent plate margins.(3) The Chenjiahe Group meta-intermediate-acidic volcanic rocks are mainly composed of metarhyolite and metadacite. The meta-intermediate-acidic volcanic rocks are of low-K — K calcalkali series. Their chondrite-normalized REE pattern are of LREE-enriched type, with relatively high ∑ REE, and the HREE pattern are relatively flat. There is a pronounced negative Eu anomaly. Their MORB-normalized trace element spidergram shows that the rocks are enriched in large-ion lithophile elements (K, Rb, Ba), observably enriched in Th and depleted in high field strength elements (P, Ti, Cr). Nb and Ta show negative anomaly. Sr-Nd-Pb isotope characteristics show that the intermediate-acidic magma roots in an intermixed source of enriched mantle metasomatized by fluid in subduction belt and continental crust. The geological and geochemical characteristics show that the meta-intermediate-acidic rocks formed in a volcanic arc tectonic environment at continental margin.(4) Consulting information of regional structure, the eastern end of Qilian orogenic belts extended in early Ordovician Period and formed Hongtubao basic volcanic rocks belt of back-arc rift-small ocean basin tectonic environment at early Palaeozoic plate tectonics evolvement stage. This ocean crust subducted northward at middle-late Ordovician Period and formed island arc type intermediate-acidic volcanic rocks represented by Chenjiahe Group at continental margin. The analyses of geological and geochemical characteristics prove basically the tectonic settings of Qingshui-Zhangjiachuan metabasic and meta-intermediate-acidic volcanic rocks and the tectonic evolving process of Yangjiasi-Hongtubao-Fenggeling metabasic rocks belt and Humao-Xinchen meta-intermediate-acidic volcanic-sedimentary belt in early Palaeozoic.(5) The comparative studies between Qingshui-Zhangjiachuan meta-volcanic rocks and Eastern Qinling early Palaeozoic Caotangou Group meta-volcanic rocks and Yinggezui Ophiolitic Melange,and Ordovician Baiyinchang orefield meta-volcanic rocks in Northern Qilian orogenic belts show that Qingshui-Zhangjiachuan meta-volcanic rock series do not extend to Northern Qinling, do not belong to Erlangping meta-volcanic rocks belt, and are not the west extension of Shangdan tectonic belt. They also do not belong to the south meta-volcanic rock series of Northern Qilian Caledonides tectonic belts. These comparative results provide necessary proof for discussing the joining relationship of the tectonic joint of Qilian and Western Qinling orogenic belts at early Palaeozoic.(6) The joining relation of Qilian and Northern Qinling tectonic belts in early Palaeozoic is that Northern Qinling extended westward to become many limited oceanic basin and small oceanic basin systems divided by microlandmasses. These oceanic basin systems went through opening and developing various ophiolite or rift volcanic rocks to subducting northward along many subduction zones, which led to many volcanic magma island arc tectonic belts formation, untill subduction, collision and orogenesis in early Palaeozoic. Early Palaeozoic Yangjiasi-Hongtubao-Fenggeling back-arc rift-small ocean basin and Humao-Xincheng intermediate-acidic volcanic island arc belts composed of Hongtubao basic volcanic rock series and Chenjiahe Group volcanic-sedimentary rock series are one of the volcanic magma island arc tectonic belts. These elementary knowledge provide important basic scientific data for discussing the joining relationship, formation and evolvement of the tectonic joint of Qilian and Western Qinling orogenic belts and the basic characteristics of continental orogenic belts, and exploring the continental dynamics in China.更多还原