【作者】 骆必继；

【导师】 张宏飞；

【作者基本信息】 中国地质大学 ， 地球化学， 2013， 博士

【摘要】 西秦岭造山带是秦岭-大别-苏鲁造山带的西延部分,西与昆仑和祁连造山带相接,南与松潘-甘孜造山带相邻一起构成中国大陆最大的构造结,是中国大陆构造东西转换和南北主体拼合的重要衔接部位。印支期是中国大陆主体拼合的主要完成时期,勉略缝合带和阿尼玛卿缝合带作为中国大陆印支期碰撞闭合最重要的缝合带,关于它们是否连通还存在争议。西秦岭造山带及其毗邻的东秦岭造山带、扬子西北缘、东昆仑造山带(柴达木陆块)及松潘-甘孜造山带以发育有大量的印支期岩浆岩为显著特征,这些岩浆作用可能是对中央造山带西段这些微陆块之间相互作用和碰撞拼合过程中的岩石圈演化和深部地球动力学过程的响应和记录。西秦岭造山带位于中国中央造山带的关键位置,对其印支期岩浆岩的时空分布,岩石组合及岩浆成因和源区的研究,对揭示西秦岭造山带印支期的构造演化和深部地球动力背景,阐明东、西秦岭造山带印支期的构造演化是否存在差异性及原因,乃至对大陆碰撞造山过程中大陆地壳的演化和分异均具有十分重要的科学意义。本论文选择西秦岭内印支期岩浆岩为研究对象,在总结整合前人资料的基础上,对西秦岭印支期岩浆岩进行详细的地质学、岩相学、锆石U-Pb年代学,地球化学和Sr-Nd-Hf同位素的系统研究。西秦岭以高钾钙碱性Ⅰ型花岗岩类分布最广,本文首先选择对西秦岭地区最大的Ⅰ型复式岩基美武岩体进行了详细的解剖,详细的探讨了其成分多样性的原因及其侵位机制；然后对西秦岭造山带内一类出露较少的岩石类型,含石榴子石岩浆岩和淡色花岗岩的成因机制进行了讨论;接着对整个西秦岭印支期岩浆作用的岩石组合特征、时空分布规律及不同成因类型岩浆岩的源区和岩石成因进行了讨论;最后建立了西秦岭印支期岩浆岩作用的年代学格架,通过与其周缘东秦岭造山带、扬子西北缘、松潘-甘孜造山带及东昆仑造山带内印支期岩浆作用的对比,揭示了西秦岭印支期岩浆形成的深部地球动力学背景。本研究获得的主要认识如下：(1)西秦岭中部合作地区美武岩基由石英闪长岩、花岗闪长岩和黑云母花岗岩三种岩性组成,其中,花岗闪长岩广泛发育有大量暗色微粒包体和少量的英云闪长质包体。岩基的侵位年龄为～240-245Ma,属于印支早期。不同的岩性单元的化学组成表现出不同的演化趋势,表明并不存在一个统一的大岩浆房,而是由多批次岩浆侵位聚集而成的,不同岩性单元的地球化学成分的变化是由源区和不同的岩浆演化过程共同控制的。美武岩基建造和成因演化的综合模型如下所述：在印支早期,富集的岩石圈地幔发生部分熔融形成含水玄武质岩浆不断以岩席状反复底侵至西秦岭下地壳,形成深部地壳热区,热的含水玄武质岩浆提供热量和挥发份诱导促使下地壳物质发生熔融,含水玄武质岩浆经历橄榄石和辉石等矿物的分离结晶作用形成的残留熔体与壳源长英质岩浆混合形成混合岩浆,这种混合岩浆多批次的侵位汇聚形成石英闪长岩。石英闪长岩外部快速冷却结晶,而岩体内部缓慢结晶形成一个均一的对流岩浆房并发生内部分异作用；镁铁质岩席状岩浆随机侵位到下地壳中,使不同深度的下地壳物质发生部分熔融作用,形成于不同深度的熔体多批次侵位聚集形成花岗闪长岩,镁质铁岩浆不仅为壳源岩浆提供了热量,还少量的参与了花岗闪长岩岩浆的形成；花岗闪长岩岩浆在就位后冷却发生部分结晶,类似于石英闪长岩的岩浆侵入花岗闪长岩中,与寄主岩发生局部的物理交换和扩散交换作用,形成暗色微粒包体,形成于加厚下地壳的埃达克质岩浆注入花岗闪长岩中形成浅色的英云闪长质包体：下地壳较浅部位的同一源区物质部分熔融程度不断增加形成的熔体不断的侵位形成黑云母花岗岩。(2)西秦岭中部合作地区德乌鲁火山岩、夏河地区赛尔钦沟含榴火山岩、含榴花岗斑岩和淡色花岗岩的形成时代为～239-243Ma,属于印支早期。德乌鲁火山岩以安山岩和英安岩为主：赛尔钦沟火山岩的岩性从安山岩到流纹岩及火山碎屑岩均有发育,在赛尔钦沟英安岩和火山碎屑岩和夏河西的花岗斑岩观察到含石榴子石斑晶。这些火山岩和次火山岩主体为钠质岩浆(K20,Na2O多<1),其中性端元安山岩随着SiO2的增加,逐渐从准铝质变为弱过铝质到强过铝质,其酸性端元(赛尔钦沟含榴英安岩,流纹岩和含榴花岗斑岩)的A/CNK均大于1.1,为强过铝质岩浆。地球化学和Sr-Nd-Hf同位素表明德乌鲁火山岩、夏河地区赛尔钦沟含榴火山岩、含榴花岗斑岩是同源岩浆演化序列,由准铝质的钙碱性安山岩质母岩浆发生分离结晶作用并同化混染泥质岩变成强过铝质的岩石。夏河淡色花岗岩具有高的Si02(-74%),高的ISr的值(～0.7103-0.7119),负的εNd(t)值(～-12)和负的锆石εHf(t)值(～-11.3),Nd和Hf二阶段模式年龄分别为1.97-2.00Ga和1.84-2.15Ga,表明其源区主要为中、下地壳古元代变沉积岩物质。夏河淡色花岗岩轻重稀土元素强列分异((La/Yb)N＝300-486),极度亏损HREE(YbN＝0.1-0.2),表明源区有大量的石榴子石残留。本次研究中的石榴子石的成因类型复杂多样,具有不同的成分环带的特征,记录了从源区、混染或不同熔体相互作用的重要的岩石成因信息。根据其内部结构、包裹体类型及成分特征可将其分为3种类型：Ⅰ型石榴子石为岩浆型石榴子石：Ⅱ型石榴子石是直接从准铝质的安山岩在高压环境下结晶出来的,I2型石榴子石是原来的准铝质安山质岩浆发生同化混染分离结晶作用形成过铝质岩浆后结晶出来的；Ⅱ型石榴子石多与黑云母集合体共生或主要成群分布在富云泥质岩包体中,具明显的成分环带,其核部为变泥质岩转熔作用形成的；Ⅲ型石榴子石多呈单颗粒、不规则状,成分多变,为捕虏晶。(3)本研究共对西秦岭地区32个样品进行LA-ICP-MS锆石U-Pb定年,对其中的4个样品又进行了SIMS测定,不同方法获得的年龄在误差范围内一致,并整理了近年来发表的19个锆石U-Pb年龄数据,建立和查明了整个西秦岭地区印支期岩浆活动的年代学格架和时空分特征。西秦岭印支期岩浆活动可主要划分为两个时期：(1)第一期为～245-230Ma,属于印支早期,主要分布在西秦岭的中部和西部：(2)第二期为～230-205Ma,属于印支晚期,在整个西秦岭地区均有分布。(4)根据西秦岭地区岩浆岩的年代学、岩相学和地球化学等,可将西秦岭印支期岩浆活动分为印支早期闪长岩类(包括暗色微粒包体)、低Sr/Y花岗岩类和高Sr/Y花岗岩类(C型埃达克岩)及印支晚期花岗岩类和暗色微粒包体等几种重要类型：印支早期闪长岩以谢坑辉石闪长岩为代表,谢坑辉石闪长岩可分为高铝闪长岩和高镁闪长岩两种类型,它们均是由先前被板片熔体交代过的富集岩石圈地幔的部分熔融形成的。高铝闪长岩是含水玄武质岩浆在侵位过程中发生橄榄石和辉石的分离结晶并伴有斜长石的优先堆晶作用形成的;高镁闪长岩是玄武质浆岩发生橄榄石的分离结晶并伴有辉石的优先堆晶作用形成的；印支早期的低Sr/Y花岗岩类属于高钾钙碱性Ⅰ型花岗岩类,ISr＝0.7063-0.7098,εNd(t)＝9.2～－3.8,T2DM＝1.46-1.74Ga,其源区为中、古元古代富钾的镁铁质下地壳。其(La/Yb)N＝10.0-30.6,Eu/Eu*=0.31-0.79,表明源区有角闪石和斜长石残留；西秦岭高Sr/Y花岗岩类与低Sr/Y花岗岩类的Sr-Nd同位素的组成范围重叠,表明它们的源区相同,轻重稀土分异明显(La/Yb)N>29,亏损HREE,(Yb)N小于7,高Ba.Sr和Sr/Y的特征,表明其源区有石榴子石残留,表明其是由加厚的下地壳部分熔融形成的,部分样品出现中等程度的铕负异常,与岩浆演化中斜长石的分离结晶有关；印支早期的低Sr/Y和高Sr/Y花岗岩类的大部分样品均表现出高Mg#、Cr和Ni的特征,是幔源物质加入并在源区发生岩浆混合作用造成的,加上同期闪长岩类的出露和暗色包体在花岗岩类中的大量发育,表明西秦岭印支早期存在着一期广泛的幔源岩浆底侵事件；西秦岭印支晚期花岗岩类整体具有比印支早期花岗岩类低MgO(Mg#),Cr和Ni和富集K2O、Rb和更高的ISr的特征,表明印支晚期的岩浆主要来自于壳源岩浆,幔源岩浆贡献较少,源区物质的成熟度更高。(5)西秦岭印支早期和晚期岩浆岩的地球化学特征也表现出明显的不同,这也反映了它们的形成可能对应着不同的演化阶段和深部地球动力学背景。结合区域地质资料,本文认为阿尼玛卿洋至少在晚二叠末已闭合,西秦岭印支早期的岩浆作用形成于后碰撞早期的构造环境之下,可能与俯冲的阿尼玛卿洋壳发生断离作用有关。板片断离模型能较好地解释西秦岭印支早期岩浆岩线性分布的特征和西秦岭中三叠纪地壳的快速抬升。印支晚期岩浆活动不仅发生在西秦岭地区,且在其周缘如东昆仑-柴北缘地区、东秦岭、扬子西北缘和松潘-甘孜造山带地区均有出现,表明在印支晚期这些地区均已共同进入后碰撞造山阶段,西秦岭印支晚期的岩浆岩的形成可能与区域上广泛的局部的岩石圈拆沉作用有关。西秦岭中部的江里沟钾长花岗岩呈现出了类似铝质A型花岗岩的性质,其形成时代为210±2Ma,与秦岭环斑花岗岩、东昆仑-柴北缘铝质A型花岗岩和在松潘-甘孜年保也则A型花岗岩的形成时代相近,该期岩浆事件的同时出现可能代表了中央造山带造山期的结束,中国大陆完成最终的拼合,从而转入区域岩石圈构造仲展体制。(6)西秦岭印支早期岩浆活动主要发育在西秦岭造山带的中、西部,然而东秦岭的岩浆活动以印支晚期为主,表明东、西秦岭在印支早期的构造演化上可能存在明显的差异。阿尼玛卿和勉略缝合带是中国大陆在印支期完成拼合过程中最重要的缝合带,西秦岭的中、西部对应阿尼玛卿缝合带所处的空间范围,而西秦岭东部和东秦岭造山带对应为勉略缝合带的位置。关于阿尼玛卿带和勉略缝合带是否连通还存在一些争论,造成分岐的原因主要是在中段玛曲-兰坪一线并没有发现与原古缝合带相关的蛇绿构造混杂岩的出露,而此段也刚好对应西秦岭岩浆岩带的中带与西秦岭东部岩浆岩带间隔缺失岩浆活动的位置,这可能意味着阿尼玛卿洋和勉略洋有着不同的演化史,在空间上可能并不相连。更多还原

【Abstract】 The West Qinling belt is the westward extension of the Qinling-Dabie-Sulu orogen in central China. It links the Kunlun and Qilian orogens to the west and the Songpan-Ganze belt to the south, making it an important tectonic syntaxis in China. The final amalgamation of China continents mainly took place during Indosinian stage, along the Mianlue and A’nimaque suture zones. The connection between these two sutures is still puzzling. One of the most profound phenomenon is that the Indosinian magmatism are widely distributed in the West Qinling and the adjacent East Qunlun and Qaidam terrans, Songpan-Ganze block, East Qinling and northwestern margin of the South Chian Block. These magmatic rocks may record the lithosphere evolution and geodynamic processes responding for the amalgamation of these micro-continental blocks in western China. The West Qinling is located on the central domain of these tectono-magma belts. An integral investigation of spatial and temporal distributions, rock types, and petrogenesis of Indosinian magmatic rocks in the West Qinling plays an important role in discussing the west Qinling Indosinian tectonic evolution, the distinct evolutions between east and west Qinling belts, and even the continent crust differentiation during the continents collisional.This thesis focuses on the Indosinian magmatism in the West Qinling. Combined with the previous relevant data, we carry out an integrated study of petrography, zircon U-Pb dating and Hf isotope, and whole-rock geochemical and Sr-Nd isotopic compositions for Indosinian magmatic rocks from the West Qinling. The West Qinling is characterized by the most widely distributed of High-K cal-alkalin I-type granitioids. Firstly, the Meiwu composite pluton, one of the largest batholiths in the middle part of the West Qinling, were choosed as the detailed study objects, to clarify the petrogenesis and emplacement mechanism for the composition diversity of I-type high-K calc-alkaline rocks; second, one type of rarely exposed rocks, garnet-bearing magmatic rocks and leucogranite, were discussed for their petrogenesis; then, the rock association and the spatial and temporal distribution of the Indosinian magmatism throughout the West Qinling were investigated for their petrogenesis and magma sources; finally, compared with the Indosinian magmatism in the adjacent areal, the geochronological framework for the Indosinian magmatism in the West Qinling were established and the geodynamic background for their formation were revealed. Our conclusions are summarized as follows:(1) The Meiwu pluton, a complex batholith, is composed of quartz diorite, granodiorite, and biotite granite, with abundant mafic enclaves and minor felsic enclaves in the granodiorites. The magma crystallization age is～240-245Ma. The different rock types exhibit distinct geochemical variation, indicating that the Meiwu pluton was constructed by multiple injections and repeated magma pulses assembly over a protracted period, rather than a single larger magma chamber. A united model for the construction and evolution of the Meiwu batholith is listed as bellow:(1) During the Early Indosinian, partial melting of the enriched lithospheric mantle generates the hydrous basaltic magmas. The high-T mafic sills were successively emplaced in the lower crust forming a Deep Crustal Hot Zones. The high-T hydrous basaltic magmas can also provide the necessary heat source and volatiles to induce melting of the lower crust;(2) The hydrous basalts that had experienced fractional crystallization of olivine and pyroxene formed the residual melt. The residual melt from hydrous basaltic magmas mixing with the melt derived from partial melting of crustal materials formed a hybrid magma; then the hybrid magma batches intruded to the upper crust and coalesced as a single magma body as the Meiwu quartz diorites. The outer part of the quartz diorites cooled rapidly preserving the primary isotopically diversity, whereas the inner part would have developed as a larger homogeneous magma chamber that underwent internal differentiation and crystallized slowly;(3) For random emplacement of the mafic sills, the single batches of melt can be generated by polybaric partial melting of the crustal materials over a range of depths through heat transfer from the cooling basalts. The melt segregation from different depths assimilated with minor the evolved mafic magma that then were successive intruded into the present lever of exposure, forming the Meiwu granodiorites;(4) The hybrid magmas analogue to the quartz diorites were injected into the host granitic magma and had experienced local mechanical exchange and diffusion exchange with the adjacent host rocks, producing various types of MME. The melts derived from the thickened mafic lower crusts that were subsequently intruded into the host granitic magma and disaggregated to the tonalitic enclaves;(5) The melts generated by increase degree of partial melting of a single source due to heat input from mantle magma were successive emplaced forming the Meiwu biotite granites.(2) The magma crystallization ages for Dewulu volcanic rocks, Sai’erqin garnet-bearing volcanic rocks and Xiahe garnet-bearing granite porphyry, and Xiahe leucogranite are～239-243Ma. Dewulu volcanic rocks are mainly composed of andesite and dacite, and Sai’erqin volcanic rocks range from andesite to rhyolite to volcaniclastic rocks. These rocks are enriched in Na2O (K2O/Na2O ratios mostly<1). The andesitic rocks range from metaluminous to weakly peraluminous with increasing SiO2contents, and the felsic rocks show strongly peraluminous (A/CNK>1.1). Geochemical and Sr-Nd-Hf isotopic compositions indicate that these rocks share a single metaluminous calc-alkaline andesitic magma source, which turned into strongly peraluminous magmas through AFC processes. The Xiahe leucogranite has high SiO2contents (～74%), high ISr values of～0.7103-0.7119, negative εNd(t)(～-12) andεNf(t)(～-11.3), with two-stages model ages of～ 1.8-2.2Ga, implying derivation from partial melting of reworked mature crust. The leucogranite contains very low HREE (YbN=0.1-0.2), with strongly fractionated REE patterns with (La/Yb)N ratios of400-486, indicating a large volume of residual garnet. Garnets from these rocks show distinct formation features. They are divided into three types:(i) crystallizing from the metaluminous to peraluminous magmas;(ii) inheriting from the melting source;(ⅲ) xenocryst.(3) To understand the temporal and spatial distribution of west Qinling Indosinian magmatism, thirty-two samples were selected for in situ LA-ICP-MS zircon U-Pb analyses. And four of them were reanalyzed by using the SIMS. We compile new results and nineteen published data. Our compilation shows two magmatism stages:(i) early Indosinian (-245-230Ma), concentrating on the central and west part of west Qinling belt;(ii) late Indosinian (-230-205Ma), widespread throughout the west Qinling belt.(4) Based on geochronology, petrology, and geochemistry data of west Qinling magmatic rocks, they are mainly composed of early Indosinian diorite (including mafic enclaves), low Sr/Y granitoids, and high Sr/Y granitoids (C-type adakite), and late Indosinian granitoids and mafic enclaves. Xiekeng pyroxene diorite is a typical example of early Indosinian diorite, including high-Al diorite and high-Mg diorite. Both of them were derived from partial melting of enriched lithospheric mantle that had been modified by slab-derived melt. The Xiekeng high-Al diorite was formed by the hydrous basaltic magmas that had experienced fractional crystallization of olivine and pyroxene and/or preferential accumulation of plagioclase, while the Xiekeng high-Mg diorite was formed by fractional crystallization of olivine and/or preferential accumulation of pyroxene. Early Indosinian low Sr/Y granitoids are high-K calc-alkaline series, and they display ISr=0.7063-0.7098, εNd(t)=-9.2--3.8, with T2DM=1.46-1.74Ga, indicating derivation from a early-middle proterozoic mafic K-rich lower crust. Their low (La/Yb)N (10.0-30.6) and Eu/Eu*(0.31-0.79) imply the residual mineral of amphibole and plagioclase in its source. The high Sr/Y granitoids show similar Sr-Nd isotopic compositions with those of the low Sr/Y granitoids, suggesting that they share a same source. Their high Ba, Sr contents and high Sr/Y and (La/Yb)N ratios imply the residual mineral face of garnet. Some samples exhibit slightly negative Eu anomalies, probably resulted from fractional crystallization of plagioclase during the emplacement. Most of the early Indosinian low Sr/Y and high Sr/Y granitoids show high Mg#, Cr, and Ni values. Besides, coeval diorites and mafic enclaves are widespread, suggesting a culmination of basalt underplating during early Indosinian. Late Indosinian granitoids commonly have low MgO, Cr, and Ni contents, and high K2O, Rb, and ISr values, indicating they were derived from more mature crust, with less juvenile material contribution.(5) Distinct geochemistry features of early and late Indosinian granitoids may imply different geodynamic setting. Combined with regional studies, we interpret this early Indosinian magmatism in the West Qinling to result from break-off of the subducted A’nimaque oceanic slab soon after collision. The slab break-off model can explain the linear distribution of the early Indosinian plutons and rapid uplift during the Middle Triassic in the West Qinling. Late Indosinian magmatic rocks are widely exposed in the Qinling orogen, the northwest part of the Yangtze craton, the Songpan-Ganzi belt, and the east Kunlun belt, suggesting all of these regions are in a post-collision setting. Widespread late Indosinian intrusions are most likely caused by delamination of thickened crust. The Jiangligou potassic granite shows similar geochemical features with those of A-type granite, and its crystallization age is210Ma. Coeval A-type granites and are also found in east Kunlun and Songpan-Ganzi. This A-type granitic magmatism may represent the end of central orogen processes, and the west Qinling belt turn into an extension setting.(6) Early Indosinian magmatism mainly concentrate on the central and west part of west Qinling belt, whereas the east Qinling was dominated by the late Indosinian magmatism, suggesting that West Qinling and East Qinling belts are in different geodynamic settings during early Indosinian. Whether the A’nimaque and Mianlue suture zones is connected or not is still in debate. The early Indosinian magmatism in the middle and west part of the West Qinling is related to the A’nimaque suture zone in spatial, and the east part (as well as east Qinling belt) magmatism related to the Mianlue suture zone in spatial. The distinct temporal evolutions of Indosinian magmatism between these two domains reveal that the A’nimaque Ocean and Mianlue Ocean were not connected before.更多还原