导　　师： 樊祺诚;李霓;孙谦

学科专业： 070904

授予学位： 博士

作　　者： ;

机构地区： 中国地震局地质研究所

摘　　要： 1前言 中国东部中-新生代以来火山活动频繁,造就了绵延5000公里宽达数百公里的陆相火山岩带,新生代玄武岩主要沿中国东部大陆边缘一系列北东向、北北东向的裂谷和断陷盆地分布,东北火山岩区位于其北端,是我国重要的新生代火山分布区。上个世纪80年代以来,我国在中国东部火山岩年代学和岩石地球化学研究方面取得丰硕成果,但迄今为止,大兴安岭地区因森林覆盖,交通不便,其中隐藏的新生代火山/(主要是第四纪火山/)鲜为人知,调查研究工作进展缓慢。当今火山学科已经成为集火山地质、地球物理、地球化学、灾害等学科于一体的综合性交叉学科。本文以火山学理论为指导,运用地质学、火山物理学和地球化学综合研究方法,透视东北大兴安岭第四纪以来大陆板内火山活动的基本特征和规律。论文主要研究以下科学问题: ①火山活动的时空演化规律 ②火山地质特征反映出什么样的活动特点 ③火山岩浆来源及其深部地球化学制约 2区域地质背景 大兴安岭第四纪火山区在区域上分为南区和北区两个组成部分。南区的哈拉哈河-绰尔河火山区位于大兴安岭中段,横跨内蒙古自治区东部兴安盟阿尔山市和呼伦贝尔市交界处。北区的诺敏河-奎勒河火山区地处大兴安岭北段,位于呼伦贝尔市东部鄂伦春自治旗境内。 中生代该地区在大兴安岭隆起的背景下,发育有强烈的岩浆活动,以中酸性的岩浆侵位和喷发为主。在新生代中国东部大陆整体处于拉张构造环境,日本海弧后盆地和中国东部裂谷系拉张的背景下,大兴安岭山体发生强烈隆升,并伴生有频繁的玄武质岩浆喷发活动。该地区地层主要为侏罗纪、白垩纪火山岩系,其次为新生代地层。侵入岩主要为燕山早期二长花岗岩;火山岩分布广泛,以中生代酸性火山岩分布最广,新生代火山岩主要为碱性玄武岩。 3火山地质 3.1火山活动历史 哈拉哈河-绰尔河地区火山总体呈北东向带状分布,以熔岩流为主的火山产物呈连续带状分布于主要河流的河谷当中。根据野外调查,确认第四纪火山35座,熔岩流分布面积约400km~2。诺敏河-奎勒河火山区火山产物以熔岩流为主,呈片状分布,根据野外调查确认火山25座,熔岩流分布面积约600km~2。根据火山地质特征、野外地层的新老覆盖关系和岩石风化程度等地质特征,结合地质测年数据对研究区的火山活动初步划分为早更新世、中更新世、晚更新世和全新世四期。 早更新世玄武岩主要分布于北区的诺敏河果楞奇附近,以及奎勒河宜里,火山产物主要是熔岩流,具有以下特点:熔岩流找不到明显的火山源头;熔岩流构成河流的二级阶地,表面已经被厚层的河流冲积物覆盖,多被开垦为农田;熔岩流不但被大的河流分割,也被更多的小型河流深度侵蚀切割为小的块体;玄武岩风化较严重,具球状风化特点。 中更新世火山产物在北区的诺敏河、奎勒河流域均有大量分布,占据着第四纪以来火山生成物的主体。哈拉哈河-绰尔河火山区内约占总数3//4的火山形成于此时。熔岩流分布面积达近300km~2。诺敏河-奎勒河地区此时形成火山占总数的2//3,相应的熔岩流分布面积达320km~2。火山产物具有以下特点:保存大量火山碎屑锥及部分火山口,火山锥风化程度相对较强,其顶部溅落锥已被不同程度剥蚀,火山口被后期崩塌物质及厚层腐质土所充填;熔岩流被大的河流切割,构成河流一级阶地,岩石风化程度较强,多被林地覆盖。 晚更新世火山产物在哈拉哈河-绰尔河地区主要是基尔果山天池火山、子宫山火山、以及卧牛泡子火山、乌苏浪子湖火山。火山数目较少,熔岩流覆盖面积只有不到40 km~2。相应的火山产物在诺敏河-奎勒河主要分布在毕拉河河谷附近厚层熔岩流的中-低层位,推测分布面积约150 km~2。这一时期产物具有以下特点:火山锥风化程度相对较弱,火口较深、内壁较陡,锥体顶部仍保存较完整的溅落锥;熔岩流风化程度较弱,其上只有薄层土壤覆盖;熔岩流侵占河道,未被河流侵蚀切割;松散、易风化的射汽岩浆火山碎屑保存较完好,也是晚更新世火山活动的重要标志之一。 全新世火山在南区哈拉哈河-绰尔河地区有焰山、高山、十号沟盆地火山、小东沟火山四座火山,熔岩流分布面积共计约100km~2。北区毕拉河上游的马鞍山、达来滨湖通以及达来毕诺熔岩流属于全新世火山产物,推测毕拉河下游河谷上部层位熔岩流也属全新世产物,覆盖面积约100-150km~2。全新世火山产物一般具有以下特点:火山锥保存最为完好,一般锥体高大陡峻,火口具漏斗状,内壁陡峭易垮塌;锥体顶部的仍保持火山喷发结束时的溅落堆积特征;熔岩流保持了刚冷却时的形貌和流动构造,表面很少有覆盖层,树木难以扎根,只在岩块隙间有稀疏林木,裸露的石塘是全新世熔岩流的标志性特征;火山产物尤其是大片的熔岩流对现代地形地貌进行了明显改造,熔岩流侵占阻塞河道,形成众多火山堰塞湖。 根据火山活动期次可以恢复研究区火山活动历史如下:早更新世时期诺敏河和奎勒河流域最先开始第四纪火山活动,这一时期火山活动强度不大,熔岩产出量不高,熔岩流分布在少数河谷中。中更新世时,大兴安岭地区火山活动趋强,无论是火山数量还是熔岩产量都占第四纪以来火山产物总量的1//2以上,火山产物既有火山锥又有熔岩流,这一时期的熔岩流分布范围广泛,产物遍布大的河流流域,构成了整个火山区的雏形。晚更新世时期火山活动趋弱,火山产物占总量比例最低,火山活动范围缩小,只局限于小范围区域。进入全新世,在继承晚更新世火山活动区域的基础上,火山活动又进入另一个高峰期,爆破式喷发造就多座较大的火山碎屑锥,熔岩流分布广泛。 3.2火山喷发类型 区内火山按照火山作用方式不同,可分为爆破式和溢流式两种火山喷发方式。爆破式喷发产物按照爆破强度和方式分为低强度岩浆爆发形成的粗粒火山碎屑堆积、高强度岩浆爆发形成的细粒火山碎屑堆积;射汽-岩浆爆发形成的火山碎屑堆积。溢流式喷发形成的火山产物主要是熔岩流,其中包括渣状熔岩,结壳熔岩,块状熔岩等,其中还发育有大量独特的熔岩构造。溢流式喷发产物主要是熔岩流,研究区内熔岩类型多样,结壳熔岩和渣状熔岩分布面积较广,另外在个别地方还发育块状熔岩。熔岩在流动及冷却过程中形成喷气锥、熔岩冢、熔岩挤入构造、柱状节理等地质现象。 爆破式火山喷发有三种喷发类型:斯通博利型喷发、高山-焰山式型喷发、射汽岩浆型喷发,产生不同类型的碎屑堆积物。溢流式喷发以夏威夷型喷发为主,产物组成盾形火山和熔岩流。几种喷发类型都属岩浆喷发作用范畴,只有射汽-岩浆型喷发除外。 斯通博利型 以低强度岩浆爆发形成的碎屑为主要特征,喷发形成的火山锥一般都是复式碎屑锥,溅落锥叠置于降落锥之上。碎屑集中在火口附近分布,构成火山碎屑锥。其中的降落锥主要由火山渣、火山弹、熔岩饼组成,有时还有薄层熔岩流分布其中。火山碎屑粒度较粗,以1~5cm为主。碎屑锥顶部的溅落锥由弱爆破下的火山弹、熔岩饼及少量熔岩等在高温下重新焊结在一起,形成熔结集块岩和碎屑熔岩。火山锥的整体结构反映出火山由初期低强度的爆破式喷发向后期溢流式喷发过渡的过程。 焰山-高山型 本文提出焰山-高山型这种新的火山喷发型式。这种喷发以高强度岩浆爆发为主要特征,形成的细粒火山碎屑,这种碎屑气孔构造非常发育,粒级较小,一般2-10mm的火山砾及2mm以下的火山灰,与斯通博利式型喷发碎屑相比,粒径小得多。火山碎屑总体上呈松散堆积状态,组成火山碎屑锥和空降火山碎屑席,这种喷发形成比斯能博利型火山更大的锥体,降落锥表层火山砾呈面状平行于锥体斜坡面分布,火山周围分布有面积较广的与火山锥同期形成的火山碎屑席。根据火山产物推测火山喷发时火口之上形成了持续时间较长的高度较高的喷发柱,火山碎屑随之上升到高空,最后降落在火山锥及周围广大范围内,形成火山碎屑锥和空降火山碎屑席。 夏威夷型 夏威夷型喷发以熔岩平静溢流为特征,产物主要是广泛分布的熔岩流。在火口周围形成小型的单喷发成因盾形火山锥,以及火口塌陷坑。盾形火山锥坡度低缓,一般小于5°,在平面上形状为圆形或椭圆形,整体上形成盾形,组成物质主要是多期熔岩流。火口塌陷坑的四周是近于直立的陡壁,反映出火山活动后期岩浆通道冷却,密度增大,体积收缩,由于重力作用,造成火口塌陷。 射汽-岩浆型 特征产物是基浪堆积物,有以下特点:由灰黑色火山碎屑组成,粒径以0.5mm-3cm为主,其中还夹杂有大量的围岩碎屑;平行层理、大型交错层理、层理下陷等构造发育,堆积物中有明显的侵蚀面;分布范围较广,距火口可远至三公里;火口邻近中生代破火口湖。这些特征说明这种喷发类型是是玄武质岩浆遇水后与水相互作用,发生的射气-岩浆爆炸。 3.3火山成因湖泊 大兴安岭火山区内湖泊众多,都与火山作用有关,之前无人对其做成因研究。本文在系统对比不同形式的湖泊,根据它们与火山作用之间的关系,划分出四种火山成因的湖泊:火山口湖、熔岩堰塞湖、低平火山口湖、熔岩塌陷湖。 3.4火山系统 火山喷发过程中不仅受控于岩浆因素/(岩浆的物理和化学属性/),还受控于非岩浆因素,后者决定着火山的形态与规模。本文从火山形态学入手,分析控制火山喷发的非岩浆因素。根据火口的多寡研究区内火山碎屑锥分为单火口锥和多火口锥,按照锥体形态分为圆马蹄形锥、长马蹄形锥和圆形锥,研究区内以马蹄形碎屑锥为主。本文提出裂隙形的岩浆通道造成碎屑锥在建造时特定方向的不稳定性,使该方向持续的垮塌,叠加熔岩流溢出破坏,从而形成马蹄形锥体。圆马蹄形锥体与长马蹄形锥体受控于裂隙形岩浆通道,圆形锥的岩浆通道是管状。多火口锥是由于裂隙岩浆通道在喷发过程中局部受岩浆冷却,在同一裂隙通道中改变喷发位置而形成。 对比研究区内57座火山锥体的形态描述参数/(Wcr-火口直径、Hco-锥体高度、Wco-锥底直径/)后得出W/_/(co/)在130-2500m之间,平均值为940m,中值为810m。其中碎屑锥W/_/(cr/)//W/_/(co/)在0.13-0.85之间,平均值为0.5,中值为0.49。H/_/(co/)//W/_/(co/)~b在0.03-0.32之间,平均值为0.12,中值为0.11。熔岩盾锥W/_/(co/)在300-2500m之间,平均值为1170m,中值为1000m;H/_/(co/)//W/_/(co/)~b在0.01-0.15之间,平均值为0.05,中值为0.04。火山锥的W/_/(cr/)-W/_/(co/)~b和H/_/(co/)-W/_/(co/)~b都分别具有线性对应关系,碎屑锥与熔岩盾在W/_/(cr/)//W/_/(co/)~b和H/_/(co/)//W/_/(co/)~b呈现明显的不同。结果反映出熔岩盾在整体直径上比碎屑锥大,而在锥体高度//锥底直径上比碎屑锥小很多。 对火山锥形态的研究显示研究区火山锥规模较小,形态也相对简单,结合对火山机构的研究,表明区内火山为单次喷发成因,由一次喷发形成或相隔时间很近的少数几次喷发即可形成。研究区的火山锥及熔岩流规模都相对较小,表明该地区火山系统的特点是岩浆的输送率和岩浆的输送频率低。输送率低造成单个火山的规模较小,岩浆输送频率低造成岩浆通道在一次火山活动之后很快冷却固结,下次岩浆活动只能重新开辟新的通道,表现在地表就是火山锥之间都有一定距离,火山喷发没有沿相同的火山通道及火口,因此研究区没有出现大型复合式火山。 4岩石地球化学特征 研究表明大兴安岭火山区南区与北区火山岩岩石学与地球化学特征呈现出显著的区别。哈拉哈河-绰尔河玄武岩为钠质系列碱性玄武岩,主要类型为碱性橄榄玄武岩。∑REE为109.5-262.6　g//g,/(La//Yb/)N介于7.9-12.4之间,显示轻重稀土的分异程度弱,大部分稀土配分曲线相互平行。δEu普遍大于1/(0.92-1.07/),呈弱的正异常。玄武岩在微量元素原始地幔配分图中,大多样品曲线相互平行且平滑,总体上表现出与OIB相似的特征,显示出Ba、Nb、Ta的正异常,Rb、Th、U显示弱的负异常。只是大离子亲石元素/(LIL/)如Ba、Rb比OIB稍低,LREE也明显低于OIB。MgO与Cr、Co、Ni都显示出相同的线性正相关,与Yb、Zr线性负相关性。不相容元素如Rb、Ba、Th、U、La、Sr、Nd、Pb与Nb都呈良好的线性相关关系。南区玄武岩稀土和不相容元素配分曲线与大同玄武岩最为接近。Sr-Nd-Pb同位素投点都投在OIB范围内,在中国东部新生代玄武岩中投在高~/(143/)Nd//~/(144/)Nd,低~/(87/)Sr//~/(86/)Sr,高~/(206/)Pb//~/(204/)Pb、~/(207/)Pb//~/(204/)Pb、~/(208/)Pb//~/(204/)Pb的一端,与大同及阳原新生代碱性玄武岩范围接近。 诺敏河-奎勒河玄武岩属于钾质系列碱性玄武岩,主要类型为碱玄岩和碧玄岩。∑REE为223.6-345.8　g//g,大部分样品/(La//Yb/)N介于21.6-41.9之间,散布范围较宽。δEu为0.84-0.96,呈弱的负异常。玄武岩不相容元素原始地幔配分曲线呈现特征性的Ba、Sr高正异常,Nb、Ta、Sr、Sm、La微弱正异常,Th、U、Zr、Hf微弱负异常。曲线的坡度相对较陡,显示出强不相容元素与弱不相容元素间较高程度的分异。MgO与Cr、Co、Ni都显示出相同的线性正相关。MgO与/(La//Yb/)N值表示出弱的正相关。不相容元素如Rb、Ba、Th、U、La、Sr、Nd、Pb与Nb都呈良好的线性相关关系,REE配分曲线及不相容元素原始地幔配分曲线介于汉诺坝玄武岩与五大连池玄武岩。 两个地区相比,诺敏河-奎勒河玄武岩以相对高K、Na,富集大离子亲石元素/(尤其是Ba,Sr/)及轻稀土为特征。而哈拉哈河-绰尔河玄武岩相对富HREE,更加接近OIB的特征。一系列证据表明岩浆期后蚀变和地壳混染对研究区岩浆成份的影响不明显,结晶分异也仅限于少量的单斜辉石和橄榄石分异。岩石地球化学性质差异表明哈拉哈河-绰尔河火山区与诺敏河-奎勒河火山区岩浆源区明显不同。 哈拉哈河-绰尔河玄武岩的REE和不相容元素配分曲线、Sr-Nd同位素、Nb//U、La//Nb、Ba//Nb都与OIB相近,证明中国东部与OIB源区性质相近的软流圈地幔在玄武岩起源中起着重要作用。另一方面,区内玄武岩Ce//Pb明显小于OIB及源于软流圈地幔的大同碱性玄武岩,Ba//Nb和La//Nb之间存在弱正相关,Sr同位素与Ce//Pb之间也表现出明显的线性相关,这些证据表明岩浆源区是软流圈与另一源区的混合。 诺敏河-奎勒河玄武岩具有低Ce//Pb,高Ba//Nb和La//Nb,远离OIB,与五大连池、二克山、科洛火山岩高度重合。推测岩浆来源与钾质交代地幔有关。 综合以上证据表明,哈拉哈河-绰尔河玄武岩岩浆源区主要是软流圈,岩浆在软流圈与岩石圈界面处与岩石圈地幔底部发生相互作用。诺敏河-奎勒河玄武岩岩浆源区与变质活化的底部岩石圈地幔有关。研究区内软流圈具有以下性质:Ce//Pb>22.7,Ba//Nb<7.30,La//Nb<0.57,87Sr//86Sr<0.7035;而岩石圈地幔底部Ce//Pb<13.7,Ba//Nb>28.3,La//Nb>1.19。这两种源区的不同的作用造成南北两区玄武岩岩石和地球化学性质的差异。 5火山灾害 综合地质及地化证据表明,研究区内软流圈及岩石圈是岩浆的主要来源,目前该地区地幔和地壳结构有利于岩浆的生成与喷发,因此未来研究区有可能再次发生火山喷发。通过对研究区火山产物分析,推测未来主要火山灾害由火山喷发空降碎屑物、火山碎屑涌流、熔岩流、火山喷发气体造成。 6主要结论 大兴安岭第四纪哈拉哈河-绰尔河火山区/(南区/)和诺敏河-奎勒河火山区/(北区/)包括约1000平方公里火山岩和60座火山,火山活动划分为早更新世、中更新世、晚更新世和全新世四期。根据火山喷发特征及其喷发产物,提出大兴安岭第四纪火山分属四种喷发类型:斯通博利型、高山-焰山型、夏威夷型和射汽岩浆型,高山-焰山型为本文新提出的喷发型式。大兴安岭第四纪火山属于单次喷发成因火山系统,岩浆通道受构造断裂控制,岩浆输送频率和输送速率比较低,非岩浆因素控制了火山的分布与形态。根据火山区内湖泊与火山作用之间的关系,划分出四种火山成因的湖泊:火山口湖、火山堰塞湖、熔岩塌陷湖和低平火山口湖/(玛珥湖/)。在综合资料基础上,编制了大兴安岭南、北区两幅火山地质图。 大兴安岭南区哈拉哈河-绰尔河和北区诺敏河-奎勒河玄武岩分别属于钠质玄武岩和钾质玄武岩,它们都具有接近原始岩浆的高Mg#特征,岩浆上升过程中极少演化和受到地壳混染。火山岩的REE、微量元素元素和Sr-Nd-Pb同位素地球化学特征表明,大兴安岭南区哈拉哈河-绰尔河岩浆源区具有软流圈与岩石地幔圈的混合特征,北区诺敏河-奎勒河岩浆起源与钾质交代地幔有关。 1 Introduction Cenozoic volcanic rocks are widely distributed in Northeast China, particularly along the valleys and mountain belts trending NNE-NE on the flanks of the Songliao basin, which comprise one of most important part of the Mesozoic-Cenozoic volcanic belt in eastern China. Although most of the volcanic fields in this region have been extensively investigated, few studies are made on the Quaternary volcanoes in the Da Hinggan Ling Mountains due to the awful work conditions. On the basis of modern volcanology theories, this thesis attempts to perform a systematic research on volcanic geology and geochemistry in this area. It focuses on the following issues: volcanic history, features of volcanic geology, geochemical constraints on magma source and volcanic processes. 2 Regional geology The Quaternary volcanic field of the Da Hinggan Ling Mountains is divided into two parts: Halaha River and Chaoer River volcanic field /(HC for short/) in the south, and Nuomin River and Kuile River volcanic field/(NK for short/) in the north. The HC, which is near the boundary of Hulunbeier City and Hinggan Meng in the northeast of Inner-Mongolia Autonomic Region, is located in the middle of the Da Hinggan Ling Mountains. The NK, in the north of Da Hinggan Ling Mountains, is situated in the Nuoming Town in the Oroquen Autonomous Banner in Hulunbeier City. This area had experienced strong silicic volcanism in Mesozoic time and widely scattered basalt volcanism in the Cenozoic era. Igneous rocks produced in the Jurassic and Cretaceous Period composed the main strata. The Cenozoic strata are dominated by alkaline basalts. 3 Volcanic geology 3.1 History of volcanic activity Field investigations have determined 35 Quaternary volcanoes distributed along a Quaternary NE strike belt in HC. The lava flow of HC, characterized by extension without break, lies in the valleys of several rivers with an area of 400 km~2. In the north, 25 Quaternary volcanoes and 600km~2 lava flows, which is separated into several parts, are confirmed in NK. Based on studies on the volcanic field stratigraphy, in conjunction with weathering extent and geological dating, it is identified that the volcanism occurred in 4 periods: Early Pleistocene, Middle Pleistocene, Late Pleistocene and Holocene. Basalts of early Pleistocene dominated by lava flows are distributed in the Guolenqi and Yili in NK. No pertinent cones are found, and they commonly compose the second fluvial terraces. The surface of lava flows has been covered by thick fluvial sediments, mostly reclaimed as farmlands. The lava flows have been dissected by several major rivers as well as small rivers into many little parts. The basalts were also deeply weathered and featured by spheroidal weathering. Volcanic products of middle Pleistocene composed the main part of the Quaternary volcanic deposits. There exist a great number of cones as well as vents generated in middle Pleistocene which remain at present. Cones are strongly weathered in many cases, and the vents are full of deep deposits. Lava flows, covered by woods, were deeply eroded by big rivers and constitute the first fluvial terraces. Moderate volcanism occurred in late Pleistocene which produced a small amount of volcanic deposits. Cones were moderately weathered and characterized by deep vents, steep inner walls, and well-reserved spatter cones. Weathering of lava flows, which were covered by thin soil mantle, is relatively weak. Four volcanoes in HC are identified as Holocene products. 2 volcanoes and upper lava flows in the Bila River in NK are thought to belong to Holocene products too. Volcanic cones generated in this period usually have large volumes and steep outlines, with funnel-like vents in which collapse occasionally happened. The cones and lava flows largely remain the same as that when the volcanoes stopped eruption. Lava flows look like fresh, with little cover and vegetation, reshaping modern rivers and causing many volcanic dammed lakes. Reconstruction of volcanic activity history: In terms of periods of volcanic activity, the volcanic history of this region can be reconstructed as follows: In the early Pleistocene, Quaternary volcanism started first in the drainage areas of the Nuoming River and Quile River. Its intensity was not large with a small amount of lava rock products which are distributed in a few valleys. During middle Pleistocene, volcanic activity in the Da Hinggan Ling tended to become intensive, of which both the volcano and lava rock amounts account for more than 1//2 of the total since Quaternary. The volcanic products include volcanic cones, as well as lava flows which are widely distributed over drainage areas of major rivers, forming the initial shape for the whole volcanic region. In late Pleistocene, the volcanic activity tended to be weak, with a small amount of products and small range. When it entered Holocene time, the volcanism culminated again with explosive eruptions that produced many big cones of volcanic fragments and extensive lava flows. 3.2 Types of volcanic eruption In this region the volcanic eruptions can be classified into explosive style and effusive style. The explosive eruptions produced coarse-grain pyroclastic accumulations associated with low-intensity magma explosion, fine-grain pyroclastic accumulations associated with high-intensity magma explosion and phreatomagmatic explosion. The eruptions of effusive style generated primarily lava flows, including aa, pahoehoe and block lava, as well as lots of distinct lava structure. These lavas have generated some special structures during their flow and cooling processes, such as fumarolic cones, lava hillock, lava intrusions and columnar joints. Four eruption types-Strombolian type, Gaoshan-Yanshan type, Hawaiian type, and Phreatomagmatic type, are found in the researching area. Strombolian eruption characterized by magmatic weak explosion, which produced The volcanic eruptions of explosion style has three types: Strombolian type, Gaoshan-Yanshan type and phreatomagmatic type, which produced different kinds of pyroclastic accumulations. The effusive eruption is dominated by the Hawaiian type which generated scutulum volcanoes and lava flows. Except the phreatomagmatic type, other types of eruptions are attributed to the range of magma eruption. Strombolian type It is characterized by fragments produced by low-intensity magma eruption, which usually left complex scoria cones where the sptter cones overlay the cinder cones. The pyroclasts was concentrated around craters, forming scoria cones. The cinder cones consists of scoriae, volcanic bombs, sometimes with thin layers of lava flows. The grain of pyroclasts is relatively coarse, mostly of 1~5cm size. The splash cones overlying the pyroclastic cones are composed of assembled block and pyroclastic lava rocks that were resulted from re-adjoining volcanic bombs, driblets and few lava rocks from weak eruptions at high temperature. As a whole, the structure of volcanic cones shows a transitional process of volcanoes from the explosion style of low intensity at early time to the effusive style in the late period. Gaoshan-Yanshan type This work suggests a new type of volcanic eruption as high mountain and flame-mountain style. It is characterized by high-intensity explosion of magma, which generates fine pyroclasts with numerous vesicular structures of small size, that exhibit in general as 2~10mm volcanic lapilli and <2mm ash. Compared with the Strombolian type, the grain size of this kind of pyroclasts is much smaller. Overall, the pyroclasts of this type looks like a loose accumulation shape, constituting pyroclastic cones and descending sheets. It can produce cones that are lager than the Strombolian type. The lapilli on the surface of cinder cones are distributed along the slopes of cones. Surrounding the volcanoes, there are broad pyroclastic sheets that were produced when the cones formed. Based on the volcanic products, it can be speculated that a rather high column was generated for a long time above the crater as the volcano erupted. Thus the pyroclasts was thrown up to the sky, and finally descended onto the surface surrounding the cone to a large range, leaving pyroclastic cones and descending sheets as seen at present. Hawaiian type The eruption of this type is featured by gentle effusive of lava rock that is widely distributed. Around a crater, there are shield-like cones produced by small single eruptions as well as collapsed pits. The shield-shaped cones have a gentle slope angle, generally less than 5°, exhibiting circles or ellipses on plane, and look like a big shield as a whole. Their composition is primarily lava flows of many periods. Nearly vertical walls are present around the collapsed pit, implying density increase and volume contraction by cooling magma paths at the late stage of the volcanic activity, and then the crater collapsed by gravity. Phreatomagmatic type Its characteristic product is base-wave accumulation which consists of grey-black pyroclasts dominated by grain size 0.5mm~3cm, with intercalated lots of country rock pyroclasts. It contains well developed structures of parallel stratification, large-scale staggered beds and bed sags. Notable erosional levels are seen in the accumulations. The pyroclasts is distributed in a big range that can be as far as 3km away from the crater which is close to the lake of produced by the Mesozoic caldera. These features indicate that such a type of eruptions was a phreatomagmatic explosion that occurred when the basalt magma met and interacted with water. 3.3 Lakes generated by volcanism In the study area, there are many lakes generated by volcanic activity. In the past time, there was no studies on their generation cause. This thesis makes a comparison of the volcanic lakes of various forms, and classifies them into four categories: crater lake, lava dammed lake, caldera lake and lava collapse lake. 3.4 Volcanic system The process of a volcanic eruption is dictated by magma nature /(physical and chemical attributes of magma/) as well as other factors that determine the shape and size of the volcano. This thesis starts with morphology, and analyzes the non-magma factors that control volcanic eruptions. In terms of the number of craters, the pyroclastic cones in the study area are divided into single crater cones and multiple crater cones. Usually volcanic cones look like circular horseshoe, long horseshoe and circular shapes. In the study area, they are dominated by horseshoe pyroclastic form. This thesis suggests that due to instability in a certain direction during formation of pyroclastic accumulation by fissure-bearing magma channels, collapse occurred steadily in this direction, leading to overlapping outflows of lava, thus horseshoe-like cones were generated. The circular and long horseshoe cones were controlled by crack-shaped magma channels while circular cones were associated with tupe-like magma paths. Multiple crater cones were produced by cooling magma at varying local positions in the along the same crack path during the eruption. This thesis gives following the parameters that describe the morphology of 57 volcanic cones in the study area, i.e. W/_/(cr/) -crater diameter, H/_/(co/)-cone height, and W/_/(co/) -diameter of cone base. The statistics show W/_/(cr/) that ranges from 130 to 2500m, 940m on average, with medium value 810m. For pyroclastic cones, the values of W/_/(cr/)//W/_/(co/) is between 0.13-0.85, with average value 0.5 and medium value 0.49; H/_/(co/)//W/_/(co/) is 0.03-0.32 with average 0.12 and medium 0.11. For shied-like lava cones, W/_/(cr/) is 300-2500m, with average 1170m and medium 1000m, is H/_/(co/)//W/_/(co/)~b 0.01-0.15 with average 0.05 and medium 0.04. Both W/_/(cr/)-W/_/(co/)~b and H/_/(co/)-W/_/(co/)~bof volcanic cones have linear relationships, while W/_/(cr/)//W/_/(co/)~b and H/_/(co/)//W/_/(co/)~bare much different for pyroclastic cones and lava shields. And the result shows that lava shields have a larger diameter than that of pyroclastic cones in general, while the values of H/_/(co/)//W/_/(co/)~b of lava shields are much smaller than that of pyroclastic cones. Overall the volcanic cones of this region have a small scale and simple shape. The study of volcanic mechanism suggests that they are attributed to single eruption in generation, i.e. generated by one time of eruption or a few eruptions that occurred successively in a short time. The volcanic cones and lava flows have relatively small scales, indicative of low transfer rates and transfer frequency of magma in the volcanic system. A low transfer rate of magma can cause small-sized individual volcanoes. And a low transfer frequency of magma makes the path cool very soon before the next volcano, that has to open new path for magma motion, as evidenced by certain distances between cones on the surface. Since the volcanic eruptions had no same paths and craters, there is no large-scale composite volcano in this region. 4 Geochemical characters of rocks This thesis suggests that the south and north of the Da Hinggan Ling volcanic field are considerably different in characters of petrology and geochemistry. The basalt at Halaha River-Chuo’er River is of alkali one in sodium series, dominated by alkali dorgalite. Its∑REE is 109.5-262.6ug//g, /(La//Yb/)N is between 7.9-12.4, indicating a weak differentiation of light and heavy rare earth /(RE/), with parallel partitioning curves of RE. The values ofδEu are mostly greater than 1.0 /(0.92-1.07/), exhibiting a weak positive anomaly. On the plot of original mantle partition of microelement of basalt, the most sample curves are mutually parallel and smooth, roughly similar to OIB in characters, expressing positive anomalies of Ba, Nb and Ta as well as weak anomalies of Rb, Th, and U. But it has a slightly lower LIL than OIB, such as Ba and Rb, and its LREE is also considerably lower than OIB. Between MgO and the elements Cr, Co and Ni is of linearly positive correlation, while linearly negative correlation with Yb and Zr. The incompatible elements, such as Rb, Ba, Th, U, La, Sr, Nd, Pb and Nb are all in good linear correlation. In the south, the partitioning curves of basalt RE and incompatible elements are very close to that of the Datong /(Shanxi Province/) basalt, where the isotopes of Sr-Nd-Pb are all projected in the range of OIB. While in eastern China, these elements of Cenozoic basalt are projected at the end of high 143Nd//144Nd, low87sr//86Sr, high206Pb//204Pb, 207Pb//204Pb and 208Pb//204Pb, that is close to the range of the alkali Cenozoic basalt in Datong and Yangyuan. The basalt at Nuoming River-Quile River is alkali one of kali series, mainly appearing as tephrite and basanite. Its∑REE is 223.6-345.8μg//g. On most samples, the values of La//Yb are 21.6-41.9, exhibiting a wide range. TheδEu is 0.84-0.96, as a weak negative anomaly. The original mantle partitioning curves of the incompatible elements express high positive anomalies of Ba and Sr, weak positive anomalies of Nb, Ta, Sr, Sm and La, and small negative anomalies of Th, U, Zr and Hf. These curves are relatively steep, indicating high differentiation of strong and weak incompatible elements. MgO and Cr, Co and Ni are of same linearly positive correlation, while MgO and /(La//Yb/)N are of weak positive correlation. The incompatible elements, such as Rb, Ba, Th, U, La, Sr, Nd, Pb and Nb are all well correlated linearly. The partitioning curve of REE and curves of original mantle partitioning curves are between those of the Hannuoba basalt and Wudalianchi basalt. Comparison of the two areas shows that relatively high K and Na, rich big ion lithophile /(especially Ba and Sr/) and light RE characterize the basalt of the Nuomong River-Quile River area. While in the Halaha River-Chuo’er River area, the basalt is relatively rich in HREE, more close to OIB in the character. A series of evidence suggests that the alteration of late magma and crustal hybridization have little influence on the magma composition of the study region. and the crystal differentiation is confined to a small amount of clinopyroxene and olivine. The difference in rock geochemistry shows that these two areas have distinct magma sources for volcanoes. For the basalt of the Halaha River-Chuo’er River area, the REE, curves of incompatible elements, Sr-Nd isotope, Nb//U, La//Nb and Ba//Nb are also close to that of OIB, suggesting a similar asthenosphere in eastern China to OIB that has played an important role in the origin of basalt. On the other hand, the Ce//Pb of basalt in this area is much less than that of OIB and the Datong basalt. Meanwhile Ba//Nb and La//Nb are weakly positively correlated, and Sr isotope and Ce//Pb are also conspicuously linearly correlated. These lines of evidence may indicate that the magma source is the mixture of the astheonsphere and another source. The basalt of the Nuoming River-Quile River has low Ce//Pb, high ba//Nb and high La//Nb, from different from OIB, while highly coincide with the volcanic rocks at Wudalianchi, Erke Shan and Keluo volcanoes. It is inferred that its magma is originated from the base of old lithospheric mantle that had experienced metamorphism and enrichment, i.e. the asthenospheric material was not dominated for the source even if it had been involved. In sum, the basalt source of the Halaha River-Chuo’er River area is primarily the asthenosphere, of which the magma interacted with the base of the lithospheric mantle. That of the Nuomimg River-Quile River area is mainly the base of the lithospheric mantle. The asthenosphere in the study area has the following nature: Ce//Pb>22.7 , Ba//Nb<7.30 , La//Nb<0.57 ,87Sr//86Sr<0.7035, while for the base of the lithospheric mantle: Ce//Pb<13.7,Ba//Nb>28.3,La//Nb>1.19 in geochemistry. It is just these two different source features that cause the distinct petrology and geochemistry of basalts in the northern area and southern area of the study region. 5 Volcanic hazards Integrated evidence from geology and geochemistry suggests that the magma of the study area is primarily originated from the asthenosphere and lithosphere. At present the structures of crust and upper mantle are favorable to generation and eruption of magma in this region. Thus it is estimated that the future volcanic eruption is possible here. Based on the analysis of volcanic products, it is inferred that the future probable volcanic hazards would be caused by descending pyroclasts, pyroclastic outpouring flows, lava flows and gases from volcanic eruptions.

关 键 词： 大兴安岭 第四纪火山 活动历史 火山地质 地球化学

分 类 号： [P317]

领　　域： [天文地球] [天文地球]