【作者】 王国强；

【导师】 徐学义；

【作者基本信息】 长安大学 ， 矿物学、岩石学、矿床学， 2015， 博士

【摘要】 北山地区位于中亚造山带南缘,其古生代构造格局的探讨是该区乃至整个中亚造山带研究的热点问题。蛇绿岩和火山岩研究作为构造单元划分和构造演化研究的重要支撑部分,在形成时代和构造环境方面存在明显争议。本文通过北山不同蛇绿岩(带)以及奥陶纪-泥盆纪火山岩的同位素年代学、岩石地球化学研究对北山蛇绿岩属性、形成时代、构造环境以及火山岩的形成环境进行探讨,结合现有研究基础,恢复和重建北山早古生代洋陆格局和演化历史。北山地区发育四条近东西向产出的蛇绿岩带,由北向南依次为红石山-百合山-蓬勃山、芨芨台子-小黄山、红柳河-牛圈子-洗肠井和辉铜山-帐房山蛇绿岩带。本论文对北山地区蛇绿岩中的辉长岩进行锆石LA-ICP-MS U-Pb定年,结果表明辉铜山蛇绿岩形成年龄为446.1±3.0 Ma,帐房山蛇绿岩形成年龄为362.6±4.0 Ma、牛圈子蛇绿岩形成年龄为446.5±4.0 Ma、芨芨台子蛇绿岩形成年龄为321.2±5.7 Ma、红石山蛇绿岩形成年龄为346.6±2.8 Ma;岩石地球化学研究表明,辉铜山、红柳河-牛圈子-洗肠井、红石山-百合山-蓬勃山蛇绿岩属MOR型。构造环境分析认为,红柳河-牛圈子-洗肠井蛇绿岩带所代表的洋盆为早古生代有限洋盆,不具有板块分割意义;帐房山、芨芨台子-小黄山、红石山-百合山-蓬勃山蛇绿岩为晚古生代红海型洋盆环境的产物。北山出露大量古生代火山-沉积岩系,其中奥陶纪火山岩主要分布于南部的花牛山、洗肠井等处和北部的伊哈托里、希热哈达等处;志留纪火山岩系出露最为广泛,主要分布在南带的公婆泉、东七一山等处;泥盆纪这一洋陆转换关键时段的火山岩主要出露于墩墩山一带。本论文岩石地球化学研究表明:(1)奥陶纪火山岩属拉斑系列,其中花牛山与洗肠井火山岩属蛇绿岩的组成部分,伊哈托里和希热哈达玄武岩表现为轻稀土富集、重稀土平坦、铕异常不明显的特征。Nb、Ta、Ti等高场强元素显示不同程度的亏损,具有类似于岛弧火山岩的特征。具原始地幔标准化Th/Nb比值(>>1)、低Nb/La比值(﹤1)、Zr/Nb比值类似于N-MORB及原始地幔标准化蛛网图中缺乏明显的Nb、Ta、Ti负异常“隆起”的特征。显示了北山北部伊哈托里、希热哈达地区火山岩浆在上升过程中遭受地壳混染作用,由此表现出岛弧火山岩的特征。构造背景分析认为,在中-晚奥陶世时伊哈托里、希热哈达处于伸展拉张的背景之下,该区火山岩属明水-旱山微陆块北缘裂谷火山岩;(2)志留纪公婆泉群火山岩除东七一山矿南样品主要落入钙碱系列外,其余绝大多数属拉斑系列。公婆泉群火山岩均表现为轻稀土富集、重稀土亏损型,伊哈托里火山岩相对于东七一山矿南、大狐狸山、四顶黑山以及双沟山北等地的火山岩而言,轻重稀土分馏程度较低。公婆泉群火山岩的大离子亲石元素(LILE)K、Rb、Ba等显示一定程度的富集,高场强元素(HFSE)如Ta、Nb、Ti等相对亏损(尤其是伊哈托里火山岩),显示了与俯冲有关的火山岩特征。志留纪公婆泉群玄武岩样品的εNd(t)值较高且集中,均为正值,而(87Sr/86Sr)t值则较低(0.70340~0.70519),部分样品Sr同位素比值明显增高的主要原因是源区中俯冲洋壳变质脱水流体的加入。结合本文微量元素和Sr-Nd同位素研究,认为北山志留纪公婆泉群玄武岩的地幔源区应为受俯冲洋壳板片沉积物流体及板片熔体双重交代的楔形亏损地幔源区;(3)泥盆纪三个井组火山岩属钙碱性系列,墩墩山群火山岩分属拉斑系列和钙碱系列。三个井组火山小学校岩表现出轻稀土富集、重稀土亏损的右倾型特征,具有微弱铕负异常特征,墩墩山群火山岩轻、重稀土分异程度比三个井组火山岩更为强烈,表现出轻稀土富集、重稀土亏损的右倾型特征,具有明显的负异常特征。墩墩山群英安岩的成岩时代为367±10 Ma,属晚泥盆世。三个井组火山岩的εNd(t)大于0,而墩墩山群火山岩εNd(t)小于0,从εNd(t)值方面显示前者属幔源源区性质,后者则具地壳或富集地幔的性质,同位素研究表明三个井组和墩墩山群火山岩并非同一岩浆源区不同演化阶段的产物,应具有不同岩浆源区。结合火山岩构造环境判别图解、三个井组和墩墩山群沉积特征以及蛇绿岩研究成果,认为北山墩墩山地区早-中泥盆世三个井组和晚泥盆世墩墩山群是北山早古生代洋盆演化过程中洋-陆转化的地质记录,三个井组火山岩为洋盆向南俯冲引发的岛弧岩浆作用的产物,而墩墩山群火山岩则是北山早古生代洋盆碰撞造山后裂谷拉伸作用的产物。本论文结合已有研究基础和本次蛇绿岩及火山岩研究成果,重建了北山古生代构造演化模式。即认为北山古生代大地构造演化依次经历了超大陆裂解及洋陆转换阶段、板内伸展及裂谷盆地演化阶段,中二叠世后则进入真正意义的陆内演化阶段。更多还原

【Abstract】 Beishan is located in the South margin of CAOB(Central Asia Orogenic Belt), the research of Paleozoic Tectonic evolution has become the hot issues in this area. The study of ophiolite and volcanic rocks are very important part of tectonic unit division and tectonic evolution, however, there is a significant dispute about the chronology and tectonic setting. The method of chronology and petrogeochemisry was used for the research of structural attribute, Formation age, tectonic setting of the several ophiolites belt and tectonic setting of volcanic rocks in this article. In addition, on the basis of existing research, the Early Paleozoic ocean-land transition and tectonic evolution history of Beishan was retrieved and reconstructed by the present author.There are four ophiolite belts(From north to south: Hongshishan-Baiheshan-Pengboshan, Jijitaizi-Xiaohuangshan, Hongliuhe-Niujuanzi-Xichangjing and Huitongshan-Zhangfangshan) which are nearly in E-W direction in Beishan region. The author conducted the chronological study of zircons from gabbros for the several ophiolite belt by means of LA-ICP-MS U-Pb, the weighted mean ages were obtained as following 446.1±3.0 Ma(Huitongshan ophiolite), 362.6±4.0 Ma(Zhangfangshan ophiolite), 446.5±4.0 Ma(Niujuanzi ophiolite), 321.2±5.7 Ma(Jijitaizi ophiolite) and 346.6±2.8 Ma(Hongshishan ophiolite); Analyses of volcanic rocks geochemistry show that the Huitongshan-Huaniushan, Hongliuhe-Niujuanzi-Xichangjing and Hongshishan-Baiheshan-Pengboshan ophiolite belts belong to the type of MORB. The limited ocean basin which represented by Hongliuhe-Niujuanzi-Xichangjing ophiolite belt was formed in the Early Paleozoic without the significance of subdivision of plates. The other ophiolite belts(Zhangfangshan, Jijitaizi-Xiaohuangshan and Hongshishan-Baiheshan-Pengboshan) are the product of the rift extention in Late Paleozoic, which are the geological record of the embryonic ocean crust.There are a lot of the Paleozoic volcanic-sedimentary rocks outcropping in Beishan area, Ordovician volcanic rocks are outcropped in Huaniushan and Xichangjing in southern Beishan, Yihatuoli and Xirehada in northern Beishan; Silurian volcanic rocks are most widely exposed in Gongpoquan and Dongqiyishan area; the Devonian volcanic rocks that is the key period of ocean-land transition are mainly exposed in Dundunshan area. The followings are the main achievement by means of petrogeochemistry:(1) the author consider that volcanic rocks in this area mainly belong to tholeiite series, the samples collected from Huaniushan and Xichangjing are the part of the ophiolite, the samples from Yihatuoli and Xirehada are enriched in LREE, flat in HREE and without obvious Eu anomaly in the chondrite-normalized REE pattern, and slightly depleted in HFSE(Nb, Ta, Ti), which are similar with those of the island arc volcanic rocks. However, they have ratio values of(Th/Nb)N>>1 and Nb/La﹤1, as well as Zr/Nb similar to the N-MORB, and no depletion in HFSE(Nb, Ta and Ti), which suggested that the volcanic rocks were contaminated by the crust. Combined with the research of tectonic evolution in this area, the author concluded that Ordovician volcanic rocks in Yihatuoli and Xirehada formed at a extensional rifting setting with the plate;(2) the samples collected from Silurian Gongpoquan Group mainly belong to tholeiite series, except Dongqiyishan samples which belong to calc-alkaline series. The volcanic rocks are chatacterized by enrichment in LREE and LILE(K,Rb and Ba),and have negative anomaly in Nb, Ta, Ti(especially Yihatuoli), which suggest a characteristic of volcanic rocks related to subduction. PositiveεNd(t)and lower(87Sr/86Sr)t values(0.70340~0.70519)suggest that both fluid-sediments and silicate melts derived from subducted oceanic slab are involved in the genesis of basalt in Gongpoquan Group.(3) the samples collected from Devonian Sangejing Formation mainly belong to calc-alkaline series, and Devonian Dundunshan Group belong to tholeiite and calc-alkaline series. The volcanic rocks of Sangejing Formation are characterized by enrichment in LREE, depletion in HREE and negative anomaly in Eu slightly, Dundunshan Group are characterized by stronger enrichment in LREE and more obvious negative anomaly in Eu than Sangejing Formation. The volcanic rocks of Dundunshan Group were formed at 367 Ma±10 Ma, corresponding to Late Devonian. In addition, the volcanic rocks from Devonian Sangejing Formation have a positive εNd(t)which suggest a depleted mantle-related source, but the volcanic rocks from Devonian Dundunshan Group have negative εNd(t)which sugggest a crust or enriched mantle source. Combined with the tectonic setting discrimination diagram with trace element, sedimentary characteristics of Sangejing Formation and Dundunshan Group, and the research of ophiolite, it’s suggested that Sangejing Formation formed in an island arc setting owing to the southward subduction of Beishan limited ocean, whereas the Dundunshan Group were produced by rift extension and entered into a new phase, i.e.d Late Paleozoic intraplate extention.Based on the existing research and this study, the Paleozoic tectonic evolution history of Beishan was retrieved and reconstructed in this paper. In other word, Beishan experienced a history of Paleozoic tectonic evolution from supercontinent breakup and ocean-continent transition, to intraplate extention and rift basin development. The real stage of intracontinental evolution started at Early Permian.更多还原