【作者】 王惠初；

【导师】 莫宣学；

【作者基本信息】 中国地质大学（北京） ， 矿物学、岩石学、矿床学， 2006， 博士

【摘要】 柴北缘早古生代碰撞造山带呈北西向展布于祁连山与柴达木盆地之间,是我国中央造山带的重要组成部分,也是我国仅有的几个存在高压-超高压变质岩的地点之一。本文在柴北缘关键部位1∶5万和1∶25万区调填图基础上,以板块构造和大陆造山带理论为指导,着重研究了造山带的内部结构、造山过程中的岩浆作用和变质作用,并进行了较系统的锆石U-Pb年代学测试分析。研究结果表明,柴北缘早古生代造山带可以划分为三个主要构造单元:(1)鱼卡河-沙柳河高压-超高压变质构造带,主要由新元古代花岗片麻岩、鱼卡河(沙柳河)岩群变质表壳岩和榴辉岩等高压-超高压变质岩组成;(2)滩间山蛇绿杂岩-岛弧构造带,由蛇绿杂岩、岛弧火山岩、弧后盆地火山-沉积建造和岛弧深成岩等岩石组合构成;(3)欧龙布鲁克陆块,具基底和盖层的双层结构,基底由古元古代的德令哈杂岩组成,盖层为全吉群等未变质盖层。与造山过程相关的岩浆作用包括:(1)蛇绿杂岩组合,以超镁铁质-镁铁质堆晶岩类为主,少量变质橄榄岩、枕状熔岩、硅质岩和浅色花岗岩,岩石地球化学特点指示其属SSZ型蛇绿岩,;(2)岛弧火山岩组合,为一套浅变质的钙碱性火山岩,以中基性火山岩为主,少量酸性火山岩,形成时代在450Ma之前;(3)弧后盆地火山岩组合,与一套类复理石建造的陆缘碎屑岩共生,在～435Ma发生了变质作用;(4)岛弧深成岩组合,为一套钙碱性的石英闪长岩-英云闪长岩-花岗闪长岩-二长花岗岩,从岛弧向大陆一侧具有碱性程度增加的趋势,形成时代上可划分为465-435Ma和435-400Ma两个阶段;后一阶段有少量同碰撞型花岗岩产出。变质作用研究表明,柴北缘造山带具有双变质带结构,即鱼卡河-沙柳河高压-超高压变质带和滩间山岛弧中低压变质带。在高压-超高压变质带中,包括变质泥质岩和花岗片麻岩在内的许多岩石都遭受过高压变质作用,许多榴辉岩达到了超高压变质条件;但变质温度和P-T-t路径在高压-超高压变质带的横向和纵向上均有所不同;纵向上,野马滩和锡铁山一带的榴辉岩折返过程中经历麻粒岩相的变质叠加,而绿梁山地区的榴辉岩仅达到角闪岩相的温度条件;横向上表现为靠近缝合线一侧榴辉岩形成的温压条件较高。岛弧构造带以中低压角闪岩相-绿片岩相为主,其中弧后盆地建造靠近高压变质带一侧为(高)角闪岩相变质,远离缝合带变质程度降为绿片岩相。通过岩石学、地球化学、同位素年代学等方面的综合分析建立了柴北缘造山带的年代事件序列,以及造山过程与岩浆作用和变质作用之间的内在联系,探讨了柴北缘造山带从洋壳俯冲到陆-陆碰撞的演化过程。更多还原

【Abstract】 The northwest-trending Early Paleozoic collisional orogenic belt located on northernmargin of the Qaidam basin is a part of the Central Orogenic belt and one of a few places inChina where HP-UHP metamorphic rocks exposed. Based on the data of 1:50 000 and 1:250000 regional mapping and the theories of plate tectonics and continental orogeny, this paperstudies basically the framework of the orogen, magmatism and metamorphism duringorogenic process and presents zircon U-Pb isotopic ages of igneous and metamorphic rocksfrom the orogen.The Early Paleozoic orogenic belt consists of three tectonic units: (1) the Yuqiahe-Shaliuhe HP-UHP metamorphic belt, which is predominantly composed of Neoproterozoicgranitoid gneisses and metamorphic supercrutal rocks (Yuqiahe/Shaliuhe Group complex)with minor HP-UHP metamorphic rocks such as eclogite;(2) the Tanjianshan ophiolitecomplex – volcanic arc belt, which consists of ophiolite complex, volcanic island arc,back-arc basin assemblage and arc granitoid rocks;(3) the Olongbruk micro-block, whichhas a double-layered structure, i.e. a metamorphic basement composed of the Delinghacomplex and a cover composed of the Quanji Group.The igneous petrotectonic assemblages formed in the orogenic process include: (1)ophiolite complex assemblage, composed predominantly of the ultramafic-mafic cumulates,minor metamorphic peridotite, pillow lava, cherts and leucocratic granites as well.Geological and geochemical characteristics indicate that ophiolite complex is classified as“SSZ”(supra-subduction zone) type;(2) island arc volcanic assemblage, a suit oflower-grade metamorphic calc-alkaline volcanics, composed mainly of intermediate-basicvolcanic rocks with minor acidic volcanic rocks and formed before ~450Ma;(3) back-arcbasin volcanic assemblage, associated with a suit of terrigenous flysch, which underwentmetamorphism at ~435Ma;and (4) arc granitoid rocks assemblage, a suit of calc-alkalinequartz-diorite, tonalite, granodiorite and granite, whose alkalinity increases toward continent,and formed during two stages of 465-435Ma and 435~400Ma, and accompanied bysyn-collisinal granites at later stage.The study of metamorphism indicates that the orogen has a paired-metamorphic beltstructure, i.e., co-existence of Yuqiahe-Shaliuhe HP-UHP metamorphic belt and Tanjianshanisland-arc lower-medium pressure metamorphic belt. Rocks in the HP-UHP metamorphicbelt, including pelites and granitic gneisses, was probably all or partly subducted into thedeep interior of the lithosphere and underwent HP metamorphism, but some eclogites wentup to the UHP metamorphic condition. Metamorphic temperatures and P-T-t paths ofHP-UHP metamorphic rocks are spatially diverse. Longitudinally, the P-T-t path of eclogitesfrom Yematan and Xitieshan differ from the one in Lüliangshan area. While the former hadbeen overprinted by granulite facies metamorphism during exhumation, the later onlyreached amphibolite facies metamorphism. Laterrally, the metamorphic temperature andpressure increase toward the geosuture. The island-arc belt underwent chiefly LP-MPamphibolite-greenschist facies metamorphism, and the metamorphic facies in the back-arcassemblage changed from high-amphibolite facies to greenschist facies from island-arc tocontinent.Based on the studies of petrology, geochemistry and zircon U-Pb isotopic dating, wehave established the sequence of chronotectonic events in the orogenic belt and the geneticrelationship among magmatism, metamorphism and orogenesis, and discussed the evolutionof the Early-Paleozoic Orogenic belt from the period of subduction of the oceaniclithosphere to that of the continent-continent collision.更多还原