【作者】 杨永峰；

【导师】 董传万；

【作者基本信息】 浙江大学 ， 构造地质学， 2008， 硕士

【摘要】 福建沿海地处中国东南活动大陆边缘,中生代岩浆作用十分强烈,是中国东南部重要的构造-岩浆活动带。在福建沿海大片晚中生代花岗岩及火山岩中,普遍出露中-基性岩墙群,它们起源于地球深部（地幔或下地壳）,侵位于不同的地壳层次,蕴藏着大量岩浆活动、壳-幔相互作用等信息。本论文以该地区晚中生代中-基性岩墙群为主要对象,研究了它们的岩石学、地球化学及同位素年代学等,对岩浆源区的性质、构造环境、成因等进行了讨论,获得以下重要认识:1、福建沿海地区晚中生代中-基性岩墙成群产出,侵入到晚中生代花岗岩及火山岩中,岩性有辉绿岩、角闪辉绿岩和石英闪长岩等,以辉绿岩类为主,产状较陡,北东、近东西及北北东走向,大多未发生变质或变形,保持了岩墙形成时的应力状态。2、岩墙群代表性样品锆石SHRIMP U-Pb同位素分析结果表明,它们的形成年龄都比较接近,形成在87～96Ma之间,说明它们是近时侵入的,都是晚中生代晚白垩世岩浆活动的产物。3、岩墙群的SiO₂含量为50.22～61.10%,属于基性和中基性岩类,以基性为主;它们的Al₂O₃含量普遍高于15%,属高铝玄武岩类;具有较低的镁指数（Mg^#=42.77～62.28）及TiO₂含量（0.97～2.41%）,说明它们经历了镁铁质矿物的结晶分异作用,也指示它们不是由原始玄武岩浆结晶;Na₂O/K₂O都大于1,显示出低钾的特点;岩石化学判别,岩墙群岩石属亚碱性系列。4、岩墙群的轻稀土元素富集,稀土配分曲线较平滑,整体向右倾斜,无明显Eu异常;明显富集大离子亲石元素及来自板块俯冲的Ce,而高场强元素显示出不同程度的亏损;Sr-Nd同位素具有高Sr、低Nd特点,Sr初始比值(⁸⁷Sr/⁸⁶Sr)_i与ε_Nd（t）呈很好的负相关,显现出典型的岛弧特征,表明玄武岩浆起源于被俯冲带流体交代过的岩石圈地幔,并受到地壳物质的混染。5、岩墙群形成于弧后拉张环境,岩浆起源于地幔楔橄榄岩的湿的部分熔融。早白垩世晚期-晚白垩世（90Ma左右）,福建沿海从拉张进入裂解或裂谷阶段,在拉张的构造环境中,底侵、滞留并经过广泛结晶分异的玄武质岩浆沿燕山晚期花岗岩及火山岩的节理迅速上侵,最终形成中-基性岩墙群。福建沿海地区晚中生代中-基性岩墙群是晚中生代岩浆活动的最后产物,它们的形成是浙闽沿海活动大陆边缘重要的构造-岩浆事件。对它们的组成和成因进行研究,有助于对中国东南部沿海地区晚中生代岩浆活动规律的深入了解?更多还原

【Abstract】 The coastland of Fujian Province is located in the active continent margin of SE China.The magmatism in the Mesozoic Era was very intense and this area was an important tectonic-magmatic activity belt of SE China.In the Late Mesozoic granite and volcanic rocks,there appears basic-intermediate dike swarms universally.They originated deep earth（mantle or lower crust） and emplaced different crustal layer, implying a lot of magmatic activity,crust-mantle mutual action information.Taking the Late Mesozoic basic-intermediate dike swarms from this area as the main object, we have studied their Petrology,Geochemistry and Isotopic Chronology,and discussed the property of their magmatic source,tectonic setting and origin as well. The significant conclusions acquired are as follows:1,The Late Mesozoic dike from the coastland of Fujian Province appears as clusters,intruding into the Late Mesozoic granite or volcanic rocks,and the lithologies are diabase,hornblende diabase and quartz-diorite etc,mainly diabase. The mode of their occurrence is mainly precipitous and they tend towards NE, approximately EW or NNE.The dike swarms did not suffer metamorphose or distortion,holding the stress state as the dike formed.2,The zircon SHRIMP U-Pb isotopic analytical results of the representative samples indicate that their forming ages are all comparatively adjacent,ranging from 87Ma to 96Ma,which illustrates they emplaced approximately the same time and they are all the products of the magmatic activities in the Late Cretaceous,Late Mesozoic Era.3,The SiO₂ content of the dike swarms varies from 50.22%to 61.10%and they are basic-intermediate rock series,mainly basic.The Al₂O₃ content is generally higher than 15%and they are high aluminum basaltic rocks.The lower magnesium index（Mg^#=42.77～62.28） and TiO₂ content（0.97～2.41%） illuminate that they su圷?ed magnesium-iron minerals’ fractional crystallization and that they did not crystallize from original basaltic magma.The values of Na₂O/K₂O are all above 1, indicating lower K character.Chemically discriminated,these dike swarms are sub-alkaline rock series.4,The dike swarms are enriched in light rare earth element and the rare earth element assemblaging curves are relatively smooth,bending rightward in general and showing no Eu abnormity.They are enriched in LILE and Ce（Coming from the plate subduction） and depleted in HFSE in different degree.The Sr-Nd isotopes are rich in Sr but poor in Nd and the original ratio of Sr(⁸⁷Sr/⁸⁶Sr)_i is in good negative correlation withε_Nd（t）.All of these indicate that they are in typical island-arc character and the basaltic magma generated from the lithosphere mantle which is metasomatic by subduction belt liquid and contaminated by the lithosphere.5,The dike swarms formed in back-arc extension setting and the magma generated from the mantle wedge peridotite’ waterish partial melt.From the late stage of the Early Cretaceous Era to the Late Cretaceous Era（about 90Ma）,the coastland of Fujian Province changed from extension setting to crack or rift valley stage.In the extension tectonic setting,the basaltic magma that suffered bottom emplacement,detention and extensive fractional crystallization emplaced upwards rapidly along the joint of the Late Yanshan Period granite and volcanic rocks,and then finally formed the basic-intermediate dike swarms.The Late Mesozoic basic-intermediate dike swarms from the coastland of Fujian Province were the last products of the Late Mesozoic magmatic activity and their formation was an important tectonic-magmatic events of the active continental margin of the coastland in Zhejiang and Fujian Province.The discussion on the composition and origin of the basic-intermediate dike swarms from the coastland of Fujian Province would help to get a profound comprehension of the Late Mesozoic magmatic activity rule of the coastland in SE China.更多还原