【作者】 黄皓；

【导师】 薛怀民；

【作者基本信息】 中国地质科学院 ， 矿物学、岩石学、矿床学， 2012， 硕士

【摘要】 大别北缘的北淮阳地区以出露大量中生代火山岩为特征。相比邻区的大别地区花岗岩和长江中下游地区火山岩,北淮阳中生代火山岩研究比较薄弱。这严重制约了对大别碰撞后演化过程和东部岩石圈减薄机制的深入认识。本文综合北淮阳中生代火山岩的已有划分方案,本文将北淮阳火山岩分为四个组进行研究,自西向东依次为,毛坦厂组、响洪甸组、金刚台组、陈棚组。在此基础上,对北淮阳中生代火山岩进行岩石学、年代学和地球化学综合研究。结合前人工作,达成了以下认识：(1)火山作用之前的北淮阳前中生代浅变质岩群对比后可分为三部分,苏家河群定远组和卢镇关群小溪河组是一套记录了新元古代华南裂谷岩浆作用的变岩浆-沉积岩；信阳群南湾组和佛子岭群上段为一套俯冲带前的复理石建造；梅山群和杨山群中的部分石炭系地层是残留海相的类磨拉石建造。(2)北淮阳东部的毛坦厂组和西部的金刚台组构成北淮阳地区火山岩的主体,两者均为一套爆发-溢流相的火山岩,且在岩石组合、地球化学特征和成岩年龄上均可进行对比。两组火山岩的相似性说明北淮阳地区的火山岩主体形成于同一期岩浆活动。(3)对北淮阳中生代火山岩的锆石LA-ICPMS的测年结果显示,毛坦厂组东部的粗安岩年龄为128.6±0.6Ma；响洪甸组的霞石正长岩年龄为135.1±0.7Ma；金刚台组熔结凝灰岩年龄为127.9±0.8Ma,粗面安山岩的年龄为127.6±0.5Ma；陈棚组上部流纹岩的年龄为125.1±0.7Ma。结合近年来发表的高精度数据,本文将火山活动分为两期。第一期火山喷发(约127Ma-131Ma)延绵区内东西部地区,形成北淮阳火山岩的主体——毛坦厂组、金刚台组和陈棚组；第二期火山喷发(约122Ma)在中部地区形成了响洪甸组的超钾质火山岩。(4)在地球化学上,北淮阳中生代火山岩普遍具有富钾,富集轻稀土元素,轻重稀土分馏强烈,富集大离子亲石元素,贫高场强元素,发育典型的Nb、Ta槽,高的Isr值(0.70756-0.71054),极低的εNd(t)值(-12.66--21.1),中、古元古代的亏损地幔Nd模式年龄(1.5Ga-2.6Ga),较低的206Pb/204Pb值(15.975-17.363%)等特征。同位素对比进一步说明,北淮阳火山岩的源区是同碰撞期时已经富集的华北岩石圈地幔,经俯冲的扬子板块下地的壳物质(相当于崆岭群)加入改造而成。(5)北淮阳地区的第二期火山喷发形成的超钾质岩与第一期火山岩地球化学特征相似,但也表现出更富集钾质和轻稀土元素,同位素受地壳物质影响更低的特征。这一特征说明在火山活动的晚期,由于地幔源区俯冲地壳物质的消耗,部分熔融的岩浆更具幔源性质。综合前人的成岩模式,本文提出了北淮阳火山岩华北岩石圈包裹俯冲扬子地壳残片的模式。(6)通过北淮阳地区火山岩和长江中下游火山岩对比发现,北淮阳地区火山岩和长江中下游火山岩均为对早白垩世中国东部岩石圈减薄事件的响应。两处火山岩在岩石组合和地球化学方面的差异说明岩石圈减薄在不同构造环境下具有不同的机制,北淮阳地区软流圈的热烘烤加厚岩石圈地幔,导致大规模的部分熔融；长江中下游地区软流圈主动的热侵蚀使岩石圈拆沉而形成混合岩浆。更多还原

【Abstract】 The Beihuaiyang belt in the northern part of Dabie orogenic belt is characterized by the outcrop of a great amount of Mesozoic volcanic rocks. Compared with the neighboring area, such as the granite outcropping area of Dabie Orogen and volcanic rocks outcropping area in the middle and lower reaches of the Yangtze River, relatively less work has been done to the Mesozoic volcanic rocks in the Beihuaiyang belt. This in turn has greatly constrained the interpretation of the Dabie orogen’s post-collision evolution process and the thinning mechanism of the lithosphere in the east part of China. After considering and colligating the pre-existing divisiory scenario, this thesis studied mainly these four formations of the Beihuaiyang volcanic rocks which are (from west to east), the maotanchang Formation, the Xianghongdian Formation, the Jingangtai Formation and the Chenpeng Formation. Through a combined study of petrology, geochronology and geochemistry, we get mainly these several new findings:(1). The pre-Mesozoic rock-groups before the volcanism, which experienced low grade metamorphism, can be divided into three parts after comparison. The Dingyuan Formation of the Sujiahe Group and the Xiaoxihe Formation of the Luzhenguan Group is a set of meta-magmatic and sedimentary rocks which recorded the Nanhua-rift-magmatism in the Neoproterozoic; The Nanwan Formation of the Xinyang Group and upper Member of the Foziling Group is a set of pre-subduction flysch; The Carboniferous strata of part of the Meishan Group and Yangshan Group is a set of the relic of sea-facies molasse formation.(2). The Maotanchang Formation in the eastern part of Beihuaiyang and Jingangtai Formation in the western part of Beihuaiyang make up of the main part of the volcanic rocks in the area. Both of them are a set of explosion-effusive facies volcanic rocks and they resemble each other in aspects of rock assemblage, geochemical characteristics and the diagenetic ages of the volcanic rocks. The similarities of the characteristics of the two volcanic-rock formations suggest that the main body of the volcanic rocks of the Beihuaiyang belt was formed in one episode of magmatism.(3). LA-ICPMS zircon U-Pb dating results of the Mesozoic volcanic rocks in the Beihuaiyang belt show that the andesitoid of the Maodanchao Formation is formed at about128.6±0.6Ma. The midalkalite of the Xianghongdian Formation is formed at about135.1±0.7Ma. The flood tuff of the Jingangtai Formation is formed at about127.9±0.8Ma, the necrolite is formed at about127.6±0.5Ma. The rhyolite in the upper part of Chenpeng Formation is formed at about125.1±0.7Ma. Based on these dating results and the results obtained by other researchers, we divided the magmatism into two stages. The first-stage eruption happened mainly in the east and west part of the area at127-131Ma, which resulted in the formation of the main part of the Mesozoic volcanic rocks:the maotanchang Formation, the Jingangtai Formation and the Chenpeng Formation. The second-stage eruption happened in the middle part of the area at about122Ma, which resulted in the formation of ultrapotassic volcanic rocks.(4). Geochemical study shows that the Mesozoic volcanic rocks in the area are high in K, LREE and LILE, strong LREE and HREE fractionation, low in HFSE. Trace element spider diagram shows typical low curve in Nb and Ta. They are high in ISr ratio, and low in sNd(t). Palaeo-and Meso-proterozoic Nd depleted mantle model age ranges from1.5to2.6Ga. Also, they are low in206Pb/204Pb ratio. Isotopic study further constrains that the source of the Beihuaiyang volcanic rocks is enriched lithosphere mantle which experienced the adding of ancient crustal materials. In other words, the source is the lithosphere mantle of the north China with the mixture of the Kongling Group of the Lower crust of the Yangtze block.(5). The ultrapotassic rocks formed during the second eruption stage in the area have similar geochemical characteristics with that of rocks formed during the first stage. They are also high in K and LREE. But they show less influence by crustal materials. This characteristic suggests that during the late stage of the volcanism, due to the great consumption of the crustal materials in the mantle source, magma formed by partial melting inherited geochemical characteristics more from the mantle materials. After carefully summarizing the existing diagenesis model, we present in this dissertation a model that the source of the Beihuaiyang volcanic rocks is the lithosphere mantle of the north with the mixture of the relic of crustal materials of the Yangtze block, which is added to the mantle by the subduction of Yangtze toward North China Craton.(6). The comparison of the volcanic rocks in Beihuaiyang and the mid-and lower reach of the Yangtze River shows that the volcanic rocks in both these two areas are the result of the thinning of lithosphere in the eastern part of China in early Cretaceous. The differences in rock assemblage and geochemistry of the two area show that the thinning of lithosphere in different tectonic backgrounds has different mechanism. In Beihuaiyang belt, the aesthenosphere heated the thickened lithosphere and resulted the extensive partial melting. In the mid-and lower reach of the Yangtze River, the aesthenosphere actively intruded into the lithosphere and this resulted in the generation of mixed magma in the area.更多还原