【作者】 段政；

【导师】 邢光福；

【作者基本信息】 中国地质科学院 ， 矿物学、岩石学、矿床学， 2013， 硕士

【摘要】 浙闽相邻区晚中生代火山活动强烈,属中国东南沿海火山岩带的重要组成部分,但由于地形复杂交通不便,长期以来地质调查与科学研究工作薄弱,尤其缺乏系统的年代学、地层学和成因学研究,严重制约了东南沿海区域晚中生代火山活动成因及地质演化过程的深入认识。本文选择该区代表性晚中生代火山岩地层组开展了详细的野外调查和路线地质剖面观察,进行了系统的锆石U-Pb年代学、岩性岩相学及地球化学研究,在此基础上解剖了火山-沉积作用地质过程,探讨了火山岩成因及与壳幔作用的关系。根据岩石组合特征、同位素年龄、古生物化石组合、地层层序及其接触关系,本区晚中生代火山-沉积地层划分为上下两套火山岩系。浙江省境内的地层层序由下而上分别为：下火山岩系高坞组和西山头组,上火山岩系馆头组、朝川组、小平田组和泰顺组；福建省境内的境内的地层层序由下而上分别为：下火山岩系南园组和小溪组,上火山岩系黄坑组和寨下组。利用锆石LA-ICPMS U-Pb法测年技术,测得本区下火山岩系高坞组底部和顶部年龄分别为136.0Ma、133.2Ma,西山头组底部和顶部年龄分别为131.9Ma、127.9Ma,南园组底部和顶部年龄分别是143.1Ma、140.1Ma,小溪组顶部年龄为126.2Ma；本区上火山岩系小平田组顶部年龄为114.9Ma,泰顺组底部和顶部年龄分别为112.7Ma和111.2Ma；侵入泰顺组的外北山晶洞花岗岩时代为96.2Ma。同位素年代学和路线地质剖面资料表明,浙闽相邻区晚中生代火山活动起止时间为143～11Ma(高峰期143～126Ma),属早白垩世,并存在较长的火山活动沉寂期(126～116Ma),最终岩浆活动结束于96Ma左右,火山-沉积作用地质过程可明显划分为5个阶段；这对于开展省际早白垩世火山活动区域对比研究、深化东南沿海火山活动时空规律认识,具有重要意义。本区下火山岩系中,高坞组岩性为流纹质熔结凝灰岩；西山头组主要为流纹质熔结凝灰岩、粗面英安岩夹多层沉积岩；南园组下部为流纹质熔结凝灰岩,顶部为厚层碎斑熔岩;小溪组主要为流纹质熔结凝灰岩与砂泥岩互层。上火山岩系中,馆头组和朝川组分别为灰黄色凝灰质砂岩和紫红色泥岩；小平田组主要为流纹质晶玻屑凝灰岩和流纹质熔结凝灰岩夹紫红色泥岩；泰顺组为本区晚中生代最晚期火山-沉积地层,为灰色流纹质晶玻屑熔结凝灰岩和深灰色泥岩互层。地球化学分析表明,本区下火山岩系为大面积展布的长英质火山岩,属弱过铝质高钾钙碱性系列,富集大离子亲石元素(如Cs、Rb、Ba, Th、K、Pb)和LREE,无明显Eu异常,亏损高场强元素Nb、Ta、Ti、P,有较低的(176Hf/177Hf)i值(0.282098～0.282577)和较低的εHf(t)值(-20.43～-4.51),锆石Hf同位素二阶段模式年龄TDM2集中在1.8-2.2Ga之间；首次在西山头组中发现东南沿海罕见的高Sr粗面英安岩,以属橄榄安粗岩系列、富集Sr、Ti、Co、Ni和极低的εHf(t)而区别于其它长英质火山岩,推断由基性岩浆经结晶分异形成。上火山岩系为局限于断陷盆地内的长英质火山岩,属偏铝质高钾钙碱性系列,负Eu异常明显,εH(t)值也高于下火山岩系。结合区域前寒武纪变质基底时代与组份,推断本区白垩纪上下火山岩系均形成于活动大陆边缘环境,主要由幔源岩浆底侵导致的古元古代陆壳基底部分熔融形成,底侵基性岩浆部分可演化形成高Sr流纹质岩浆,喷发形成西山头组粗面英安岩;比较而言,上火山岩系形成过程中经历了更强烈的壳幔源岩浆混合作用。根据锆石εHf(t)值对比发现,区域上下两套火山岩系的成因与壳幔作用关系密切,而壳幔作用存在明显的时空差异性,具体表现为：时间上由早白垩世早期到晚期，上火山岩系形成过程中的壳幔作用明显强于下火山岩系；空间上由南往北,壳幔作用均表现为逐渐减弱趋势,可能反映古太平洋板块的俯冲作用是由南往北依次展开的。更多还原

【Abstract】 Late Mesozoic volcanism was intense in the area of Zhejiang-Fujian boundary, which constitutes an important part of China southeast costal volcanic rock belt. However, due to its geographical inconveniencies, geological investigations and scientific researches in this area have long been in a low degree, particularly systematic studies of chronology, stratigraphy and petrogenesis. Therefore, understanding of late Mesozoic volcanism and geological evolution of southeast coast is seriously hampered. The thesis chooses typical late Mesozoic volcanic formations in this area for field investigations and observations of route geological profiles. It carries out systematic studies of zircon U-Pb chronology, lithology, petrography and geochemistry, accordingly analyses geological process of volcano-sedimentary, and discusses the relationship between petrogenesis of volcanic rocks and crust-mantle interaction.Based on the characteristics of rock composition, isotopic ages, fossil assemblages, stratigraphic sequence and contact relation between strata, late Mesozoic volcano-sedimentary strata in the area of Zhejiang-Fujian boundary could be divided into upper and lower volcanic rock series. From bottom to top, stratigraphic sequence inside Zhejiang Proince are Gaowu Formation and Xishantou Formation of lower volcanic rock series, Guantou Formation, Chaochuang Formation, Xiaopingtian Formation and Taishun Formation of upper volcanic rock series; while for Fujian province, it is respectively Nanyuan Formation and Xiaoxi Formation of lower volcanic rock series, Huangkeng Formation and Zhaixia Formation of upper volcanic rock series.Using zircon LA-ICPMS U-Pb chronology, the paper gets different ages for the two series in the study area. Respectively, the bottom and top ages of Gaowu Formation of lower volcanic rock series are136.0Ma,133.2Ma; the bottom and top ages of Xishantou Formation131.9Ma,127.9Ma; the bottom and top ages of Nanyuan Formation143.1Ma,140.1Ma; the top age of Xiaoxi Formation126.2Ma. While for the upper volcanic rock series, the top age of Xiaopingtian Formation is114.9Ma; the bottom and top ages of Taishun Formation112.7Ma,111.2Ma; the age of outside Beishan miarolite intruded into Taishun Formation96.2Ma. The isotopic chronology and route geological profiles show that, volcanic activities in the study area took place during143～111Ma (with a peak between143～126Ma), i.e., Early Cretaceous, in which occurred a long intermission in126～116Ma and an end of magmatic activity in ca.96Ma. The volcano sedimentation process can be divided into five stages, which is conducive to understand temporal and spatial evolutions of Early Cretaceous volcanism in coastal Southeast China.The lithology for the lower volcanic rock series are mainly rhyolitic ignimbrite for Gaowu Formation; rhyolitic ignimbrite, trachyandesite bedded with sedimentary rocks for Xishantou Formation; rhyolitic ignimbrite and thick layer of porphyroclastic lava for the bottom and top of Nanyuan Formation respectively; rhyolitic ignimbrite interbedded with sand and shale for Xiaoxi Formation. For the upper volcanic rock series, Guantou Formation and Chaochuang Formation are respectively sallow tuffaceous sandstone and red mud; Xiaotian Formation mainly rhyolitic vitric tuff and rhyolitic ignimbrite bedded with red mud; Taishun Formation, which is the most advanced Late Mesozoic volcano-sedimentary strata in this area, mainly rhyolitic crystal vitric ignimbrite interbedded with Dark grey mudstone. Geochemical analyses imply that the lower volcanic rock series are widely-spread felsic volcanic rocks, weakly peraluminous high-k calc-alkaline, enriched in large ion lithophile elements (such as Cs、Rb、Ba、Th、K、Pb) and LREE, without evident Eu anomaly, short of high field-strength elements Nb、Ta、Ti、P, low (176Hf/177Hf), value (0.282098～0.282577) and low εHf.(t) value (-20.43～4.51), Zircon Hf isotopic model age of stage two TDMt concentrating between1.8and2.2Ga. For the first time, low Sr trachyandesite rarely seen in southeast coast is found in Xishantou Formation. It belongs to Shoshonite series, enriched in Sr, Ti, Co, Ni, with low εHf (t) value, which clearly differentiate it from other felsic volcano rocks. It is probably formed by the fractional crystallization of basalt magma. The upper volcanic rock series are felsic volcano rocks restricted in graben basin, high K calc-alkaline metaluminous, obvious negative Eu anomaly, higher εHf(t) value than the lower volcanic rock series. According to the age and composition of the pre-Cambrian metamorphosed basement of the study area, the paper holds that the cretaceous upper and lower volcanic rock series all developed in active continental margin environment. The underplating mantle-derived magma caused partial melting of Proterozoic basement, and underplating magma could evolve into low Sr Rhyolitic magma, the effusion of which formed trachyandesite of Xishantou Formation. Comparatively, the development of upper volcanic rock series underwent more intense crust-mantle-derived magmatic mixing.Through comparison of zicron εHf(t) values, this thesis discovers that the formation of the two volcanic rock series in the study area is closely related to crust-mantle interaction, which has evident spatio-temporal differences:temporally, from the early period of early Cretaceous to the late period, the crust-mantle interaction is obviously more intense in the formation of upper volcanic rock series; spatially, from south to north, the crust-mantle interaction weakens gradually, probably in accordance with the tendency of the subduction of the ancient Pacific plate.更多还原