Post-Collisional Transition from Subduction- to Intraplate-type Magmatism in the Westernmost Mediterranean: Evidence for Continental-Edge Delamination of Subcontinental Lithosphere

Post-collisional magmatism in the southern Iberian and northwesternAfrican continental margins contains important clues for theunderstanding of a possible causal connection between movementsin the Earth's upper mantle, the uplift of continental lithosphereand the origin of circum-Mediterranean igneous activity. Systematicgeochemical and geochronological studies (major and trace element,Sr–Nd–Pb-isotope analysis and laser ⁴⁰Ar/³⁹Ar-agedating) on igneous rocks provide constraints for understandingthe post-collisional history of the southern Iberian and northwesternAfrican continental margins. Two groups of magmatic rocks canbe distinguished: (1) an Upper Miocene to Lower Pliocene (8·2–4·8Ma), Si–K-rich group including high-K (calc-alkaline)and shoshonitic series rocks; (2) an Upper Miocene to Pleistocene(6·3–0·65 Ma), Si-poor, Na-rich group includingbasanites and alkali basalts to hawaiites and tephrites. Maficsamples from the Si–K-rich group generally show geochemicalaffinities with volcanic rocks from active subduction zones(e.g. Izu–Bonin and Aeolian island arcs), whereas maficsamples from the Si-poor, Na-rich group are geochemically similarto lavas found in intraplate volcanic settings derived fromsub-lithospheric mantle sources (e.g. Canary Islands). The transitionfrom Si-rich (subduction-related) to Si-poor (intraplate-type)magmatism between 6·3 Ma (first alkali basalt) and 4·8Ma (latest shoshonite) can be observed both on a regional scaleand in individual volcanic systems. Si–K-rich and Si-poorigneous rocks from the continental margins of southern Iberiaand northwestern Africa are, respectively, proposed to havebeen derived from metasomatized subcontinental lithosphere andsub-lithospheric mantle that was contaminated with plume material.A three-dimensional geodynamic model for the westernmost Mediterraneanis presented in which subduction of oceanic lithosphere is inferredto have caused continental-edge delamination of subcontinentallithosphere associated with upwelling of plume-contaminatedsub-lithospheric mantle and lithospheric uplift. This processmay operate worldwide in areas where subduction-related andintraplate-type magmatism are spatially and temporally associated. KEY WORDS: post-collisional magmatism; Mediterranean-style back-arc basins; subduction; delamination; uplift of marine gateways     

青海西秦岭地区晚中生代基性火山岩地球化学特征及构造意义

对青海省同仁县多禾茂地区火山岩进行系统的元素同位素地球化学研究表明:该地区白垩纪地层中发育的中基性火山岩,属于碱性火山岩系,岩石类型主要为玄武岩,该套玄武岩具有低且变化较小的Si O2质量分数(44.98%~48.20%),低MgO的质量分数高,变化较大(8.15%~10.98%),具有较高的Cr(208×10-6~418×10-6)和Ni(166×10-6~231×10-6)质量分数。所有样品都表现出轻稀土富集的右倾平滑分布模式,轻重稀土分异中等,(La/Yb)CN=10.09~27.2,重稀土弱分异,(Gd/Yb)CN=2.92~4.67,Eu异常不明显,Eu*/Eu=0.99~1.04。Ba,Nb-Ta和Th-U不亏损,Rb,K相对亏损,Sr正异常。该区火山岩具有亏损的Sr-Nd同位素组成,Sr-Nd同位素组成变化范围不大[87Sr/86Sr(t)=0.7033~0.7039,εNd(t)=6.13~8.03,t=110 Ma],与Hawaii-OIB的Sr-Nd同位素组成相似。其所有地球化学特征表明该火山岩具有类似OIB的特征,可能是含石榴子石橄榄岩低度部分熔融的产物。结合对西秦岭地质构造背景和演化历史的分析,暗示该地区晚中生代岩浆源区来源于软流圈,其起源可能与岩石圈拆沉作用,软流圈地幔上涌和岩石圈伸展减薄有关,可为秦岭大别造山带中生代岩石圈演化提供了有利的直接证据。     

Constraints from Thorium/Lanthanum on Sediment Recycling at Subduction Zones and the Evolution of the Continents

Arc magmas and the continental crust share many chemical features,but a major question remains as to whether these features arecreated by subduction or are recycled from subducting sediment.This question is explored here using Th/La, which is low inoceanic basalts (<0·2), elevated in the continents(>0·25) and varies in arc basalts and marine sediments(0·09–0·34). Volcanic arcs form linear mixingarrays between mantle and sediment in plots of Th/La vs Sm/La.The mantle end-member for different arcs varies between highlydepleted and enriched compositions. The sedimentary end-memberis typically the same as local trench sediment. Thus, arc magmasinherit their Th/La from subducting sediment and high Th/Lais not newly created during subduction (or by intraplate, adakiteor Archaean magmatism). Instead, there is a large fractionationin Th/La within the continental crust, caused by the preferentialpartitioning of La over Th in mafic and accessory minerals.These observations suggest a mechanism of ‘fractionation& foundering’, whereby continents differentiate intoa granitic upper crust and restite-cumulate lower crust, whichperiodically founders into the mantle. The bulk continentalcrust can reach its current elevated Th/La if arc crust differentiatesand loses 25–60% of its mafic residues to foundering. KEY WORDS: arc magmatism; continental crust; delamination; thorium; sediment subduction     

扬子东部冶山和山里陈埃达克质侵入岩年代学与地球化学:岩石成因和动力学意义

冶山和山里陈岩体构造上属于扬子地块东部,毗邻郯庐断裂带东侧,主要岩石类型为石英闪长玢岩。它们的单矿物(角闪石,黑云母)40Ar/39Ar坪年龄分别为131.22±0.77 Ma和130.07±0.48 Ma。冶山和山里陈岩体具有与埃达克岩类似的地球化学特征,如w(SiO2)>56%,高Al2O3[w(Al2O3)=14.95%～17.67%]、Sr[w(Sr)=495×10-6～2086×10-6]、Sr/Y(44～159)和La/Yb(26～68),低重稀土元素,如w(Yb)=0.82×10-6～1.56×10-6),w(Y)=9.14×10-6～20.32×10-6,无明显-正Eu异常(δEu=0.90～1.11)。另外,样品普遍具有较高的MgO[w(MgO)=2.01%～4.98%]、IMg(45～71)和Cr[w(Cr)=19.2×10-6～199×10-6]、Ni[w(Ni)=13.8×10-6～58.8×10-6]。它们的Sr-Nd-Pb同位素特征为:(87 Sr/86 Sr)i=0.7059～0.7062,εNd(t)=-14.75～-12.15,(206 Pb/204 Pb)t=16.082～16.847,(207 Pb/204 Pb)t=15.303～15.461,(208Pb/204Pb)t=35.889～36.919。冶山和山里陈埃达克质侵入岩可能是由于扬子与华北地块在三叠纪的碰撞及扬子地块东部的地壳拆离作用,导致下地壳增厚并发生拆沉熔融,岩浆在上升过程中与地幔橄榄岩发生了反应。郯庐断裂带两侧早白垩世时期的埃达克(质)岩可能形成于断裂带早白垩世时期由左行平移向伸展活动转变的阶段,其源动力很可能是受到滨太平洋板块构造的影响。     

北祁连加里东造山带从挤压到伸展造山机制的转换

早古生代早期，北祁连造山带发生强烈的挤压变形作用，形成加里东期的俯冲－增生杂岩、高压变质岩，并使造山带岩石圈地壳加厚缩短。志留纪末期，加厚的造山带岩石圈由于垮塌作用及根部的拆沉作用，使造山带从挤压造山机制转换为伸展造山机制，并进入后造山伸展作用阶段，增厚的岩石圈开始减薄，发生不同层次的伸展作用，同时伴随花岗岩及Ａｎ∈变质岩的穹隆以及泥盆纪磨拉石盆地上叠盆地（Ｃ－Ｔ）的形成。石炭纪末，北祁连造山带岩石圈地壳已基本减薄到正常厚度。而现今的北祁连造山带的缩短和抬升则为喜马拉雅期再造山作用的产物     

断陷盆地覆盖区不整合结构量化判识新方法——以济阳坳陷古近系不整合为例

地层不整合结构的量化判识是油气藏,特别是地层不整合油气藏成藏研究的关键问题。针对断陷盆地覆盖区不整合结构研究相对局限、量化判识不够的现状,研究了济阳坳陷第三系不整合纵向结构的发育特征,将不整合结构划分为Ⅰ1,Ⅰ2 和Ⅱ三种类型,它们可通过实验室岩芯样品的分析测试量化判识。由于不整合各层结构在测井曲线上都有一定的响应,因而根据多种测井曲线构建了综合分层曲线,并用最优分割法确定了地层中不整合面的位置。在此基础上,根据GR,AC和SP 曲线建立了主成分曲线,并综合岩芯地质和分析测试研究成果,提出了量化判识不整合结构的量化标准图版。该方法应用于济阳坳陷地层不整合油气藏勘探中,取得良好效果。     

关于底侵作用和拆沉作用在南海中的指示意义

南海位于印度板块、欧亚板块和太平洋板块之间,是世界上最大的边缘海。许多学者对其成因模式和动力学机制进行过多方面的探讨,但往往只注重地壳浅部和侧向碰撞的研究,而忽视了深部的壳幔相互作用。从地形地貌、岩石地球化学、地球物理学、岩相古地理学等地质事实出发,论述了南海存在着大规模的底侵作用和拆沉作用,并在前人研究的基础上,建立了简单的以底侵作用和拆沉作用为主的壳幔相互作用模型来模拟南海的形成演化过程,指出底侵作用和拆沉作用是南海形成演化过程中深部的主要推动力,对南海的形成演化有着非常重要的影响。     

从藏南陆-陆碰撞带深部结构构造演化探讨斑岩铜矿的成岩成矿问题

本文以INDEPTH项目对印度大陆与欧亚大陆碰撞带深部成像结果为基础,从构造演化角度探讨藏南陆-陆碰撞带冈底斯斑岩铜矿带的成矿作用问题。深部探测给出的碰撞带深部结构与侯增谦等地质学家提出的深部结构有较大的异同,如何协调起来以深化对藏南陆-陆碰撞条件下成矿作用的认识,这是本文讨论的中心。藏南碰撞带成矿实际上是在新特提斯大洋岩石圈俯冲形成的冈底斯岩浆弧成矿作用的基础上,再经过陆-陆碰撞挤压强烈改造后的再成矿。碰撞带的深部结构构造演化的特点是:(1)新特提斯大洋岩石圈板块向北连续俯冲了约120 Ma,形成的冈底斯陆缘火山岩浆弧带,这导致了陆缘带地壳增厚并含有大量的地幔岩浆流体物质(如南美安第斯成矿带那样);(2)在印度大陆与冈底斯陆缘弧接近碰撞时,在对挤中新特提斯大洋洋壳与大洋岩石圈地幔发生向上挤出与向下拆沉,并使部分洋壳残片和大洋岩石圈物质保存在中上地壳内;(3)两大陆岩石圈碰撞对接后,印度岩石圈地幔加深达70~80 km并沿地壳底部向北推进,并将加厚地壳内大量的成矿物质、钙碱性岩浆,洋壳及新生的下地壳,以及部分地幔物质从地壳底部将其围限起来,成为后期再成矿的物质基础;(4)查明了碰撞带深部壳/幔间产生了一层中间速度层(相当于MASH层),在中上地壳部位出现一层巨大的部分熔融层;(5)在碰撞挤压下冈底斯带内产生多组断裂构造,大型逆冲断裂系与背冲断裂,并引发了含矿岩浆的再活动,并在浮力(下地壳内)和挤压力作用下多次活动上升生成斑岩型铜矿床;(6)成矿后地表遭受过强烈的风化剥蚀作用,使矿床出露地表。     

基于ComGIS的校园导航系统

分析了ComGIS的特点,并结合其特点,利用VisualBasic6.0,SuperMap和SQLServer2000等软件实现了校园导航系统的空间数据和属性数据的存储、可视化查询、测量地图等,同时介绍了系统空间数据库和空间地理模型的建立,以及在系统的设计中所涉及到的多媒体等技术。     

锑矿床研究若干问题初探

锑矿床研究的重点包括：锑矿床分类;不同构造环境下锑矿庆的成因模式;锑的来源。今后锑矿床分类将以锑矿庆形成的构造背景为主,结合锑矿床成因方式和容矿主岩特点来进行。文章介绍了几种典型构造环境下（大型走滑、造山挤太和造山崩坍）锑矿床形成机理和成因模式。现有的资料表明锑矿床受壳幔相互作用所制约（如拆沉作用,ｄｅｌａｍｉｎａｔｉｏｎ）,这无疑会成为锑矿研究的热点和难点。