Trace Element Geochemistry of Hannuoba Ultramafic Inclusion—bearing Alkali Basalts

Presented in this paper are the trace element abundances of 16 samples of Hannuoba ultramafic inclusion-bearing aldali basalts,which were determined by instrumental neutron activation analysis and X-ray fluorescence spectrometry.The Petrogenesis of the alkali basalt suite has been modeled by batch partial melting and and Rayleigh fractional crystallization processes,The geochemical characteristics of the mantle source from where alkali basalts were derived are described in terms of variations in trace element abundances of the alkali basalt suite.     

西藏冈底斯带石炭纪陆缘裂陷作用：火山岩和地层学证据

冈底斯作为重要的中、新生代岛弧岩浆岩带,历来是青藏高原最热门的地质研究领域,但是对晚古生代火山岩的性质及其形成的构造背景仍缺乏研究。本文在区域地质调查资料的基础上,对冈底斯带石炭纪火山-沉积岩系进行了系统的地层学和岩石地球化学研究,测量了重点剖面,对火山岩进行了常量、微量元素和Sr、Nd、Pb同位素地球化学测试。研究表明,石炭系与下伏泥盆系或前寒武系之间普遍存在不整合或岩性、沉积相的突变面,代表重要的构造转换面。石炭纪的沉积环境大致有两次滨海-深海或深水斜坡-滨海沉积演化旋回,深海-深水斜坡沉积或冰海相含砾板岩与两次火山活动伴生。石炭纪火山岩主要为安山玄武岩和英安岩、流纹岩类,略具双峰式火山岩特点。安山玄武岩类的成分与典型MORB和岛弧玄武岩相比,具有MgO含量低,TiO2、Al2O3、P2O5含量高等特点,稀土和微量元素为LREE和LILE富集型分配模式,与大陆拉斑玄武岩相似。石炭纪酸性火山岩的稀土和微量元素地球化学特征与陆内流纹岩相似。岩石地球化学示踪和地层学研究表明,冈底斯带石炭纪为伸展背景下的冈瓦纳陆缘裂陷环境。火山岩的源区地幔具有典型的Dupal异常,发生过复杂的混合作用,涉及到原始地幔、富集地幔EMII和地壳成分等,说明发生过冈瓦纳古陆壳俯冲、再循环进入古老地幔等过程。玄武岩类成分的某些特殊性,可能与源区混合作用有关。     

皖东上第三系玄武岩的地球化学特征和成因

郯庐断裂带东侧的皖东地区上第三系花果山组和桂五组玄武岩，是我国东部大陆大陆新生代玄武岩带中，尚未作详细地球化学研究的少数岩区之一。本文报道了该岩区２２个样品的常量元素和稀土等２０多种微量元素的丰度，以及其中１１个样品的Ｎｄ，Ｓｒ，Ｐｂ同位素组成，比较系统地提示了该大陆玄武岩套的地球化学特征。依据同位素和元素地球化学的制约，讨论了该岩套的成因及其地幔源区的性质和过程。该岩套来源于具ＥＭＩ型大洋玄武岩     

Geochemistry and Petrogenesis of Three Series of Cenozoic Basalts from Southeastern China

Cenozoic basaltic volcanism in southeastern China was related to the lithospheric extension and asthenospheric upwelling at the eastern Eurasian continental margin. The cenozoic basaltic rocks from this region can be grouped into three different series: tholeiitic basalts, alkali basalts, and picritic-nephelinitic basalts. Each basalt series has distinctive geochemical features and is not derived from a common source rock by different degrees of partial melting or from a common parental magma by fractional crystallization. The mineralogy, petrography, and major and trace-element geochemistry of the tholeiites are similar to oceanic island basalts, implying that the mantle source for these Chinese continental tholeiites was similar to that of the oceanic island basalts—an asthenospheric mantle. The alkali basalts and picritic-nephelinitic basalts are enriched in incompatible trace elements, and their geochemical features can be interpreted as a result of partial melting of an enriched lithospheric mantle, or the mixing products of an asthenospheric magma with a component derived from an enriched lithospheric mantle through thermal erosion at the base of the lithosphere. But the lack of a transitional rock type and continuous variational trends among these basalts suggests that the mixing between asthenospheric magmas and lithospheric magmas probably was not significant in the petrogenesis of the basalts from SE China. Low-degree partial melting of enriched lithospheric mantle alone can account for the observed geochemical data from these basalts.     

阿尔金南缘帕夏拉依档沟斜长角闪岩年代学、地球化学及其构造意义

阿尔金造山带南缘帕夏拉依档沟零星发育一些斜长角闪岩,主要呈透镜状或豆荚状产于阿尔金岩群大理岩中。文章对这些斜长角闪岩开展了岩相学、年代学以及地球化学方面的研究,获得斜长角闪岩LA-ICP-MS锆石U-Pb年龄为(857.4±7.1)Ma和(432.5±5)Ma,前者代表其形成年龄,后者代表其变质年龄。地球化学特征显示斜长角闪岩轻稀土微弱富集,轻重稀土分异不显著,并有部分消减带玄武岩地球化学特征,源区为古俯冲带再循环的富集地幔源区,并受到流体或熔体的改造。结合判别图解和区域构造演化,分析得出斜长角闪岩原岩形成于初始裂谷环境,是大陆拉张向裂谷转换机制下形成的过渡岩石类型,较新年龄(432.5±5)Ma为早志留世陆壳伸展和软流圈底辟作用引起的变质年龄。本次研究为新元古代中晚期阿尔金南缘基性岩浆活动提供了新的年代学和地球化学方面的证据,这期基性岩浆活动可能为Rodinia超大陆初始裂解事件在阿尔金南缘的响应。     

Geochemical characteristics of lava-field basalts from eastern Australia and inferred sources: Connections with the subcontinental lithospheric mantle?

 A large new database of major, trace elements and Sr-Nd isotopic ratios from 11 lava-field provinces in New South Wales and Queensland, eastern Australia allows detailed interpretation of the origin of these basaltic magmas. Isotopic signatures and trace element patterns identify an OIB-type (oceanic island basalt) source as a dominant component for most of these and some provinces appear to have additional significant components derived from the subcontinental lithospheric mantle (SCLM). The SCLM components have geochemical characteristics that overlap those observed in spinel lherzolite xenoliths (samples of shallow lithospheric mantle) from eastern Australia. These SCLM components show geochemical provinciality that indicates the occurrence of distinct geochemical lithospheric domains reflecting the timing and style of tectonic evolution of different regions. One component reflects modification by subduction-related processes during the late Paleozoic and Mesozoic, one records enrichment by fluids during old metasomatic events and another suggests a metasomatic event involving a distinctive amphibole and apatite-style enrichment. The composition and age distribution of volcanic lava-field provinces older than 10 Ma are consistent with a model involving a regional upwelling (elongated N–S along eastern Australia) of deep hot mantle related to marginal rifting and with OIB-type source geochemical characteristics. Thermal inhomogeneities within this plume swath resulted in small diapirs which may have undergone melt segregation at about 100 km and incorporated varying amounts of SCLM components there or from higher levels of the SCLM during ascent. Subsequent hot-spot generated central volcanoes overprinted this lava-field volcanism, tapped a similar OIB-type source component and truncated the thermal events. Accepted: 15 March 1995     

大别造山带前陆阳新盆地古近纪玄武岩地球化学特征与成因研究

大别造山带前陆阳新盆地古近纪玄武岩的成分从石英拉斑玄武质到橄榄拉斑玄武质,以相对较高的大离子亲石元素(Rb、Ba、K、Th、U等)丰度和弱到中等亏损高场强元素(Nb和Ta)为特征,岩石地球化学的总体特征类似于初始裂谷的拉斑玄武岩,而与大陆拉斑玄武岩所表现出的强烈亏损高场强元素Nb和Ta的地球化学特征明显不同。该拉斑玄武质岩浆喷发前曾经历过较小程度(约4．5％)的以橄榄石为主的矿物分离结晶,玄武岩的母岩浆则是源区物质经过约15％的部分熔融形成的,源区物质可能包含了来自下地幔的FOZO和富集型大陆岩石圈地幔两种组分。大别造山带内及前、后陆地区古近纪拉斑玄武岩的地球化学特征具有可比性,意味着随着大别造山带山根的拆沉,来自下地幔的FOZO岩浆(可能以地幔柱的形式)上涌,对大陆岩石圈的地幔部分发生了大规模的改造,使得原性质明显不同的两大岩石圈地幔在会聚带附近其地球化学分区性变得模糊。所形成的玄武岩总体具有较多的FOZO特征,但(除造山带内部)也不同程度地保留有岩石圈地幔的性质。     

华北东部中生代晚期-新生代软流圈上涌与岩石圈减薄

现今的地幔是由软流圈地幔(热的,主元素饱满、微量元素亏损的,塑性流变性质的)、古老岩石圈地幔(地幔1,冷的,主元素贫瘠、微量元素富集的,刚性的,以方辉橄榄岩为代表)以及现今的岩石圈地幔(地幔2,主元素饱满、微量元素亏损,以二辉橄榄岩为代表,可能包括多时期形成的)组成。古老岩石圈地幔与地幔2样品的共存、100~4·3Ma在地幔内部持续发生的古老岩石圈与软流圈的相互作用以及上述作用的时空不均一性,都表明了岩石圈减薄是软流圈呈“蘑菇云状”大规模上涌的结果。上述事件发生于100Ma以后。软流圈来源的玄武岩大范围喷发并伴随了岩石圈的强烈拉伸是事件发生的主要标志,岩石圈减薄是一个深部地质过程,不像是突发事件。     

Cryptic metasomatic agent measured in situ in Variscan mantle rocks: Melt inclusions in garnet of eclogite,Granulitgebirge, Germany

Garnet of eclogite (formerly termed garnet clinopyroxenite) hosted in lenses of orogenic garnet peridotite from the Granulitgebirge, NW Bohemian Massif, contains unique inclusions of granitic melt, now either glassy or crystallized. Analysed glasses and re-homogenized inclusions are hydrous, peraluminous, and enriched in highly incompatible elements characteristic of the continental crust such as Cs, Li, B, Pb, Rb, Th, and U. The original melt thus represents a pristine, chemically evolved metasomatic agent, which infiltrated the mantle via deep continental subduction during the Variscan orogeny. The bulk chemical composition of the studied eclogites is similar to that of Fe-rich basalt and the enrichment in LILE and U suggest a subduction-related component. All these geochemical features confirm metasomatism. In comparison with many other garnet+clinopyroxene-bearing lenses in peridotites of the Bohemian Massif, the studied samples from Rubinberg and Klatschmühle are more akin to eclogite than pyroxenites, as reflected in high jadeite content in clinopyroxene, relatively low Mg, Cr, and Ni but relatively high Ti. However, trace elements of both bulk rock and individual mineral phases show also important differences making these samples rather unique. Metasomatism involving a melt requiring a trace element pattern very similar to the composition reported here has been suggested for the source region of rocks of the so-called durbachite suite, that is, ultrapotassic melanosyenites, which are found throughout the high-grade Variscan basement. Moreover, the Th, U, Pb, Nb, Ta, and Ti patterns of these newly studied melt inclusions (MI) strongly resemble those observed for peridotite and its enclosed pyroxenite from the T-7 borehole (Staré, České Středhoři Mountains) in N Bohemia. This suggests that a similar kind of crustal-derived melt also occurred here. This study of granitic MI in eclogites from peridotites has provided the first direct characterization of a preserved metasomatic melt, possibly responsible for the metasomatism of several parts of the mantle in the Variscides.     

峨眉山大火成岩省东区普安玄武岩系年代学、地球化学及成因研究

广泛分布于中国西南川、滇、黔三省的峨眉山玄武岩是我国最早被国际认可的大火成岩省,受到了国内外学者的广泛关注.前人对大火成岩省西区玄武岩已达成多项共识,而对东区玄武岩的岩石组合、火山活动时限、岩石成因等方面还存在诸多争议.本文以峨眉山大火成岩省东区贵州普安玄武岩系为研究对象,通过解析典型剖面,明确该区玄武岩系岩石类型从底...     

川西新元古代双峰式火山岩成因的微量元素和Sm-Nd同位素制约及其大地构造意义

系统的微量元素和Sm-Nd同位素分析表明,川西地区早震旦世苏雄组双峰式火山岩中的大多数玄武岩具有高的正ε_Nd(T)值(+5～+6)、大离子亲石元素和LREE富集,与现代典型的洋岛玄武岩和大陆溢流玄武岩省中的碱性玄武岩有非常相似的地球化学和同位素组成特征。酸性火山岩的ε_Nd(T)值较低(+1.1～+2.6),地球化学特征总体上与A₂-型花岗岩相似,它们是受地壳混染的OIB型玄武质岩浆在地壳中部的一个“双扩散”岩浆房通过结晶分异形成的。苏雄组双峰式火山岩形成于典型的大陆裂谷环境,非常类似于现代与地幔柱活动有关的高火山活动型裂谷火山岩,扬子块体西缘 800Ma前的裂谷作用和火山活动应是约825Ma前的华南地幔柱活动引发的结果。     

东准噶尔奥依托浪格地区石炭纪双峰式火山岩地球化学特征及其构造意义

本文对东准噶尔造山带三塘湖盆地西北缘奥依托浪格地区石炭纪巴塔玛依内山组进行了火山岩地质、岩相学、地球化学特征研究,并结合相关年代学成果,探讨了火山岩成因、形成环境与东准噶尔造山带构造演化。研究认为：该区石炭纪巴塔玛依内山组火山岩具明显的陆相火山岩、双峰式火山岩特征,岩石组合基性端元以玄武岩为主,酸性端元为珍珠岩、英安岩;主量元素特征显示基性端元为钙碱—碱性性质,酸性端元珍珠岩、英安岩分别属于碱性、拉斑性质;微量元素特征显示基性端元相对富集Ba、K、Sr等大离子亲石元素（LILE）,亏损Th、Nb、Nd等高场强元素（HFSE）,酸性端元强烈亏损Ba、Nb、Sr、Sm、Ti等元素,富集K元素;稀土元素显示两端元均为轻稀土元素（LREE）相对富集、重稀土元素（HREE）相对亏损特征;珍珠岩主、微量元素特征与整体具有明显差异。综合研究表明：该双峰式火山岩基性端元来源于地幔岩的部分熔融,酸性端元英安岩来源于基性端元的分离结晶作用,珍珠岩则来源于下地壳的部分熔融;地球动力学背景为俯冲洋壳的拆沉作用,造成软流圈热源上涌,引发地幔岩的部分熔融与岩石圈的伸展减薄,进而诱发下地壳镁铁质岩的减压熔融;构造环境为后碰撞伸展阶段晚期,并指示了板块链式缝合的特点。     

中国东部新生代玄武岩记录古太平洋俯冲带壳幔相互作用

大陆玄武岩通常具有与洋岛玄武岩相似的地球化学成分,其中含有显著的壳源组分.对于洋岛玄武岩来说,虽然其中的壳源组分归咎于深俯冲大洋板片的再循环,但是对板片俯冲过程中的壳幔相互作用缺乏研究.对于大陆玄武岩来说,由于其形成与特定大洋板片在大陆边缘之下的俯冲有关,可以用来确定古大洋板片俯冲的地壳物质再循环.本文总结了我们对中国东部新生代玄武岩所进行的一系列地球化学研究,结果记录了古太平洋板片俯冲析出流体对地幔楔的化学交代作用.这些大陆玄武岩普遍具有与洋岛玄武岩类似的地球化学成分,在微量元素组成上表现为富集LILE和LREE、亏损HREE,但是不亏损HFSE的分布特点,在放射成因同位素组成上表现为亏损至弱富集的Sr-Nd同位素组成.在排除地壳混染效应之后,这些玄武岩的地球化学特征可以由其地幔源区中壳源组分的性质来解释.俯冲大洋地壳部分熔融产生的熔体提供了地幔源区中的壳源组分,其中包括洋壳镁铁质火成岩、海底沉积物和大陆下地壳三种组分.华北和华南新生代大陆玄武岩在Pb同位素组成上存在显著差异,反映它们地幔源区中的壳源组分有所区别.中国东部新生代玄武岩的地幔源区是古太平洋板片于中生代俯冲至亚欧大陆东部之下时,在>200 km的俯冲带深度发生壳幔相互作用的产物.在新生代期间,随着俯冲太平洋板片的回卷引起的中国东部大陆岩石圈拉张和软流圈地幔上涌,那些交代成因的地幔源区发生部分熔融,形成了现今所见的新生代玄武岩.      

内蒙古月牙山蛇绿岩的岩石学、地球化学特征及其地质意义

内蒙古月牙山蛇绿岩分布于红柳河-洗肠井蛇绿岩带东部的月牙山一带,出露较完整,自下而上由超基性杂岩、辉长岩、层状玄武岩和枕状玄武岩及放射虫硅质岩组成。岩石学、岩石化学、地球化学资料显示,蛇绿岩套中的主要单元均源于幔源,接近于现代洋壳蛇绿岩特征。与蛇绿岩单元伴生的斜长花岗岩属于幔源型大洋斜长花岗岩,且为蛇绿岩组成的一部分;辉长岩、玄武岩均形成于正常洋中脊(N-MORB)环境。     

东天山企鹅山群火山岩地球化学特征及其地质意义

根据主量元素、微量元素和Sr-Nd同位素地球化学研究结果,觉罗塔格构造带中企鹅山群火山岩企鹅山群火山岩分为拉斑玄武岩和钙碱性玄武岩,拉斑玄武岩和钙碱性玄武岩均具有高铝、低钛和相对富集轻稀土元素（LREE）、大离子亲石元素的特征,与典型的岛弧玄武岩相似。钙碱性玄武岩具有显著的Eu的负异常,其不相容元素含量显著高于拉斑玄武岩,在La/Sm-La图解中,钙碱性玄武岩为结晶分异作用的产物,拉斑玄武岩则为平衡部分熔融的产物。Sr-Nd同位素特征表明企鹅山群火山岩源自亏损地幔。在玄武岩构造环境判别图中样品均落入岛弧玄武岩范围内。通过综合该区岩石组合特征、地球化学特征、地球物理特征,认为企鹅山群为康古尔洋壳向北俯冲在大南湖岛弧南侧形成的石炭纪岛弧火山岩。     

Triassic rift-related komatiite, picrite and basalt, Pelagonian continental margin, Greece

A wide range of Triassic rift-related volcanic rocks is preserved on the extensional continental margin of the central part of the Pelagonian micro-continental block of Greece, in the region of northern Evia Island and Othris. This part of the continental margin is unusual for the abundance of lavas, the wide range of lava types, and the presence of komatiitic lavas. The predominant rock types are subalkaline basalt and basaltic andesite, mildly alkaline basalt and picrite, and minor komatiite. Four groups of mafic rocks are distinguished from the 32 samples analyzed on the basis of variation in incompatible trace elements, with all but one group including both picritic and basaltic rocks. The geochemical character of the volcanic rocks suggests derivation from inhomogenous, spinel-bearing, lithospheric mantle. This mantle source was variably depleted and moderately refertilized by subduction-related fluids during earlier Hercynian subduction. The picrites were generated by variable degrees of partial melting, which are inferred to inversely correlate with pressure, thus suggesting decompression melting of their source. Evidence is lacking for a major mantle plume, but the enhanced magmatism in this sector of the rifted margin suggests that melting was induced by a large amount of hydration, which is appropriate to generate melts at lower temperatures. Consequent fractionation coupled with crustal assimilation generated the wide range of subalkaline to mildly alkaline mafic rocks.     

中国东部部分地区新生代岩石圈地幔的成因:主量和微量元素制约

为厘定中国东部新生代岩石圈地幔的成因,进而探讨岩石圈减薄的方式,本文选择汉诺坝、女山、盘石山和方山尖晶石二辉橄榄岩作为研究对象,系统分析了其主、微量元素。结果表明,这些地区新生代岩石圈地幔具有"新生"的性质,为上涌的软流圈物质经过小于10%的部分熔融程度后的残留相形成,后又经历了类似新生代玄武岩性质的富硅熔体的交代,交代介质的物质贡献小于3%。新生代岩石圈地幔与古老克拉通岩石圈地幔在主、微量元素上的显著差异以及新生代岩石圈地幔微量元素特征的拟合,支持拆沉为华北克拉通东部减薄的主要方式。     

六合—仪征第三纪大陆碱性玄武岩微量元素地球化学

本文报道了六合-仪征第三纪大陆碱性玄武岩十八个样品的REE、Rb、Ba、Sr、Nb、Zr、Ni、Cr、V、Sc、Y、Ga、Zn、Cu等痕量元素含量,讨论了该岩套的成因及其地幔源区的特征。石榴石橄榄岩型地幔源区经较小程度部分熔融形成了基性原始岩浆;其后经过橄榄石和单斜辉石为主的结晶分异作用,演化后的岩浆喷出地表形成玄武岩套。本区碱性玄武岩的地幔源区曾受近期富集作用影响,具有富集LREE等不相容元素的特征。     

大陆溢流玄武岩的地球化学特征及起源

快速上涌的大陆溢流玄武岩（CFB），与大陆裂开存在密切的成因关系。CBF总体岩石及地球化学成分均一，富集同位素及不相容元素，但一些样品含有明显的亏损成分，反映出普遍的地幔不均一性，来自上下地幔边界及软流圈的地幔柱提供了CFB所需的主要物质和能量来源，地壳混染作用对CBF的成分影响不大，而受俯冲带脱水流体以及热地幔柱自身与围岩发生的交代作用影响。交代岩石圈地幔对CBF产生重要影响，很好地解释了CFB所具备的微量元素和同位素特征。     

大别造山带南坡晚白垩世玄武岩源区地幔特征

大别造山带南坡中生代断陷盆地中出露大量晚白垩世碱性玄武岩类。因岩浆受结晶分异和陆壳混染影响微弱,其成分基本可代表本区原生玄武岩浆。在微量元素原始地幔标准化蛛网图上,本区玄武岩具有地壳富集组分Pb、K、Rb、Ba等的正异常和Nb、P、Hf等高场强元素的负异常。同位素和微量元素特征显示,玄武岩浆来自亏损地幔(DM)和富集地幔(EM+EM)混合源区。玄武岩源区地幔中Pb的富集和Nb、P、Hf等的亏损基本可由中国东部亏损地幔与榴辉岩的混合获得,由此揭示富集地幔端元组成特征可能与晚白垩世前造山带根部榴辉岩拆沉并参与地幔再循环有关。