北祁连冷龙岭蛇绿岩地质地球化学特征及构造意义

在北祁连造山带冷龙岭地区新发现一条蛇绿岩带,该蛇绿岩从下向上由地幔橄榄岩、辉长辉绿岩、玄武岩、硅质岩组成。岩石地球化学特征表明,玄武岩可分类高Ti和低Ti两类,其中高Ti玄武岩具有LREE富集的稀土配分型式,富集K、Rb、Ba、Th、Nb、Ta等不相容元素,呈现隆起型(驼峰式)分布型式,显示了典型的洋岛火山岩地球化学特征,为板内岩浆作用的产物。低Ti玄武岩具有LREE亏损,类似N-MORB的稀土配分模式,同时又具有相对于N-MORB富集的大离子亲石元素,亏损Nb、Ta等高场强元素的岛弧火山岩的地球化学特征,代表了弧后盆地环境岩石组合。蛇绿岩岩石组合和岩石地球化学特征显示该蛇绿岩套形成于弧后盆地环境,是早奥陶世北祁连洋的残留。     

西藏泽当蛇绿岩玄武岩SHRIMP锆石U-Pb年龄 及其地质意义

雅鲁藏布江缝合带中各蛇绿岩体的准确定年对待提斯洋演化和青藏高原隆升的研究具有重要意义.泽当蛇绿岩是雅鲁藏布江缝合带东段最大的蛇绿岩块体,关于其形成年龄目前仍存在不同的认识.通过SHRIMP锆石U-Pb测年得到蛇绿岩中玄武岩的形成年龄为154.9Ma±2.0Ma(95％置信度,MSWD=0.98).蛇绿岩中的玄武岩是洋脊扩张的产物,其形成年龄代表了扩张事件的时间,也代表了蛇绿岩的形成时代.结合已有的雅鲁藏布江缝合带蛇绿岩的形成年龄,该年龄进一步反映出雅鲁藏布江缝合带蛇绿岩形成时间具有东早西晚的特点.泽当蛇绿岩与含有埃达克质英云闪长岩的泽当岛弧火成岩基本为同期形成的.地球化学特征显示定年的玄武岩形成于俯冲带之上,且具有指示洋内俯冲环境的地球化学特征.因此,泽当SSZ型蛇绿岩可能形成于洋内俯冲机制.     

西藏东巧蛇绿岩的地球化学特征及其形成的构造环境

西藏东巧蛇绿岩主要由变质橄榄岩、辉长岩及玄武岩等组成。变质橄榄岩以富Mg、Fe、Cr,贫Ti、ΣREE为特征。辉长岩和玄武岩的主量元素、微量元素特征显示其具有扩张洋脊拉斑玄武岩的地球化学特征,其中高场强元素Nb、Ta、zr、Hf等亏损,大离子亲石元素Rb、sr、Ba等相对富集;在球粒陨石标准化稀土元素配分模式图上为LREE亏损的平坦型,无负Eu异常,与洋中脊玄武岩的特征类似。根据其地球化学属性,推测该区蛇绿岩形成于大洋盆地扩张的构造环境。     

西藏日喀则白马让蛇绿岩:亚洲大陆边缘的小洋盆

藏南雅鲁藏布蛇绿岩被认为是新特提斯大洋岩石圈的残留。该带中段的日喀则白马让蛇绿岩是保存较完整的蛇绿岩岩块之一。该蛇绿岩主要由橄榄岩、蛇纹岩、镁铁质侵入岩和玄武岩组成,缺堆晶岩系。镁铁质侵入岩主要呈辉绿岩脉、岩床和少量的岩墙产出。辉绿岩脉在整个蛇绿岩层序中均有分布,侵入橄榄岩的部分岩脉已经变为变辉绿岩和异剥钙榴岩。辉绿岩床(墙)向上逐渐过渡为玄武岩。局部可见日喀则群整合覆盖在玄武岩之上。地球化学分析显示不同产状的镁铁质岩均属于低钾或中钾的拉斑玄武岩,亏损Nb、Ta、Ti和LREE,具有弧前玄武岩(FAB)或弧后盆地玄武岩(BABB)的特征,它们的Ti/V和Yb/V的比值与BABB或正常大洋中脊玄武岩(N-MORB)相似,Sr-Nd-Pb同位素数据指示了亏损地幔(DM)与富集地幔(EM)过渡的源区。镁铁质岩野外产出关系和地球化学特征表明,白马让蛇绿岩的镁铁质岩组合可能形成于SSZ环境。考虑到超镁铁质岩、镁铁质岩和日喀则群在空间上的连续性,认为白马让蛇绿岩可能是起源于亚洲大陆边缘俯冲带上的洋盆,属于原地系统,而非外来的构造岩片。     

甘孜－理塘蛇绿混杂岩带特征及其构造意义

本文系统总结了甘孜－理塘蛇绿混杂岩带的地质学、岩石学和地球化学特征。该蛇绿岩由变质橄榄岩、堆积岩、席状岩墙、枕状玄武岩与块状玄武岩和放射虫硅质岩组成。岩石地球化学特征表明，蛇绿岩形成于洋脊扩张环境。该蛇绿混杂岩带是义敦碰撞造山带大地构造相中的弧前混杂带相。     

西藏雅鲁藏布江缝合带西段东波蛇绿岩的构造背景特征

西藏东波蛇绿岩位于雅鲁藏布江缝合带西段,由地幔橄榄岩、辉石岩和辉长岩等组成。地幔橄榄岩主要为方辉橄榄岩、纯橄岩和少量二辉橄榄岩。岩体的边界出露玄武岩和硅质岩等。地幔橄榄岩中有少量辉石岩和辉长岩的脉岩,宽约1 m,走向北西,与岩体的构造线方向基本一致。各岩相岩石地球化学研究结果表明,东波蛇绿岩的岩相存在较大的差异,玄武岩具有与洋岛玄武岩(OIB)相似的地球化学特征,而地幔橄榄岩中辉石岩、辉长岩脉与洋中脊玄武岩(MORB)相似,形成于洋中脊环境,并受后期俯冲流体作用的改造。东波岩体中二辉橄榄岩具有与深海地幔橄榄岩较一致的轻稀土亏损特征,而方辉橄榄岩和纯橄岩的地球化学特征显示出岩体形成于MOR环境,后受到SSZ环境的改造。东波蛇绿岩的岩石地球化学特征显示其洋中脊叠加洋岛的构造背景。     

大兴安岭中生代两类流纹岩与玄武岩的成因联系

根据地质产状和地球化学特征大兴安岭中生代流纹岩类可划分为高Ti流纹岩和低Ti流纹岩。高Ti流纹岩类与亚碱性系列玄武岩类紧密伴生 ,两者在地球化学上构成连续变异系列。低Ti流纹岩类则与碱性系列玄武岩类构成地球化学双峰态。岩相学和地球化学研究表明 ,这两类流纹岩与该区同期玄武岩类有着密切的成因联系 :起源于地幔柱亏损成分的亚碱性系列玄武岩浆经过单斜辉石、斜长石、磷灰石、锆石的分离结晶 ,形成亚碱性系列低钾玄武岩→高钾玄武岩→高Ti流纹岩演化系列 ;起源于地幔柱富集成分的碱性系列玄武岩浆侵入下地壳 ,使下地壳岩石发生部分熔融 ,形成碱性系列玄武岩 -低Ti流纹岩双峰态组合     

西昆仑山库地蛇绿岩的特征及其构造意义

库地蛇绿岩套由下部超镁铁质岩、辉长岩、幔源型花岗岩和上部基性火山岩、复理石等成分构成,形成于震旦纪-寒武纪。通过基性的辉长岩、玄武岩的矿物学、岩石学和地球化学研究得出,辉长岩表现出与典型蛇绿岩套中的堆晶辉长岩的特征相一致;而玄武岩则与大洋中脊玄武岩的特征一致,认为该蛇绿岩套是形成于洋中脊的蛇绿岩套。结合库地岛弧花岗岩的特征,提出塔里木盆地南缘在震旦纪-早古生代时期存在一个广阔的大洋,这一大洋在志留纪末期关闭。     

武山蛇绿岩的特征、同位素年代及其地质意义

武山蛇绿岩位于北秦岭西段、商丹断裂西延之北 ,岩石组成主要为超基性岩、变质的基性侵入岩、枕状玄武岩及大理岩。变玄武岩的岩石地球化学特征显示为钙碱性玄武岩 ,形成于大陆裂谷至初始洋盆环境 ,其Sm -Nd等时线年龄为 15 70Ma ,反映原始岩浆形成于中元古代 ,属大陆地壳演化早期的蛇绿岩     

商州蛇绿岩带与晋宁运动

商州蛇绿岩带具有完善的蛇绿岩套岩石组合;细碧岩与堆晶岩系渡型洋中脊玄武岩地球化化学特征。     

河南信阳地区龟山岩组新元古代变质玄武岩的地球化学特征、Sr-Nd同位素组成及地质意义

对信阳地区商丹断裂带南侧龟山岩组新元古代变质玄武岩进行了岩石学、地球化学及Sr-Nd同位素研究,分析结果显示该套玄武岩为亚碱性拉斑玄武系列,分为低Ti及高Ti两种类型：低Ti型较富Mg,不相容元素富集程度及稀土分馏程度较低,具有E-MORB的微量元素地球化学特征,Sr-Nd同位素组成相对富集,可能来自地幔柱引发的岩石圈地幔的部分熔融,并受到一定程度的地壳混染;高Ti型较富Fe,强烈富集不相容元素,具有OIB的地球化学特征,Sr-Nd同位素组成较为亏损,可能来自地幔柱的部分熔融,并较少受到地壳物质的影响。综合构造判别显示该套玄武岩可能为地幔柱伸展背景下的岩浆活动产物,可能为区域上沿商丹断裂带分布的中—新元古代局部伸展背景岩浆活动产物的组成部分。     

北祁连乌鞘岭蛇绿混杂岩地球化学特征及其构造环境

乌鞘岭蛇绿混杂岩位于北祁连造山带东段,具有相对完整的蛇绿岩序列,包括：变质地幔橄榄岩单元（蛇纹岩＋辉橄岩）,镁铁质一超镁铁质堆晶岩单元（橄辉岩＋辉石岩）,镁铁质侵入岩单元（辉长岩）,及基性火山岩单元（玄武岩）。依照TiO,的含量,本文从乌鞘岭蛇绿混杂岩中分出两类玄武岩,即：低n玄武岩（Ti02=0．55％～0．76％）和高Ti玄武岩（Ti02=1．35％-1．99％）。低Ti玄武岩大离子亲石元素含量波动较大,具明显n负异常,LREE呈略富集的配分模式,整体上具有弧火山岩的特征;高Ti玄武岩大离子亲石元素含量在小范围内变化,未见Nb、Ti负异常,LPtEE呈略亏损的配分模式,具有典型N—MORB的性质。在构造环境判别图上,低n玄武岩和高Ti玄武岩分别落入陆缘弧和大洋中脊环境。高Ti玄武岩是鸟鞘岭蛇绿混杂岩的一部分,源于亏损地幔的部分熔融,与陆缘弧型低rri玄武岩构造混杂在一起。乌鞘岭蛇绿混杂岩大概于中一晚奥陶世形成于北祁连造山带老虎山一毛毛山弧后盆地。     

A reappraisal of the high-Ti and low-Ti classification of basalts and petrogenetic linkage between basalts and mafic–ultramafic intrusions in the Emeishan Large Igneous Province, SW China

Based on published data, we reappraise the classification of high-Ti and low-Ti basalt from the Emeishan large igneous province (ELIP) and the correlations between basalts and mafic–ultramafic intrusions. Because of the lack of clear spatial and temporal variations of different types of basalts, we suggest that the basalts in the ELIP cannot be classified into high-Ti and low-Ti groups, by TiO₂ contents and/or Ti/Y ratios. The distinctive characteristics of these high-Ti and low-Ti lavas probably result largely from the different fractionating assemblages. Whether or not fractional crystallization of the Fe–Ti oxides occurred probably is the key factor that controls the Ti abundances and Ti/Y ratios in the residual melts, e.g., lavas, although the nature of the mantle sources, variable degrees of partial melting of mantle and crustal contamination also influence the geochemical signatures of the lavas. Therefore, neither Ti abundance nor Ti/Y ratios in basalts can reflect the nature of their mantle source. Moreover, the different types of mafic–ultramafic intrusions in the ELIP cannot simply be attributed to be genetically related special types of basalts, either high-Ti or low-Ti basalts. It is likely that they are merely the cumulus phases, i.e. chamber or conduit of the basaltic lavas. Hence, caution should be exercised in the use of high-Ti or low-Ti basalts as prospecting vectors for ore deposits in the region. Potential implications are proposed that both the Fe–V–Ti oxide and Cu–Ni–(PGE) sulfide mineralization in the ELIP intrusions is largely due to the variable differentiation and crustal contamination during magmatic processes.     

Origin of Late Palaeoproterozoic Great Vindhyan basin of North Indian shield: Geochemical evidence from mafic volcanic rocks

On the western and southern margins of the sickle shaped Vindhyan basin of north Indian shield, there are basal Vindhyan mafic volcanic rocks referred to as Khairmalia volcanics and Jungel volcanics respectively. These volcanics vary in composition from low-Ti tholeiite to high-Ti alkali basalt showing close affinity with continental flood basalts (CFB) and ocean island basalts (OIB) respectively. The parental magmas of Khairmalia and Jungel alkali basalts were formed by different degrees of partial melting of a garnet lherzolite. The magma of Khairmalia tholeiites was generated by a relatively higher degrees of partial melting of a garnet + spinel lherzolite. The geochemical data coupled with available geological and geophysical data favour a rift type origin of this basin which evolved as a peripheral basin showing many similarities with Paleogene Himalayan foreland basin. The existing radiometric age data suggest that the origin of Vindhyan basin is linked with Aravalli–Satpura orogeny. At about 1800–1600 Ma collision occurred along the Aravalli-Delhi fold belt (ADFB) and Central Indian Tectonic Zone (CITZ) with west and south subduction respectively. During this process the subducting lithosphere suffered extensional deformation on its convex side and some pre-existing large faults in the already thin leading edge of subducted plate also reactivated and tapped magma generated by decompressional melting of the subcontinental mantle. The simultaneous processes such as flexural subsidence, reactivation of pre-existing faults, heating, thermal cooling and contraction during volcanism, resulted in the formation of curvilinear warp parallel to the emerging mountain front. The Lower Vindhyan volcano–sedimentary succession was deformed and exposed to erosion before the deposition of Upper Vindhyan rocks. The orogenic forces were active intermittently throughout the Vindhyan sedimentation.     

峨眉山大火成岩省：地幔柱活动的证据及其熔融条件

对苦橄岩中橄榄石斑晶及其中熔体包裹体的电子探针分析表明，峨眉山大火山岩省的原始岩浆具高镁（ MgO > 16％）特征。玄武岩的 REE反演计算揭示，参与峨眉山玄武岩岩浆作用的地幔具有异常高的潜能温度（ 1 550℃）。这些特征以及峨眉山玄武岩的大面积分布和一些熔岩所显示的类似于洋岛玄武岩 (OIB)的微量元素和 Sr- Nd同位素特征均为地幔热柱在能量和物质上参与峨眉山溢流玄武岩的形成提供了确凿证据。峨眉山两个主要岩类（高钛和低钛玄武岩）可能是不同地幔源区物质在不同条件下的熔融产物。低钛玄武岩形成于温度最高、岩石圈最薄的地幔柱轴部。地幔（ ISr≈ 0.705,ε Nd(t)≈＋ 2）熔融始于 140 km，并一直延续到较浅的深度（ 60 km,尖晶石稳定区 ),部分熔融程度为 16％，这类岩石可能代表了峨眉山玄武岩的主体。而高钛玄武岩的母岩浆的形成基本局限在石榴子石稳定区（ > 70 km），其源区特征为 : ISr≈ 0.704,ε Nd(t)≈＋ 5，可能代表了热柱边部或消亡期地幔小程度部分熔融（ 1.5％）的产物。     

Two melting regimes during Paleogene flood basalt generation in East Greenland: combined REE and PGE modelling

Previously published platinum group element (PGE) and rare earth element data (REE) from a sample suite of the Palaeogene flood basalts of the East Greenland rifted margin are used to approximate primary magma compositions by numerical models of mantle melting. Both high-Ti and low-Ti basalts are found intercalated in the coastal section “the Sortebre Profile” in central East Greenland, and the apparent lack of mixing between the two series indicates coexistence of two geographically separated melting regions and plumbing systems during continental breakup above the Palaeogene Iceland plume. The lavas show little or no sign of crustal contamination and the limited variation in La/Sm and Cu/Pd ratios can be interpreted to reflect mantle source composition and melting processes. Numerical modelling indicate that the low-Ti series formed by F~20% melting in a columnar melting regime from a slightly depleted upper mantle source with a relatively normal S-content (~180 ppm S). In contrast, the high-Ti series formed by much lower degrees of melting (F~6%) in a spreading-related, triangular melting regime from a relatively S-poor (~100 ppm S) source. The low-Ti suite was S-undersaturated at the stage of melt segregation from a shallow mantle source due to the high degree of melting. In contrast, the high-Ti suite probably formed from a S-poor source where some low degree melt batches were S-saturated at the stage of deep segregation in distal parts of the triangular melting regime. This suite shows a geochemical high pressure garnet-signature and adiabatic decompression could therefore have played a role in keeping the mantle-derived S in solution before Fe-enrichment related to fractional crystallisation also increased the S-capacity of these melts. An erratum to this article can be found at     

白银厂矿田玄武岩地球化学特征及其形成地质环境

白银厂矿田玄武岩主要由玄武岩和碱性玄武岩组成 ,其中玄武岩属于钙碱性系列和拉斑系列 ,碱性玄武岩属于钾质碱性玄武岩系列和钠质碱性玄武岩系列。相对于N -MORB ,本区钙碱性系列和拉斑系列玄武岩明显富集Ba、Rb、Th、U ,而亏损Ti;碱性系列玄武岩高度富集K、Ba、Rb、Th、U ,而Ti、Zr、Ce相对亏损 ,表明该区玄武岩的形成与板块俯冲作用有关。钙碱性玄武岩系列和拉斑玄武岩系列玄武岩具有低的REE含量和亏损的LREE配分型式 ,表明它是由LREE亏损和HREE略有富集的地幔部分熔融形成 ;碱性系列玄武岩的REE含量和 (La/Yb) N 比值高 ,LREE和HREE的分馏程度较高 ,表明其形成于演化的岩浆 ,可能来自于富集LREE的地幔源区或地幔橄榄岩较低程度的熔融。痕量元素地球化学特征表明 ,本区玄武岩应是与板块俯冲作用有关的地幔部分熔融形成 ,成岩环境为火山弧环境 ,是岛弧向成熟岛弧转化过程中的产物。     

西昆仑库地蛇绿岩地质、地球化学及其成因研究

西昆仑库地蛇绿混杂岩由方辉橄榄岩和纯橄榄岩等地幔变质橄榄岩、豆荚状铬铁矿、堆晶橄榄岩、堆晶辉石岩和辉长岩、辉绿岩墙、块状和枕状玄武岩等组成。强亏损方辉橄榄岩为主的地幔岩组合,二辉石的低Al含量和铬尖晶石的高Cr^#,以及岩石的富Mg、Ni和贫Al、Ca特征一致表明地幔橄榄岩类是经较高程度部分熔融后的地幔残余,与消减带之上蛇绿岩中的同类岩石相近。岩石富Rb、Ba、U、Th、LREE,说明地幔残余岩石受到了来自消减带的洋壳重熔组分的混染。堆晶岩以辉石岩和辉长岩为主,可能属PPG系列,指示岩浆是在消减带环境和含水条件下熔融的。辉长岩为低Ti蛇绿岩型,代表洋内弧后盆地早期环境或弧前环境。辉绿岩和玄武岩为洋中脊拉斑玄武岩和岛弧拉斑玄武岩的过渡类型;玄武岩和辉绿岩相比富Ba、Th、LREE,贫Ta,指示玄武岩较辉绿岩更多地受到来自消减带洋壳重熔组分的影响。库地蛇绿岩形成时的古构造环境是消减带之上的弧间或弧后盆地。     

Age and composition of dykes emplaced before and during the opening of the Tasman Sea—source implications

Abstract

Dykes are common in the wave-cut platforms along the coast from Newcastle to Sydney. According to some authors, they may be related to the opening of the Tasman Sea that commenced ca 84?Ma ago. However, there are few detailed radiogenic dating and geochemical studies to evaluate this. We attempt to resolve this by K–Ar dating of plagioclase in and geochemical studies of, basaltic dykes intruding Permo-Triassic sequences on the wave-cut platforms and Carboniferous and Permo-Triassic sequences inland. The plagioclase separated from the dykes give K–Ar ages ranging from 266 to 53?Ma with the majority older than 84?Ma indicating that most dykes were emplaced before the Tasman Seafloor formation. The dykes are generally mildly alkaline, high-Ti basalts; fewer are tholeiitic and calc-alkaline, low-Ti basalts. Strongly light rare earth element (LREE)-enriched patterns typify the former and flat, LREE-depleted or slightly to moderately enriched LREE patterns, the latter. High-Ti basalts have ocean-island-basalt-like and low-Ti basalts, calc-alkaline or mid-ocean ridge basalt (MORB)-like patterns. Most high-Ti and some low-Ti basalts show plume-like characteristics, others N-type MORB and arc-like characteristics. Dykes intruding the Carboniferous sequences show a distinct contamination signature that could be crustal or due to subduction-related metasomatism of the subcontinental lithospheric mantle. The sources of the basaltic magmas vary substantially and in places changes with time. All alkali basalts are derived from enriched asthenospheric sources at varying depths (90–147?km) and most tholeiitic, low-Ti basalts have been extracted from asthenospheric and depleted asthenospheric–lithospheric sources indicating substantial compositional heterogeneity of the mantle. Further, Nd model ages varying from Neoproterozoic (940–580?Ma) to Paleozoic (460–370?Ma) suggest variation in the age of mantle sources for the basalts.     

桂西那坡基性岩地球化学:峨眉山地幔柱与古特提斯俯冲相互作用的证据

华南板块西南缘、越北地块以北桂西那坡县城以西及西南一带发育一套晚二叠世基性岩,由层状、似层状次火山岩相辉绿岩、辉绿玢岩及球状岩组成。根据岩石地球化学特征,那坡基性岩可划分为高Ti(TiO_22.8%和Ti/Y500)和低Ti两部分。高Ti基性岩为碱性玄武岩,而低Ti基性岩为拉斑玄武岩。与低Ti基性岩相比,高Ti基性岩整体具有相对较低的SiO_2、MgO和较高的FeO_t、P_2O_5,轻、重稀土分馏明显,富集大离子亲石元素(LILE)和高场强元素(HFSE),显示出似OIB地球化学特征,与峨眉山高Ti玄武岩具高度亲缘性;低Ti基性岩具有相对较高的SiO_2、MgO和较低的FeO_t、P_2O_5,稀土配分曲线较平坦,富集LILE,严重亏损HFSE(Nb、Ta),与岛弧玄武岩地球化学特征类似。从微量元素比值及相关图解对岩浆源区和构造环境判别,那坡高Ti基性岩来自富集OIB地幔源区,而低Ti基性岩兼具OIB和岛弧岩浆源区的过渡特征。结合岩石地球化学特征及区域地质背景,认为那坡高Ti基性岩可能为峨眉山地幔柱岩浆作用的产物,低Ti基性岩为古特提斯俯冲与峨眉山地幔柱共同作用的产物,揭示了那坡地区晚二叠世同时受到峨眉山地幔柱和古特提斯俯冲相互作用的影响。