西昆仑普鲁新生代火山岩的矿物化学特征及其对岩浆演化过程的约束

对西昆仑普鲁新生代火山岩的矿物学进行了系统的研究。结果表明：该地区火山岩主要由橄榄石、单斜辉石和斜长石组成,并有少量的斜方辉石、黑云母、角闪石、碱性长石和铁钛氧化物。其矿物学特征指示了岩浆的性质有点类似于碱性岩浆,但与典型的碱性玄武岩又有明显的区别,属于橄榄安粗岩系列。利用橄榄石-熔体平衡原理估算了进入高位岩浆房中的熔体的MgO含量约为6．2％,Mg^#为0．57,说明其不是地幔熔融形成的原始岩浆,而是经历了深部岩浆房的分离结晶过程。由单斜辉石估算的高位岩浆房的深度约7～9km。岩浆在高位岩浆房中发生了较长时间的强烈分离结晶作用,分离结晶相主要为橄榄石、单斜辉石和斜长石以及少量的斜方辉石、黑云母、角闪石、碱性长石和铁钛氧化物。不同时期形成的铁钛氧化物指示了分离结晶过程由相对高温高氧逸度向相对低温低氧逸度演化。与此相对照的是岩浆在深部岩浆房中可能只发生了橄榄石和辉石等铁镁矿物的分离结晶作用,且分异作用时间较短。深部岩浆房可能存在于岩石圈地幔或壳幔过渡带中,岩浆由深部岩浆房上升到高位岩浆房中的过程是近绝热的,从浅部岩浆房到地表是快速上升的过程。     

甘肃北山红柳沟基性-超基性岩体岩石成因及成矿条件

甘肃北山红柳沟基性-超基性岩体位于塔里木板块北缘北山裂谷带, 岩体侵位于敦煌岩群,主要岩石类型有辉长岩、橄榄辉长岩、橄榄苏长辉长岩、橄榄角闪苏长岩、橄榄辉长苏长岩、二辉橄榄岩、橄榄辉石岩和辉石岩等。橄榄石Fo介于66.97%~82.92%之间,属贵橄榄石,斜方辉石En成分范围为68.49~77.65,属古铜辉石;单斜辉石En成分范围为45.85~48.81,主要为斜顽辉石和透辉石;斜长石An为58.70~72.69,以拉长石为主;角闪石以普通角闪石为主。岩体母岩浆Mg^#值为0.59~0.62,属于高镁拉斑玄武质岩浆,岩浆演化过程中主要发生了橄榄石、斜方辉石、单斜辉石和斜长石的分离结晶作用,主要分离结晶矿物受单斜辉石和斜方辉石的控制,岩浆上升侵位过程中遭受到下地壳物质混染。从构造环境、母岩浆、岩体类型、岩浆分异程度、同化混染等方面综合分析认为红柳沟岩体具有形成铜镍硫化物矿床的较大潜力。     

东天山香山中镁铁-超镁铁质岩体岩石学与矿物学特征:对成岩成矿过程的约束

香山中镁铁-超镁铁岩体产出于新疆东天山地区的黄山—镜儿泉镁铁-超镁铁岩带。主要的岩石类型为二辉橄榄岩、(含长)单辉橄榄岩、橄榄辉石岩及角闪辉长岩。矿物学特征显示,岩体造岩矿物的结晶顺序为铬尖晶石→橄榄石→斜方辉石→单斜辉石→斜长石→角闪石。电子探针分析显示,同一侵入期次中各岩相具有良好的过渡渐变关系,钻孔剖面上橄榄石含量与颗粒大小、单斜辉石的成分有随深度连续变化的特征,铬尖晶石则具有良好的Cr→Fe3+、Ti的演化趋势,由此反映岩体的形成主要受结晶分异作用的控制;含长单辉橄榄岩中橄榄石和单斜辉石成分的突然变化显示岩浆结晶过程中受到了地壳物质阶段性的混染;模拟计算表明,橄榄石分离结晶伴随着硫化物的熔离;钻孔ZK3693深度为295~325 m,含长单辉橄榄岩中橄榄石、斜长石、单颗粒单斜辉石成分突然性的变化指示在区间内有新鲜的基性程度更高的岩浆注入,而此区间内矿石的Cu、Ni品位明显高于区间外同一岩相岩体,说明新鲜岩浆的注入是香山中岩体硫化物富集的重要原因。     

中天山白石泉镁铁-超镁铁质岩体岩石学与矿物学研究

白石泉地区镁铁一超镁铁质岩体处于塔里木板块前缘活动带与中天山地块接合部位，是中天山地块华力西中期岩浆活动的产物。主要岩石类型有辉石橄榄岩（斜方辉石橄榄岩、斜长二辉橄榄岩）、橄榄辉石岩、橄长岩、辉长岩及角闪辉长岩等，主要造岩矿物为橄榄石、斜方辉石、单斜辉石、角闪石、斜长石及黑云母。橄榄石均为贵橄榄石，其Fo值（78-85）位于含铜镍硫化物矿橄榄石的Fo值范围之内；辉石主要有顽火辉石、古铜辉石、紫苏辉石、透辉石等；斜长石的环带构造较为发育；角闪石的FeO含量随着岩浆的演化逐渐增加。它们与造山带环境中的东疆型镁铁一超镁铁杂岩中的造岩矿物具有相同的特征。这些特征表明了白石泉地区的镁铁一超镁铁质岩体的原始岩浆为高镁的拉斑玄武质岩浆。     

新疆黄山杂岩体矿物学特征及其意义

黄山杂岩体造岩矿物有橄榄石、斜方辉石、单斜辉石、斜长石、角闪石和黑云母。其中斜方辉石的多色性不是由Fe~(2+)引起,而是由Ti~(4+)引起。单斜辉石结晶后常与岩浆继续发生化学反应,形成一种岩浆岩结构——角闪反应补丁结构。橄榄石中镍含量的亏损程度反映杂岩体曾经历过硫化物熔离作用。单斜辉石Si,Al~Ⅳ,Ti的变化表明杂岩体属于拉斑玄武岩系列。超镁铁岩中K,Ba,Th,Hf等元素反常的地球化学行为是由该岩石中黑云母矿物引起。     

层状侵入体形成过程中的AFC过程:以塔里木大火成岩省小海子岩体为例

地幔柱是地球动力学系统中重要的组成部分,不仅形成规模巨大的大火成岩省,也形成了众多和层状基性侵入体相伴生的具有经济价值的矿床类型。因其成矿类型独特(比如PGE、Cr、Ni、V、Ti),层状岩体一直受到非常广泛的关注和深入的研究;其典型的克拉通内部产出环境,也使得它们成为研究克拉通演化和壳-幔相互作用非常重要的岩石组合。大多数层状岩体是地幔岩浆多期次注入到地壳岩浆房中结晶形成,因此其母岩浆不可避免的在某种程度上与岩石圈地幔(SCLM)以及地壳发生相互作用,分离结晶-同化混染作用(AFC)对开放岩浆体系的理解非常重要。小海子侵入体主体是橄榄石-辉石堆晶岩(单辉橄榄岩),主要有堆晶橄榄石(Fo=69~75)和单斜辉石(Mg#=75~84)、粒间的斜长石(An=53~86)和Fe-Ti氧化物组成,被大量的同期岩脉所穿插。岩相学和矿物地球化学均表明橄榄石和单斜辉石早于斜长石和Fe-Ti氧化物。ZK4202钻孔顶和底两个单辉橄榄岩样品给出协和的锆石SIMS U-Pb年龄,分别为(278.8±2.1)Ma和(278.3±2.1)Ma,表明小海子单辉橄榄岩侵入体属于塔里木大火成岩省第二期岩浆活动。穿插堆晶岩的岩脉具有较低的Mg#(35~39),在成分上属于碱性玄武岩-粗面安山岩。受混染程度最小的岩脉显示轻重稀土分异、Nb-Ta富集和Pb亏损。它们具有与洋岛玄武岩(OIB)相似的微量元素特征,并且具有正的εNdi(+4.3~+4.8),暗示其来源于富集的软流圈地幔源区。单辉橄榄岩中单斜辉石具有上凸的稀土元素球粒陨石标准化模式。根据单斜辉石的微量元素计算的与其平衡的熔体微量元素组成与穿插的岩脉非常相似,暗示小海子单辉橄榄岩侵入体和岩脉来自于相似的源区。小海子单辉橄榄岩侵入体中单斜辉石和斜长石之间存在普遍的同位素不平衡现象:与单斜辉石(87Sr/86Sr8i=0.703 8~0.704 1,εNdi=+1.0~+1.9)相比,斜长石具高的7Sr/86Sri(0.704 2~0.704 3)和低的εNdi值(+0.4~+1.0)。单斜辉石的87Sr/86Sri随着Zr/Nb比值的升高而升高,εNdi随着Zr/Nb比值的升高而降低,表明小海子单辉橄榄岩侵入体形成过程中受到不同程度的同化混染。斜长石Ca含量与87Sr/86Sri呈正相关,与εNdi呈负相关,表明An牌号越高的斜长石受到的同化混染程度越大。这种现象可以用基性岩浆在紊流上升过程中混染地壳物质(ATA过程—assimilation during turbulent ascend,导致越原始的岩浆受到混染程度越大,与AFC过程截然相反)来解释。然而ATA模式不能解释单斜辉石Mg#与εNdi呈正相关:即越原始的岩浆受到混染程度越小,与ATA模式预测趋势相悖。更可能的解释是,在分离结晶过程中基性岩浆同化混染塔里木岩浆岩基底和碳酸盐物质。碳酸盐物质的加入,会导致岩浆Ca O含量升高,从而结晶出更高An牌号的斜长石,而且具有更大程度的同化混染。小海子地区侵入到石炭系—二叠系的岩脉中含有大量的单斜辉石巨晶,这些单斜辉石巨晶具有明显的溶蚀结构,其Mg#(80~89)明显高于与寄主岩脉平衡的单斜辉石的Mg#(73)。因此这些单斜辉石与寄主岩脉在结构和化学成分上不平衡,表明它们并非斑晶。这些单斜辉石巨晶和小海子钻孔单辉橄榄岩中单斜辉石的Mg#与Al2O3、Ti O2、REE等具有明显的相关性,具有相似的REE配分模式,表明它们具有成因联系。小海子岩脉中的单斜辉石巨晶为再循环晶(antecryst),是同一个岩浆房中早期结晶的单斜辉石,被后期岩脉所捕获。与单辉橄榄岩中单斜辉石相比,单斜辉石巨晶具有低的87Sr/86Sri(0.703 5~0.703 7)和高的Mg#和εNdi(4.5~4.8),可以用AFC过程来解释。对小海子辉石橄榄岩侵入体和岩脉中单斜辉石再循环晶(antecryst)系统的矿物化学、微量元素和同位素研究表明,在层状岩体形成过程中,AFC过程扮演了非常重要的角色。     

东天山二叠纪镁铁-超镁铁质成矿岩体造岩矿物特征及其成因意义

东天山造山带与镁铁-超镁铁质岩体有关的铜镍硫化物矿床数量多,分布集中,是我国重要的铜镍成矿带之一。成矿岩体多以小岩体群形式产出,由西向东分布有白鑫滩、黄山和图拉尔根3个岩体群,大型矿床主要赋存在黄山岩体群内。本文对3个岩体群内成矿岩体的主要造岩矿物进行了系统的对比研究。成矿岩体的主要造岩矿物为贵橄榄石、古铜辉石、单斜辉石、斜长石和角闪石以及少量的铬尖晶石和金云母。橄榄石Al温度计计算结果表明,黄山岩体群内成矿岩体的母岩浆结晶温度介于1 143~1 257℃之间,略低于白鑫滩和图拉尔根岩体群(1 283~1 301℃)。单斜辉石压力计算表明成矿岩体的结晶压力相似,介于0.31~0.33 GPa之间。成矿岩体中均富含含水矿物,且单斜辉石结晶早于斜长石,指示成矿岩体的原始岩浆形成于富水环境。成矿岩体的单斜辉石具有较高的Al/Ti值,铬尖晶石和橄榄石具有与岛弧火山岩相似的矿物学特征,结合区域构造演化,认为东天山镁铁-超镁铁质成矿岩体的原始岩浆是被消减板片交代过的地幔部分熔融的产物。矿物学特征对比显示了黄山岩体群内成矿岩体更富斜方辉石和中性斜长石,且具有较低的铬尖晶石Cr~#值和橄榄石Ca含量。结合前人研究成果和相关地球化学数据,认为相对高的混染程度导致了黄山岩体群的母岩浆富SiO_2和Al_2O_3,同时降低了岩浆的结晶温度。     

攀西白马岩体的矿物结晶顺序与钒钛磁铁矿成因

距今约260 Ma的白马岩体位于上扬子板块西缘的攀西裂谷中,是一个大型的含钒钛磁铁矿镁铁质-超镁铁质杂岩体,是峨眉山大火成岩省的重要组成部分。含矿岩体主要由磁铁橄长岩和橄榄辉长岩组成,主要工业矿体赋存在下部的橄长岩岩相带中。显微镜下显示橄榄石和角闪石均存在2种不同的结构状态,岩浆具有多次脉动的侵位特点。矿物结构特点及磁铁矿、钛铁矿、橄榄石、角闪石及斜长石等矿物电子探针成分测定显示,矿物的结晶顺序大致为斜长石+橄榄石+辉石→角闪石+磁铁矿+钛铁矿→角闪石。根据角闪石和斜长石成分计算角闪石最低结晶温度为1090℃,斜长石的最高结晶温度是1120℃,推测磁铁矿的结晶温度介于1090~1120℃之间。橄榄石的Fo值由下部的磁铁橄长岩向上部的橄榄辉长岩呈逐渐降低的变化趋势,表明随着岩浆的结晶分异进程,系统的氧逸度是逐步变化的,暗示整个结晶分异过程系统处于封闭状态。磁铁矿中w(V2O3)变化于0.72%~1.37%之间,可近似看成是岩浆演化过程氧逸度较低的量化标志(FMQ+0.5),这种低氧逸度条件下硅酸盐矿物的结晶,会导致粒间熔体氧逸度逐步升高且成分向着富Fe的方向演化。岩浆的这种成分演化特点,是晚期形成不混溶熔浆及富Fe-Ti矿浆的主要原因。     

阿拉斯加型岩体的基本特征、成岩过程及成矿作用

阿拉斯加型岩体是一类具有独特的岩性环带状结构的镁铁-超镁铁质侵入体,常呈链状分布于汇聚板块边缘。其形成时代跨度较大,从元古代到新生代均有分布,以中生代最为发育。大部分阿拉斯加型岩体规模较小,出露面积约12～14 km~2或更小,平面上呈近似同心环状结构,垂直剖面上呈管道状。岩体中心为纯橄岩,向外依次包括异剥橄榄岩、橄榄单斜辉石岩、单斜辉石岩、角闪单斜辉石岩、角闪石岩和辉长岩。造岩矿物为橄榄石、单斜辉石、角闪石等,副矿物为铬铁矿、磁铁矿、钛铁矿等,超镁铁质岩石中少或无斜方辉石,斜长石仅出现在边缘的辉长质岩石中。磁铁矿在单斜辉石岩和角闪石岩中为常见矿物,含量最高达15%～20%。阿拉斯加型岩体的主量元素成分揭示所有岩石均为与拉斑玄武质岩浆分异有关的亚碱性堆晶岩。微量元素成分上显示平坦的稀土元素配分型式和较低的微量元素含量,且富集大离子亲石元素,亏损高场强元素。矿物化学特征上,橄榄石富镁且Fo值变化较大;单斜辉石主要为富Ca的透辉石,其成分变化具有弧堆晶趋势;角闪石主要是镁角闪石和韭角闪石;铬铁矿富集Fe-Al,贫Cr。这些特征揭示,该类岩体成因明显不同于层状岩体和阿尔卑斯型岩体。综合岩石学、矿物学和地球化学分析表明,阿拉斯加型岩体形成于与板块俯冲作用有关的岛弧或者活动大陆边缘背景下,其母岩浆为受到熔/流体交代的地幔楔部分熔融产生的含水玄武质岩浆。各岩相为未受明显地壳混染的同源母岩浆在地壳深度结晶分异的产物。阿拉斯加型岩体的岩浆体系具有含水且高氧逸度的特征,其通常为铂族元素和铬铁矿矿床的重要载体,无或少铜镍矿化。     

新疆且末县几克里阔勒镁铁—超镁铁岩体 地球化学特征及岩石成因

新疆且末县几克里阔勒镁铁—超镁铁岩体位于塔里木板块南缘活动带之喀拉米兰晚古生代沟弧系中段北侧,侵入下石炭统满达拉恰普组第三段海相碎屑岩、碳酸盐岩及中酸性火山岩建造中。主要岩石类型有纯橄岩、二辉橄榄岩、单辉橄榄岩、含长橄榄二辉岩、二辉岩、橄榄辉长岩、淡色辉长岩。岩石地球化学及岩相学特征表明:岩浆作用早期,分离结晶作用主导了岩浆演化过程和岩体形成过程,橄榄石和斜方辉石呈分离/堆晶相;矿物结晶顺序是:尖晶石/橄榄石→斜方辉石→单斜辉石→单斜辉石+斜长石→褐色普通角闪石/黑云母。原生岩浆可能来自原始地幔或中等程度熔融的高镁玄武质岩浆,属拉斑玄武岩系列,形成过程中受到了一定程度同化混染作用的影响,但硫化物熔离程度较弱。岩浆源区位于尖晶石稳定域。     

The Serra da Graciosa A-type Granites and Syenites, southern Brazil: Part 2: Petrographic and mineralogical evolution of the alkaline and aluminous associations

The Serra da Graciosa Granites and Syenites comprise five distinct plutons in the Brasiliano/Pan-African Graciosa A-type Province, southern Brazil. Six petrographic series can be identified in these plutons: (1) Alkaline series 1, composed of amphibole-bearing alkali feldspar syenites with varied mafic mineralogy and quartz contents, from alkali feldspar syenites with calcic amphibole, clinopyroxene, olivine and allanite to alkali feldspar quartz syenites with sodic–calcic amphibole and chevkinite–perrierite and to alkali feldspar granites with sodic amphibole; (2) Alkaline series 2, characterized by amphibole-bearing alkali feldspar granites, with limited modal variations but amphibole compositions also varying from calcic to sodic; (3) Alkaline series 3, of limited occurrence, which includes alkali feldspar syenites with olivine and clinopyroxene and no amphibole; (4) Aluminous series 1, of widespread occurrence, with various petrographic facies of biotite granites with amphibole; (5) Aluminous series 2, characterized by alkali feldspar granites with biotite and only minor amphibole; (6) Monzodiorites, typically with biotite, calcic amphibole and augitic clinopyroxene, partially mingled with granitic magmas. The mafic minerals present are, in general, Fe-rich with correspondingly low Mg and Al contents. In Alkaline series 1, amphiboles crystallized in progressively more oxidizing and alkaline conditions, while in Alkaline series 2, the initial conditions were somewhat more oxidizing and shifted to reducing in the final stages. In Aluminous series 1 and Aluminous series 2, amphiboles are calcic and comparatively homogeneous. The amphiboles in the monzodioritic rocks, while also homogeneous, are more Mg-rich and show compositions quite distinct from the calcic varieties in the other associations, and this is also the case for clinopyroxene. Mg# in biotite decreases from the monzodioritic rocks to Aluminous series 1 and further to Aluminous series 2. Contrasting evolution of the various associations suggests that several coeval magmatic series are present in the Serra da Graciosa granites.     

Crystal fractionation in the petrogenesis of an alkali monzodiorite–syenite series: the Oshurkovo plutonic sheeted complex, Transbaikalia, Russia

The Oshurkovo Complex is a plutonic sheeted complex which represents numerous successive magmatic injections into an expanding system of subparallel and subvertical fractures. It comprises a wide range of rock types including alkali monzodiorite, monzonite, plagioclase-bearing and alkali-feldspar syenites, in the proportion of about 70% mafic rocks to 30% syenite. We suggest that the variation within the complex originated mainly by fractional crystallization of a tephrite magma.

The mafic rocks are considered as plutonic equivalents of lamprophyres. They exhibit a high abundance of ternary feldspar and apatite, the latter may attain 7–8 vol.% in monzodiorite. Ternary feldspar is also abundant in the syenites. The entire rock series is characterized by high Ba and Sr concentrations in the bulk rock samples (3000–7000 ppm) and in feldspars (up to 1 wt.%). The mafic magma had amphibole at the liquidus at 1010–1030 °C based on amphibole geothermometer. Temperatures as low as this were due to high H₂O and P₂O₅ contents in the melt (up to 4–6 and 2 wt.%, respectively). Crystallization of the syenitic magmas began at about 850 °C (based on ternary feldspar thermometry). The series was formed at an oxygen fugacity from the NNO to HM buffer, or even higher.

The evolution of the alkali monzodiorite–syenite series by fractional crystallization of a tephritic magma is established on the basis of geological, mineralogical, geochemical and Sm–Nd and Rb–Sr isotope data. The geochemical modeling suggests that fractionation of amphibole with subordinate apatite from the tephrite magma leaves about 73 wt.% of the residual monzonite melt. Further extraction of amphibole and plagioclase with minor apatite and Fe–Ti oxides could bring to formation of a syenite residuum. Rb–Sr isotopic analyses of biotite, apatite and whole-rock samples constrain the minimum age of basic intrusions at ca. 130 Ma and that of cross-cutting granite pegmatites at ca. 120 Ma. Hence the entire evolution took place in an interval of ≤10 My. Initial ⁸⁷Sr/⁸⁶Sr ratios for the mafic rocks range from 0.70511 to 0.70514, and for syenites from 0.70525 to 0.70542. Initial _Nd (130 Ma) values for mafic rocks vary from −1.9 to −2.4, and for syenites from −2.9 to −3.5. In a _Nd(T) vs. (⁸⁷Sr/⁸⁶Sr)_i diagram, all rock types of the complex fall in the enriched portion of the Mantle Array, suggesting their derivation from a metasomatized mantle source. However, the small but distinguishable difference in Sr and Nd isotopic compositions between mafic rocks and syenites probably resulted from mild (10–20%) crustal contamination during differentiation. Large negative Nb anomalies are interpreted as a characteristic feature of the source region produced by Precambrian fluid metasomatism above a subduction zone rather than by crustal contamination.     

Enriched Subcontinental Lithospheric Mantle in the Northern Part of the South Indian Granulite Terrain: Evidence from Yelagiri and Sevattur Syenite Plutons, Tamil Nadu, South India

In the southern part of the Indian Peninsula, there are a number of alkaline plutons of Proterozoic age. In the northern part of the South Indian granulite terrain, the Yelagiri (syenite, pyroxenite) and Sevattur (syenite, pyroxenite, carbonatite) plutons intrude Archaean epidote-hornblende gneisses. Geochemical and isotopic characteristics of the Yelagiri and Sevattur plutons indicate that the syenitic magmas formed from highly differentiated mantle-derived alkali basalts. The Yelagiri and Sevattur syenites are characterized by evolved Sr and Nd isotopic compositions, pronounced enrichment in LILE and large negative Nb anomalies. Trace element and Sr-Nd isotope characteristics of the Yelagiri and Sevattur syenites are similar to those of the subduction-related alkaline rocks. The scarcity of geological evidence for subduction activity at the time of syenite intrusion during Neoproterozoic does not support a link between the alkaline magmatism and subduction. However, our data are consistent with the model of derivation of the Yelagiri and Sevattur syenites from the subcontinental lithospheric mantle, which was previously enriched by slab derived component. The geochemical and isotopic signatures of other mantle-derived intrusive rocks in the northern part of the South Indian granulite terrain with ages ranging from ca. 2.5 Ga to 0.75 Ga also support the above idea.     

The M?gantic Unstrusive Complex, Qu?bec: a Study of the Derivation of Silica-Oversaturated Anorogenic Magmas of Alkaline Affinity

The Cretaceous M?gantic intrusive complex of southern Qu?beccontains early noritic gabbrodiorites which represent cumulatesfrom crustally contaminated hawaiite to syenite magmas. Wholerock and mineral chemistry, as well as textural evidence, indicatethat post-cumulus recrystallization and reaction were important,and most of the amphibole and biotite are thought to have formedin this way. A younger plutonic quartz-syenite ringdyke maynot be cogenetic with the gabbro-diorites sice it lacks orthopyroxene.It may, however, be cogenetic with basaltic to riebeckite granitedykes. Fractionation of olivine, plagioclase, aluminous clinopyroxene,and minor Ti-magnetite from critically undersaturated alkalibasaltic magmas generated hawaiitic magmas. The developmentof quartzbearing mugearitic and syenitic residua from the hawaiitescan best be modelled by fractionation of amphibole, plagioclase,olivine, oxides, and apatite. Attempts to model fractionationusing observed phenocrysts (including clinopyroxene) were unsuccessful.Amphibole fractionation is interpreted to have taken place througha reaction with still-porous, higher-temperature cumulates onthe walls of the magma chamber. The plutonic syenites probablyrepresent alkali feldspar cumulates from the residual syeniticmelts. Magnesian calc-alkaline lamprophyres exhibit olivineto phlogopite reaction textures, are enriched in Cr, Ni, K,Rb, Nb, Y, Zr, and Si relative to the basaltic dykes, yet havesimilar incompatible element ratios. Their relation to the basaltsis problematical. The late biotite-granite core to the complexis identical to typical White Mountain granites and may haveformed as an anatectic cap on rising, fractionating, mantlederivedmagmas.     

French Creek Granite and Hohonu Dyke Swarm,South Island,New Zealand: Late Cretaceous alkaline magmatism and the opening of the Tasman Sea*

The Hohonu Dyke Swarm and French Creek Granite represent contemporaneous and cogenetic alkaline magmatism generated during crustal extension in the Western Province of New Zealand. The age of 82 Ma for French Creek Granite coincides with the oldest oceanic crust in the Tasman Sea and suggests emplacement during the separation of New Zealand and Australia. The French Creek Granite is a composite A‐type granitoid, dominated by a subsolvus biotite syenogranite with high silica, low CaO, MgO, Cr, Ni, V and Sr and elevated high‐field‐strength elements (Zr, Nb, Ga, Y). Subordinate varieties of French Creek Granite include a hypersolvus alkali amphibole monzogranite and a quartz‐alkali feldspar syenite. Spatially associated rhyolitic dykes are considered to represent hypabyssal equivalents of French Creek Granite. The Hohonu Dyke Swarm represents mafic magmatism which preceded, overlapped with, and followed emplacement of French Creek Granite. Lamprophyric and doleritic varieties dominate the swarm, with rare phonolite dykes also present. Geochemical compositions of French Creek Granite indicate it is an A₁‐subtype granitoid and suggest derivation by fractionation of a mantle‐derived melt with oceanic island basalt ‐ like characteristics. The hypothesis that the French Creek Granite represents fractionation of a Hohonu Dyke Swarm composition, or a mantle melt derived from the same source, is tested. Major‐ and trace‐element data are compatible with derivation of the French Creek Granite by fractionation of amphibole, clinopyroxene and plagioclase from mafic magmas, followed by fractionation of alkali and plagioclase feldspar at more felsic compositions. Although some variants of the French Creek Granite have Sr and Nd isotopic compositions overlapping those of the Hohonu Dyke Swarm, most of the French Creek Granite is more radiogenic than the Hohonu Dyke Swarm, indicating the involvement of a radiogenic crustal component. Assimilation‐fractional crystallisation modelling suggests isotopic compositions of French Creek Granite are consistent with extreme fractionation of Hohonu Dyke Swarm magmas with minor assimilation of the Greenland Group metasediments.     

Geology of Pulikonda and Dancherla alkaline complexes,Andhra Pradesh

Geological studies on saturated to oversaturated and subsolvus aegirine-riebeckite syenite bodies of the Pulikonda alkaline complex and Dancherla alkaline complex were carried out. The REE distribution of the Dancherla syenite shows a high fractionation between LREE and HREE. The absence of Eu anomaly suggests source from garnet peridotite. The Pulikonda syenite shows moderate fractionation between LREE and HREE as reflected by enrichment of HREE and moderate enrichment of LREE. The negative Eu anomaly indicates role of plagioclase fractionation.Three distinct co-eval primary magmas i.e. mafic syenite-, felsic syenite- and alkali basalt magmas — all derived from low-degrees of partial melting of mantle differentiates and enriched metasomatised lower crust played a major role in the genesis and emplacement of the syenites into overlying crust along deep seated regional scale trans-lithospheric strike-slip faults and shear zones following immediately after late-Archaean calc-alkaline arc magmatism at different time-space episodes i.e. initially at craton margin and later on into the thickened interior of the Eastern Dharwar craton. The ductile sheared and folded Pulikonda alkaline complex was evolved dominantly from the magmas derived from partial melting of lower crust and minor juvenile magmas from mantle. Differentiation and fractionation by liquid immiscibility of mafic magma and commingling-mixing of intermediate and felsic magmas followed by fractionational crystallisation under extensional tectonics during waning stages of calc-alkaline arc magmatism nearer to the craton margin were attributed as the main processes for the genesis of Pulikonda syenite complex. Commingling and limited mixing of independent mantle derived mafic and felsic syenitic magmas and accompanying fractionation resulting into soda rich and potash rich syenite variants was tentatively deduced mechanism for the origin of Dancherla, Danduvaripalle, Reddypalle syenites and other bodies belonging to Dancherla alkaline complex at the craton interior. The Peddavaduguru syenite was formed by differentiation of alkali mafic magma (gabbro to diorite) and it’s simultaneous mingling with fractionated felsic syenitic magma under incipient rift. Vannedoddi and Yeguvapalli syenites were derived due to desilicification and accompanying alkali feldspar mestasomatism of younger potash rich granites along Guntakal-Gooty fault and along Singanamala shear zone respectively.     

西南天山托云中新生代火山岩盆地响岩的岩石成因研究

响岩代表了陆内火山岩省钠质碱性岩浆最终端的演化产物,揭示其成因机制对认识陆内火山岩省的岩浆演化过程具有重要意义。文章对出露在我国境内西南天山托云中新生代火山岩盆地的响岩开展了系统的矿物学、岩石学和地球化学研究,以揭示托云火山岩岩浆系列特征和岩浆演化过程。托云响岩以发育碱性长石、单斜辉石、铁橄榄石和角闪石斑晶为特征,在基质中可见霞石。在地球化学组成上,托云响岩以高的Na₂O含量(7.9%～8.6%)和K₂O含量(5.2%～6.3%)为特征,∑REE变化范围为408×10^-6～470×10^-6,稀土元素配分曲线具有明显的Eu异常,在原始地幔标准化的微量元素蛛网图上,样品显示出Ba、Sr、P、Ti的负异常和Nb、Ta、Zr、Hf的正异常特征。亏损的Nd同位素组成表明其成因与共生的碧玄岩有紧密的成因联系,经MELTS热力学模拟,响岩可由碧玄岩经碱性长石(36%)、单斜辉石(21%)、尖晶石(10%)、铁橄榄石(6%)和角闪石(5%)的分离结晶作用形成,进一步的EC-AFC模拟表明,响岩在形成过程中受到一定程...     

华北晚中生代和新生代玄武岩中单斜辉石巨晶来源及其对寄主岩岩浆过程的制约:以莒南和鹤壁为例

本文对华北克拉通晚中生代和新生代碱性玄武质岩石中的单斜辉石巨晶进行了主、微量元素和Sr-Nd同位素的综合研究,发现晚中生代和新生代单斜辉石巨晶存在明显的主、微量元素和同位素组成上的差异。新生代单斜辉石巨晶有Al-普通辉石和次透辉石两类;而中生代单斜辉石巨晶只有Al-普通辉石。新生代单斜辉石SiO_2含量高、REE配分型式为上凸型、LILE和放射性元素含量高,并具有比寄主碱性玄武岩更亏损的Sr和Nd同位素组成;而中生代单斜辉石SiO_2含量低、REE配分型式为LREE富集型、LILE和部分HFSE以及放射性元素含量低,并具有比寄主碱性玄武岩稍富集的Sr和Nd同位素组成;巨晶的结构、矿物成分和地球化学特征,以及Mg-Fe在熔体与单斜辉石间的分配状况皆说明,新生代碱性玄武岩中单斜辉石巨晶是碱性玄武岩浆在高压下结晶的,因此二者是同源的;而中生代单斜辉石巨晶是被寄主岩浆偶然捕获的捕虏晶,是不同源的。华北新生代单斜辉石巨晶存在于碱性玄武岩和拉斑玄武岩中,它们具有比寄主碱性玄武岩更亏损的Sr和Nd同位素组成,说明即使是碱性玄武岩也不能完全代表软流圈来源的原始岩浆,其在上升过程中或多或少存在同位素组成富集的物质的混入。同时,拉斑玄武岩不是碱性玄武质岩浆直接结晶分异的产物,亦不是完全由部分熔融程度的不同造成的。拉斑玄武岩中存在岩石圈地幔物质的贡献或是岩浆房内碱性玄武质岩浆受地壳混染作用的结果。     

Fe isotope systematics of coexisting amphibole and pyroxene in the alkaline igneous rock suite of the Ilímaussaq Complex,South Greenland

The Ilímaussaq intrusion, South Greenland, provides an exceptional test case for investigating the changes of stable Fe isotope fractionation of solidus phases with changes in the Fe³⁺/∑Fe ratio of an evolving melt. The intrusion comprises a sequence of four melt batches that were fed from the same parental alkali basaltic magma. Differentiation produced cumulate rocks that range from augite syenite (phase I) over peralkaline granite (phase II) to agpaitic syenites (phases IIIa and IIIb). Fe³⁺/∑Fe ratios in amphiboles increase substantially from phase I to phase II and III rocks and mark a major change in the parental magma composition from augite syenites to peralkaline granites and agpaitic syenites. Before this transition, olivine, clinopyroxene, and amphibole in augite syenite, the most primitive rock type in the Ilímaussaq Complex, have a uniform Fe isotope composition that is identical to that of the bulk of igneous crustal rocks and approximated by the average isotopic composition of basalts (δ^56/54Fe_IRMM-014 = 0.072 ± 0.046‰). After the transition, amphiboles in the peralkaline granites and agpaitic syenites yield significantly heavier Fe isotope compositions with δ^56/54Fe_IRMM-014 values ranging from 0.123 to 0.237‰. Contamination of the Ilímaussaq magma by ongoing crustal assimilation as cause for this increase can be excluded on the grounds of Nd isotope data. Large-scale metasomatic overprint with an external fluid can also be dismissed based on amphibole O and Li isotope systematics. Rather, the increase towards heavy Fe isotope compositions most likely reflects the change in chemical compositions of amphiboles (calcic in augite syenite to sodic in the agpaitic syenites) and their Fe³⁺/ΣFe ratios that mirror changes in the chemical composition of the melt and its oxygen fugacity. A sensitive adjustment of equilibrium Fe isotope fractionation factors to amphibole ferric/ferrous ratios is also supported by beta-factors calculated from Mössbauer spetroscopy data. Comparison of the measured isotope fractionation between clinopyroxene and amphibole with that predicted from Mössbauer data reveal Fe isotope systematics close to equilibrium in augite syenites but Fe isotopic disequilibrium between these two phases in phase IIIa agpaitic syenites. These results are in agreement with O and Li isotope systematics. While amphiboles in all Ilímaussaq lithologies crystallized at temperatures between 650 and 850 °C, textural evidence reveals later clinopyroxene crystallization at temperatures as low as 300–400 °C. Therefore, isotopic equilibrium at crystallization conditions between these two phases can not be expected, but importantly, subsolidus reequilibration can also be dismissed.     

Mineralogical constraints on the petrogenesis of trachytic inclusions,Carpenter Ridge Tuff,Central San Juan volcanic field,Colorado

Although bulk-rock normative analyses of the trachytic inclusions from the Carpenter Ridge Tuff yield abundant quartz and minor corundum, a portion of the phenocryst assemblage is indicative of an alkaline parentage. Sanidine and biotite contain up to 8 and 5 wt% BaO respectively. In addition, both amphibole and clinopyroxene compositions are compatible with having crystallized from a mildly silica-undersaturated magma. Amphibole is magnesiohastingsite with 3 wt% TiO₂ and less than 0.3 mole fraction vacancies in the A site. Clinopyroxene compositions straddle the calcic augite-salite boundary. Chrondite-normalized REE patterns are similar for both inclusions and rhyolites. The inclusions are slightly poorer in REE and have a positive Eu anomaly versus the negative anomaly of the rhyolites. The similarity in REE patterns would seem to indicate that the two rock types are genetically related with the positive Eu anomalies resulting from feldspar accumulation. However, this possibility is denied by the antithetic alkaline and subalkaline phenocryst assemblages of the two rock types. We suggest that the best explanation for these discrepancies is that a mildly silica-undersaturated magma was the parent for the phenocrysts. This magma intruded the Carpenter Ridge chamber, and because the crystallization temperatures of both magma overlapped, the alkaline magma mixed with the ambient rhyolite to form a hybrid. This hybrid consisted of a portion of the phenocryst assemblage from the alkaline magma but the bulk-rock chemistry depended upon the proportions of the endmember liquids. The abundance of normative quartz, the minor normative corundum, and the similarity of REE patterns indicates that the inclusions are mixtures dominated by the rhyolitic component. Additional processes such as liquid-state diffusion, crystal accumulation, and alkali loss may have contributed to obscure the compositions of the initial liquids involved in the inferred mixing process.