The Origin of Marginal Compositional Reversals in Basic-Ultrabasic Sills and Layered Intrusions by Soret Fractionation

Marginal reversals—a common feature of many basic differentiatedigneous bodies regardless of their size and bulk composition—areremarkable in being a mirror of the Layered Series. These aredistinguished by: (1) an apparent lack of mass balance betweenthe lower part of the marginal reversals, including chilledmargins, and the bulk composition of the intrusions; (2) mineralcrystallization sequences and (3) mineral compositional trends,which are both essentially the opposite of those in the LayeredSeries; (4) the cotectic composition of rocks composing themarginal reversals; (5) the capacity to form from both phenocryst-richand phenocryst-free parental magmas; (6) the capability to developalong the floor, subvertical walls and even the roof of magmachambers. None of the current models of magma chamber evolutioncan provide an adequate explanation for the characteristic featuresof the marginal reversals. The problem can be resolved in thecontext of a model combining Soret diffusion in thin liquidboundary layers at the magma chamber margins and vigorous convectionin the main magma body. The key proposal is that the formationof marginal reversals takes place through the non-equilibriumevolution of liquid boundary layers as a result of a temperaturegradient imposed by the cold country rock. The fundamental explanationfor the mirror image of a marginal reversal is that the non-equilibriumSoret fractionation works in a manner opposite to that of theequilibrium crystal–liquid fractionation that producesthe Layered Series. KEY WORDS: marginal compositional reversals; sills; layered intrusions; Soret fractionation     

金川岩体母岩浆成分及其分离结晶过程的熔浆热力学模拟

金川铜镍硫化物矿床是仅次于加拿大Sudbury和俄罗斯Noril’sk-Talnakh 的世界第三大在采镍矿床。金川岩体主要由含二辉橄榄岩、二辉橄榄岩、斜长二辉橄榄岩和橄榄辉石岩组成,岩相学观察表明主要造岩矿物的结晶顺序为：橄榄石→斜方辉石→单斜辉石→斜长石。为了进一步探讨金川岩体母岩浆成分及其分离结晶过程,本文在前人工作基础上根据主要造岩矿物结晶顺序及其电子探针成分,借助熔浆热力学软件“MELTS”的计算,获得金川岩体更为准确的母岩浆成分为：48.2% SiO2,1.00% TiO2,11.3 % Al2O3,12.9% FeO,1.30% Fe2O3,12.6% MgO,10.1% CaO,1.51% Na2O,0.72% K2O,0.04% NiO。MELTS模拟计算表明金川岩体母岩浆的分离结晶经历了两个阶段,在深度约为10.9~12.5km的深部岩浆房经历了约5%的橄榄石以及约4%的斜方辉石分离结晶,并伴随硫化物熔离。在重力作用的影响下,橄榄石、斜方辉石和硫化物向下沉降,形成由下至上的分层：橄榄石-斜方辉石-硫化物-硅酸盐熔浆层,橄榄石-斜方辉石-硅酸盐熔浆层和硅酸盐熔浆层。硅酸盐熔浆首先挤出形成贫硫化物的岩体或喷出地表,之后橄榄石-斜方辉石-硅酸盐熔浆被挤入到7.6~9.2km的浅部岩浆房,经重力分异形成金川I号岩体的上部岩相带和II号岩体顶部的含辉橄榄岩。橄榄石-斜方辉石-硫化物-硅酸盐熔浆层最后被挤入金川岩体,并再次结晶出橄榄石、单斜辉石、斜长石,形成金川岩体的主要岩相和硫化物矿体。这些计算结果不仅与野外和室内岩相学观察吻合,也与硫化物熔离过程的最新研究结果相一致。     

论南京娘娘山碱性岩浆房的梯度及其成因机制

本文详细研究了娘娘山碱性火山杂岩体中矿物斑晶和化学成分的变化情况,发现它们定位前在岩浆房中就存在着物质成分的梯度和分带性。提出并论证矿物的分离结晶是该岩浆房中梯度形成的主要机制,但由对流所控制的热重力扩散作用对岩浆房中成分梯度的形成也具一定的影响。     

论南京娘娘山碱性岩浆房的梯度及基成因机制

本文详细研究了娘娘山碱性火山杂岩体中矿物斑晶和化学成分的变化情况，发现它们 定位前在岩浆房中就存在着物质成分的梯度和分带性。提出并论证矿物的分离结晶是该岩浆房中 梯度形成的主要机制，但由对流所控制的热重力扩散作用对岩浆房中成分梯度的形成也具一定的 影响。     

The Pleasant Bay Layered Gabbro--Diorite, Coastal Maine: Ponding and Crystallization of Basaltic Injections into a Silicic Magma Chamber

The Pleasant Bay layered gabbro–diorite intrusion, locatedon the coast of Maine between Bar Harbor and Machias, is roughlyoval in plan, measuring 12 km by 20 km. Gravity data, contactrelations, and internal layering suggest that it is basinformin structure with a maximum thickness of {small tilde}3 km.Its roof and upper parts have been lost through erosion. Whereit is in contact with underlying granite, the base of the intrusiontypically consists of strongly chilled gabbro with convex-downwardlobate forms, suggesting that the granite was incompletely solidifiedwhen the gabbro was emplaced. Roughly 90% of the exposed rocksare weakly layered gabbro and mafic diorite, both of which varywidely in grain-size and texture. Layers and lenses of medium-grainedleucocratic diorite to granodiorite are widely intercalatedwith the chilled mafic rocks and commonly contain partly digestedmafic inclusions; they also commonly contain zones of pillow-likebodies of gabbro chilled on all margins. The dioritic rocksare consistently topped by gabbroic layers with chilled lobatebases and commonly appear to feed granitic pipes and diapirsinto overlying gabbro. Much of the intrusion can be subdividedinto hundreds of macrorhythmic units (from 1 to 100 m thick)consisting of basally chilled gabbro that grades upward to dioriteor highly evolved leucocratic silicic cumulates. Basaltic dikesare abundant both in the underlying granite and in the layeredgabbro–diorites; they have appropriate compositions tobe feeders for chilled gabbroic layers in the Pleasant Bay intrusion. The layered rocks of the Pleasant Bay intrusion record hundredsof basaltic injections into a chamber with resident silicicmagma. Small injections produced chilled gabbroic layers andpillows within silicic cumulates. Larger infusions of basalticmagma produced temporary compositional stratification and episodesof double-diffusive convection within the chamber. Althoughfractional crystallization produced compositional variationin much of the gabbro, units that grade from chilled gabbroat the base to highly silicic cumulates at the top provide cumulaterecords of magma stratification and hybridization along a double-diffusiveinterface between basaltic and silicic magmas. The intrusionprovides a superb plutonic record of events that have oftenbeen inferred for silicic eruptive centers. Mafic–siliciclayered intrusions comparable with the Pleasant Bay are morewidespread than has generally been appreciated.     

Origin of Phenocrysts and Compositional Diversity in Pre-Mazama Rhyodacite Lavas, Crater Lake, Oregon

Phenocrysts in porphyritic volcanic rocks may originate in avariety of ways in addition to nucleation and growth in thematrix in which they are found. Porphyritic rhyodacite lavasthat underlie the eastern half of Mount Mazama, the High Cascadeandesite/dacite volcano that contains Crater Lake caldera, containevidence that bears on the general problem of phenocryst origin.Phenocrysts in these lavas apparently formed by crystallizationnear the margins of a magma chamber and were admixed into convectingmagma before eruption. About 20 km³ of pre-Mazama rhyodacite magma erupted during arelatively short period between400 and 500 ka; exposed pre-Mazamadacites are older and less voluminous. The rhyodacites formedas many as 40 lava domes and flows that can be assigned to threeeruptive groups on the basis of composition and phenocryst content.Phenocryst abundance decreases (from 32 to 8 vol.%) and SiO₂content increases (from 68 to 73 wt.%) in the apparent orderof eruption. Phenocrysts (plagioclase, orthopyroxene, augite,and Fe-Ti oxides) are commonly fragmental or form polycrystallineaggregates with interstitial glass. Discrete phenocrysts withcomplete euhedral outlines are rare except for small elongatedcrystals. The abundance of discrete phenocrysts increases withthat of aggregates. The grain-size of minerals in the aggregatescovers the range of discrete phenocrysts (0.2–4.2 mm).Rim compositions of phenocrysts and the range of chemical zoningare almost uniform among the three rhyodacite groups, regardlessof whether crystals are discrete or in aggregates. However,a small fraction of phenocrysts, especially small elongatedcrystals, have different compositions: plagioclase with Fe-richcores and augite with Wo-poor cores, both of which are characteristicof crystals in undercooled andesite enclaves in the rhyodacites.The majority of phenocrysts were derived by disintegration ofpolycrystalline aggregates; rare, small phenocrysts crystallizedin andesitic magma similar to that represented by the andesiteenclaves. The modal and chemical compositions of the rhyodacites can beexplained by different degrees of admixing of crystals, representedby the aggregates, into magma having 4 vol.% ‘true’phenocrysts, mainly plagioclase. The aggregates may be partsof the rind formed by in situ crystallization near the walland roof of the magma chamber. The rind was disrupted duringor just before eruption, and pieces were variably disaggregatedand incorporated into erupting magma. The amount of rind incorporateddeclined during the sequence of eruptions. Owing to vesiculationof interstitial liquid and shearing during flow, crystals inthe aggregates were separated and became phenocrysts. Pre-Mazamarhyodacite was erupted dominantly as lava, as opposed to thecompositionally similar rhyodacite pumice of the Holocene caldera-formingeruption of Mount Mazama, apparently because its source chamberwas crystallizing inward rather than actively growing.     

辽宁赛马碱性岩体层硅铈钛矿化学成分变化及其对碱性岩浆演化的指示意义

辽宁赛马岩体是我国典型的产铀碱性杂岩体,但其稀土矿化机制尚不明确.通过光学显微镜、扫描电镜和电子探针分析,得知该岩体从霞石正长岩经霓霞正长伟晶岩至晚期异霞正长岩,代表性稀土矿物层硅铈钛矿[Na₂Ca₄REETi（Si₂O₇）₂OF_3]不断富集,Nb、Zr和REE（特别是HREE）等高场强元素含量不断升高,部分颗粒具Zr、REE等元素成分环带,以上成分变化与稀土等不相容元素性质、碱性岩浆成分和岩浆结晶分异过程密切相关.此外,层硅铈钛矿经历了一系列的热液蚀变,蚀变部分Ti、Ca、Sr、Na含量增加而Zr、REE含量降低,最终形成由残余层硅铈钛矿+方解石+萤石+铈硅磷灰石组成的假晶,可能与富碱质、F和CO₂的自交代流体作用有关.该研究揭示了碱性岩浆演化过程中,层硅铈钛矿成分变化及热液蚀变组合对指示岩浆结晶分异程度、探究稀土元素分馏及其热液活动性的具有重要意义.      

甘肃北山红石山基性-超基性岩体的成因类型及构造意义

红石山基性-超基性岩体是板块构造研究者重点关注的岩体之一,通过岩体地质特征及系统的岩石化学、微量元素、稀土元素等研究,探讨了红石山基性-超基性岩体的成因类型和形成构造环境。根据岩体的岩石类型组合、相带产出关系、纯橄岩离析体的发育、岩体内部流动构造、晚期多种岩脉和铬铁矿矿化类型等特征研究表明,岩体应是一种岩浆类型的侵入体;从岩石化学、微量、稀土元素地球化学特征上可反映出该岩体是晚古生代(石炭—二叠纪)时期与古亚洲大洋在向南消之闭合其间,于雀儿山-英安山岛弧带南缘发育起来的一种弧后盆地裂谷类型的地幔岩浆作用产物,其机制或动力学环境为一种弧后扩张,地幔上隆的大陆裂谷岩浆作用机制。     

Cyclicity in the Main and Upper Zones of the Bushveld Complex, South Africa: Crystallization from a Zoned Magma Sheet

The major element composition of plagioclase, pyroxene, olivine,and magnetite, and whole-rock ⁸⁷Sr/⁸⁶Sr data are presented forthe uppermost 2·1 km of the layered mafic rocks (upperMain Zone and Upper Zone) at Bierkraal in the western BushveldComplex. Initial ⁸⁷Sr/⁸⁶Sr ratios are near-constant (0·7073± 0·0001) for 24 samples and imply crystallizationfrom a homogeneous magma sheet without major magma rechargeor assimilation. The 2125 m thick section investigated in drillcore comprises 26 magnetitite and six nelsonite (magnetite–ilmenite–apatite)layers and changes up-section from gabbronorite (An₇₂ plagioclase;Mg# 74 clinopyroxene) to magnetite–ilmenite–apatite–fayaliteferrodiorite (An₄₃; Mg# 5 clinopyroxene; Fo₁ olivine). The overallfractionation trend is, however, interrupted by reversals characterizedby higher An% of plagioclase, higher Mg# of pyroxene and olivine,and higher V₂O₅ of magnetite. In the upper half of the successionthere is also the intermittent presence of cumulus olivine andapatite. These reversals in normal fractionation trends definethe bases of at least nine major cycles. We have calculateda plausible composition for the magma from which this entiresuccession formed. Forward fractional crystallization modelingof this composition predicts an initial increase in total iron,near-constant SiO₂ and an increasing density of the residualmagma before magnetite crystallizes. After magnetite beginsto crystallize the residual magma shows a near-constant totaliron, an increase in SiO₂ and decrease in density. We explainthe observed cyclicity by bottom crystallization. Initiallymagma stratification developed during crystallization of thebasal gabbronorites. Once magnetite began to crystallize, periodicdensity inversion led to mixing with the overlying magma layer,producing mineralogical breaks between fractionation cycles.The magnetitite and nelsonite layers mainly occur within fractionationcycles, not at their bases. In at least two cases, crystallizationof thick magnetitite layers may have lowered the density ofthe basal layer of melt dramatically, and triggered the proposeddensity inversion, resulting in close, but not perfect, coincidenceof mineralogical breaks and packages of magnetitite layers. KEY WORDS: layered intrusion; mineral chemistry; isotopes; magma; convection; differentiation     

川西义敦岛弧中生代典型花岗岩体矿物学、地球化学特征及岩浆来源探讨

义敦岛弧是位于松潘—甘孜褶皱带和羌塘地体之间的三叠纪火山岛弧。稻城岩体和海子山岩体分别为义敦岛弧上出露的晚三叠世和白垩纪花岗质岩体。结合岩石地球化学,以及偏光显微镜和电子探针(EPMA)、扫描电镜(SEM)对上述岩体进行了系统的矿物学研究,结果表明:两个岩体的主要造岩矿物为斜长石(中长石—更长石),钾长石(正长石)、石英和黑云母(铁叶云母—铁质黑云母),副矿物为锆石、榍石、磁铁矿和磷灰石等;黑云母的成分表明两个岩体的源区均为壳幔混源。稻城岩体属高钾钙碱性系列,具过铝质特征的花岗岩,海子山岩体为高钾钙碱性—钾玄岩系列,同样具过铝质特征。根据锆元素饱和浓度温度计和稀土元素饱和浓度温度计对两个岩体进行了温度限定,稻城岩体岩浆形成的平均上限温度为783℃,海子山岩体岩浆形成时的平均上限温度为844℃。结合前人研究结果,笔者等认为稻城岩体为高分异I型花岗岩,可能为与扬子克拉通有关的晚古元古代至早中元古代的下地壳物质,在甘孜—理塘洋向西俯冲和闭合之后的弧—陆同碰撞背景下,因幔源岩浆的底侵作用而发生部分熔融,同时伴有少量的亏损地幔成分加入,因密度上升至中上—上地壳深度侵位,并且侵位后经历了快速的冷却过程,在侏罗纪之初冷却至300℃以下;海子山岩体为与俯冲有关的造山后伸展环境下形成的白垩纪A2型花岗岩,岩浆来源同样为地壳物质伴有少量地幔物质混合而成,且在地壳中侵位深度较浅,之后经历快速的冷却过程。     

Origin and Intensive Parameters in the Crystallization of Ultrapotassic Syenites: The Serra Do Man Pluton,Northeastern Brazil

The late Neoproterozoic ultrapotassic Serra do Man pluton intruded supracrustal schists and igneous rocks of the Alto Pajeu tectonostratigraphic terrane, in the Borborema province of northeastern Brazil. This quartz-alkali feldspar syenite is composed of microcline, albite, normally zoned aegirine-augite, winchite (formed after pyroxene), titanite and apatite, with the major-element geochemistry in consonance with early fractionation of pyroxene-titanite-apatite assemblage. Chondrite-normalized REE-patterns are LREE-enriched (La/Lu ∼ 20) with discrete negative Eu anomaly, and primitive mantle-normalized elemental concentration diagrams display enhanced negative Nb, P and Ti anomalies that are also observed in patterns of similar rocks in NE Brazil. Whole-rock d¹⁸O values are rather high (avg. +8.7%_SMOW) and in the +7.8 to +8.6%_SMOW range when clinopyroxene-corrected. A five-point whole-rock Rb-Sr isochron yields an age of 580±28 Ma with high Sr_o (0.7108), which together with eNd (0.58 Ga) value of -17 and t_chur of 2.3 Ga allow hypothesizing a Paleoproterozoic enriched-mantle source for the syenites in the ultrapotassic province of the Alto Pajeu terrane. The aegirine-augite, titanite, and quartz assemblage attests to a high oxygen fugacity prevailing during magmatic crystallization. The almost complete absence of primary hydrated phases assures that the magma was rather anhydrous, and its viscosity, at the liquidus temperature, was relatively high. Density is slightly higher than expected for syenitic magmas (2.8 g/cm³), being explained by a relatively high iron content of the magma. Geothermobarometry suggests shallow emplacement and temperatures of 700-750°C and 850-900°C, respectively, for the solidus and liquidus of the magma.     

Structural and Magmatic Evolution of a Magma Chamber: The Newark Island Layered Intrusion, Nain, Labrador

The Newark Island Layered Intrusion occurs in the ProterozoicNain anorthosite complex of Labrador. It contains an exceptionalsuite of cumulates ranging from troctolites and gabbros to quartzmonzonites and intermediate hybrid rocks. These layered rocksformed in a chamber that was periodically fed by a wide rangeof basic and acid magmas, the compositions of which are preservedin numerous feeder dikes. Where basic magmas commingled withcooler granitic magma, they commonly formed chilled pillows.Because of periodic injections of both acid and basic magmasthe magma chamber was compositionally stratified for much ofits existence. At times, granitic cumulates formed along thechamber walls while mafic to intermediate hybrid cumulates formedon the floor. Stratigraphic and structural relations indicatethat the magma chamber grew upward during deposition, and thatit evolved from a west-dipping sheet to a north-plunging synform.Three major episodes of expansion can be linked to injectionsof large (e.g., 20km³) volumes of acid magma. The entry of thisacid magma into the chamber disrupted previously formed cumulates,creating enlarged feeders down which resident basic magma collapsed.The resultant structures (troughs) contain strongly chilledpillows of resident basic magma that existed near the bottomof the chamber at the time of acid replenishment.     

A Melt Inclusion Record of Volatiles, Trace Elements and Li-B Isotope Variations in a Single Magma System from the Plat Pays Volcanic Complex, Dominica, Lesser Antilles

Glass inclusions in plagioclase and orthopyroxene from daciticpumice of the Cabrits Dome, Plat Pays Volcanic Complex in southernDominica reveal a complexity of element behavior and Li–Bisotope variations in a single volcanic center that would gounnoticed in a whole-rock study. Inclusions and matrix glassesare high-silica rhyolite with compositions consistent with about50% fractional crystallization of the observed phenocrysts.Estimated crystallization conditions are 760–880°C,200 MPa and oxygen fugacity of FMQ + 1 to +2 log units (whereFMQ is the fayalite–magnetite–quartz buffer). Manyinclusion glasses are volatile-rich (up to 6 wt % H₂O and 2900ppm Cl), but contents range down to 1 wt % H₂O and 2000 ppmCl as a result of shallow-level degassing. Sulfur contents arelow throughout, with <350 ppm S. The trace element compositionof inclusion glasses shows enrichment in light rare earth elements(LREE; (La/Sm)_n = 2·5–6·6) and elevatedBa, Th and K contents compared with whole rocks and similaror lower Nb and heavy REE (HREE; (Gd/Yb)_n = 0·5–1·0).Lithium and boron concentrations and isotope ratios in meltinclusions are highly variable (20–60 ppm Li with ⁷Li= +4 to +15 ± 2; 60–100 ppm B with ¹¹B = +6 to+13 ± 2) and imply trapping of isotopically heterogeneous,hybrid melts. Multiple sources and processes are required toexplain these features. The mid-ocean ridge basalt (MORB)-likeHREE, Nb and Y signature reflects the parental magma(s) derivedfrom the mantle wedge. Positive Ba/Nb, B/Nb and Th/Nb correlationsin inclusion glasses indicate coupled enrichment in stronglyfluid-mobile (Ba, B) and less-mobile (Th, Nb) trace elements,which can be explained by fractional crystallization of plagioclase,orthopyroxene and Fe–Ti oxides. The ⁷Li and ¹¹B valuesare at the high end of known ranges for other island arc magmas.We attribute the high values to a ¹¹B and ⁷Li-enriched slabcomponent derived from sea-floor-altered oceanic crust and possiblyfurther enriched in heavy isotopes by dehydration fractionation.The heterogeneity of isotope ratios in the evolved, trappedmelts is attributed to shallow-level assimilation of older volcanicrocks of the Plat Pays Volcanic Complex. KEY WORDS: subduction; volcanic arcs; igneous processes; melt inclusions; SIMS; trace elements; lithium and boron isotopes; diffusion     

长白山天池火山双岩浆房岩浆作用与互动式喷发

广义的长白山火山在我国境内包括天池火山、望天鹅火山、图们江火山和龙岗火山,是我国最大的第四纪火山岩分布区。长白山各个火山区的火山活动具有此起彼伏的穿时性特征,天池火山之下地壳和地幔两个岩浆房具有上下呼应、互动式喷发之特点。一方面来自地幔的钾质粗面玄武岩浆直接喷出地表,在天池火山锥体内外形成诸多小火山渣锥;另一方面钾质粗面玄武岩浆持续补给地壳岩浆房,发生岩浆分离结晶作用和混合作用,形成双峰式火山岩特征并触发千年大喷发。西太平洋板块俯冲-东北亚大陆弧后引张是长白山天池火山喷发的动力学机制。     

Magma Mixing Origin of Yamatu Granite in Nuoergong-Langshan Area,Western Part of the Northern Margin of North China Craton:Petrological and Geochemical Evidences

华北地块北缘西段巴音诺尔公—狼山地区的牙马图岩体以二长花岗岩为主,岩体中广泛发育岩浆暗色包体,二者界线明显.包体为岩浆结构,大多数具有塑性外形,发育淬冷边、反向脉,存在多种不平衡结构和矿物组合,如斜长石环带、石英眼斑、针状磷灰石等,显示岩浆混合特征;包体的SiO2含量为48.40％～55.40％,寄主花岗岩SiO2含量为65.03％～72.85％,具有明显的SiO2含量间隔;与寄主花岗岩相比,包体的Fe、Mg、Ca、Ti含量较高;包体和寄主花岗岩的主要氧化物之间具有很好的线性关系,微量元素和稀土元素特征相似.包体和寄主花岗岩的这些地球化学特征显示出明显的岩浆混合趋势.岩相学和元素地球化学特征表明暗色包体是基性岩浆侵入到酸性岩浆淬冷的产物,牙马图岩体存在两种岩浆的混合作用.     

The Traigh Bhan na Sgurra Sill, Isle of Mull: Flow Localization in a Major Magma Conduit

Field evidence points to localization of magma flowing in aTertiary doleritic sill on the Isle of Mull, Inner Hebrides,Scotland. Regions of the sill in which flow was short-livedhave chilled margins, a narrow or absent metamorphic aureole,and alignment of plagioclase crystals resulting in pronounced,flow-parallel, lineations on fracture surfaces (Type I regions).Prolonged flow resulted in blocky, coarse-grained dolerite withno chilled margins, and an extensive metamorphic aureole (TypeII regions). The distribution of Type I and Type II regionsshows no spatial pattern, with stagnant solidifying sectionsimmediately adjacent to sections with contemporaneous activeand sustained flow. The transition between Type I and Type IIregions occurs as the sill thickness exceeds 3·5 m. Ourobservation that sustained flow was only possible in regionsof the sill thicker than 3.5 m is consistent with previouslypublished theoretical models which predict a critical sill thicknessin the range 2·2–5 m. Regions of the sill narrowerthan this experienced a single injection of magma. Simple modelsto determine flow duration from the width of the metamorphicaureole demonstrate that progressive focusing of flow into widerparts of the conduit created discrete channels active for upto 5 months. KEY WORDS: flow localization; magma; Mull; sill; Traigh Bhàn na Sgùrra     

Magma Generation at a Large, Hyperactive Silicic Volcano (Taupo, New Zealand) Revealed by U-Th and U-Pb Systematics in Zircons

Young (<65 ka) explosive silicic volcanism at Taupo volcano,New Zealand, has involved the development and evacuation ofseveral crustal magmatic systems. Up to and including the 26·5ka 530 km³ Oruanui eruption, magmatic systems were contemporaneousbut geographically separated. Subsequently they have been separatedin time and have vented from geographically overlapping areas.Single-crystal (secondary ionization mass spectrometry) andmultiple-crystal (thermal ionization mass spectrometry) zirconmodel-age data are presented from nine representative eruptiondeposits from 45 to 3·5 ka. Zircon yields vary by threeorders of magnitude, correlating with the degrees of zirconsaturation in the magmas, and influencing the spectra of modelages. Two adjacent magma systems active up to 26·5 kashow wholly contrasting model-age spectra. The smaller systemshows a simple unimodal distribution. The larger system, usingdata from three eruptions, shows bimodal model-age spectra.An older 100 ka peak is interpreted to represent zircons (antecrysts)derived from older silicic mush or plutonic rocks, and a youngerpeak to represent zircons (phenocrysts) that grew in the magmabody immediately prior to eruption. Post-26·5 ka magmabatches show contrasting age spectra, consistent with a mixtureof antecrysts, phenocrysts and, in two examples, xenocrystsfrom Quaternary plutonic and Mesozoic–Palaeozoic metasedimentaryrocks. The model-age spectra, coupled with zircon-dissolutionmodelling, highlight contrasts between short-term silicic magmageneration at Taupo, by bulk remobilization of crystal mushand assimilation of metasediment and/or silicic plutonic basementrocks, and the longer-term processes of fractionation from crustallycontaminated mafic melts. Contrasts between adjacent or successivemagma systems are attributed to differences in positions ofthe source and root zones within contrasting domains in thequartzo-feldspathic (<15 km deep) crust below the volcano. KEY WORDS: zircon; U-series dating; rhyolite; Taupo Volcanic Zone; Taupo volcano     

西藏冈底斯带东段早侏罗世英云闪长岩的岩浆起源及其对拉萨地体地壳演化的意义

西藏冈底斯岩浆岩带除了大范围分布的晚白垩世以来的花岗岩类以外,还发育少量晚三叠世—侏罗纪花岗岩,目前关于其岩石成因还存在很多争议。为了进一步认识冈底斯早侏罗世花岗岩的岩浆起源,本文选择代表性的加查县香木村英云闪长岩开展了岩相学、岩石地球化学、锆石U-Pb年代学和Hf同位素研究。所研究的英云闪长岩主要由斜长石、石英、黑云母和少量的角闪石组成。LA-ICP-MS锆石U-Pb定年结果表明其结晶年龄为184±4Ma。锆石εHf(t)值均为正值,变化范围为7.9~10.3。在全岩地球化学组成上,英云闪长岩为中钾钙碱性、准铝质,并具有埃达克质岩石特征:高SiO2、Al2O3、Na2O/K2O(1)和Sr/Y(35.5~106),低Y(4.46×10-6~12.9×10-6)和Yb(0.51×10-6~1.39×10-6),富集Rb、Ba、K、Pb等大离子亲石元素,亏损Nb、Ta、Ti等高场强元素。在稀土元素配分图中呈现轻稀土元素富集、中稀土亏损的右倾"凹"型的式样,无明显Eu异常。岩石地球化学和锆石Hf同位素特征表明其岩浆可能起源于下地壳基性物质的部分熔融,残留相主要为角闪石、石榴子石,无斜长石。综合前人资料,冈底斯晚三叠世—早侏罗世花岗岩的锆石Hf同位素具有由西向东、由南向北逐渐富集的规律,反映古老地壳组分的贡献逐渐增多,这暗示了南冈底斯带东部也可能存在古老的地壳基底。     

安徽沙溪含和不含铜(金)矿斑岩斜长石斑晶粒度统计及其岩浆结晶动力学和成矿意义

通过对沙溪矿区含矿与不含矿斑岩中斜长石斑晶的粒度统计,计算出平均矿物粒度（Ｓ）、矿物分布密度（Ｄ）及矿物分布均匀度（Ｈ）;再根据晶体大小分布理论（ＣＳＤ）,计算出斜长石的晶体生长速率、成核密度及成核速率等岩浆结晶动力学参数;发现含矿斑岩较无矿斑岩有高的成核密度和成核速率,而晶体生长速率则较低;指出成分相近的斑岩在岩浆结晶动力学特征上的极细差别可能造成岩体含矿性明显不同     

Intrusion and Crystallization of a Spinifex-Textured Komatiite Sill in Dundonald Township, Ontario

Although komatiite has been defined as an ultramafic volcanicrock characterized by spinifex texture, there is a growing recognitionthat similar textures can also form in high-level dykes andsills. Here, we report the results of a petrological and geochemicalinvestigation of a 5 m thick komatiite sill in Dundonald Township,Ontario, Canada. This unit forms part of a series of komatiitesand komatiitic basalts, some of which clearly intruded unconsolidatedsediments. The komatiite sill is differentiated into a spinifex-texturedupper part and an olivine cumulate lower part. Features characteristicof the upper sections of lava flows, such as volcanic brecciaand a thick glassy chilled margin, are absent and, instead,the upper margin of the sill is marked by a layer of relativelylarge (1–5 mm) solid, polyhedral olivine grains that gradesdownwards over a distance of only 2 cm into unusually large,centimetre-sized, skeletal hopper olivine grains. This is underlainby a 1 m thick zone of platy spinifex-textured olivine and coarse,complex, dendritic, spinifex-textured olivine. The texture ofthe olivine cumulate zone in the overlying unit is uniform rightdown to the contact and a lower chilled margin, present at thebase of all lava flows, is absent. The textures in the silland the overlying unit are interpreted to indicate that thesill intruded the olivine cumulate zone of the overlying unit.Thermal modelling suggests that soon after intrusion, a narrowinterval of the overlying cumulate partially melted and thatthe liquid in the upper part of the sill became undercooled.The range of olivine morphologies in the spinifex-textured partof the sill was controlled by nucleation and crystallizationof olivine in these variably undercooled liquids. KEY WORDS: komatiite; intrusion; spinifex texture; olivine