单斜辉石中石英出溶体的显微结构和成因机制

矿物出溶结构保存有早期母体矿物的物理化学条件信息,对其开展研究不仅有助于了解寄主岩石的来源深度,而且有助于研究减压折返的动力学演化过程.在世界许多高压-超高压带的榴辉岩和石榴辉石岩中,人们普遍发现单斜辉石中有定向排列的针状或棒状SiO₂析出物,其矿物相主要为α石英,有时会伴生钙质角闪石等含水矿物.这些定向针状或棒状体通常平行于单斜辉石c[001]轴方向延伸,石英长轴可以为其c[0001]轴或a[1120]轴.电子背散射衍射(EBSD)测试结果表明,多数石英(96%)析出物与寄主单斜辉石具有结晶学取向关系:(1) 50%的石英c轴平行,并且[0001]_Qz//[001]_Cpx;(2)35%的石英至少有一个a轴平行,并且[1120]_Qz//[001]_Cpx;(3)11%的石英至少有一个s{1121}面平行,并且(1121)_Qz//(100)_Cpx.钙质角闪石析出物与寄主单斜辉石也具有密切结晶学取向关系:(100)_Amp//(100)_Cpx、[010]_Amp//[010]_Cpx、[001]_Amp//[001]_Cpx、[100]_Amp∧[100]_Cpx≈32°.上述定量显微构造证据表明,单斜辉石中定向石英析出物是由出溶作用所形成,并且多数石英出溶体形成于α石英稳定域.已有高温高压实验研究数据表明,单斜辉石中空位的形成和钙埃斯科拉组分(CaEs)的含量均受化学组成、压力、温度等多种因素综合影响:单斜辉石中CaEs含量对化学组成非常敏感,并受到共生矿物体系中自由SiO₂相和蓝晶石的共同缓冲;相同化学组成和等压条件下,CaEs含量总体上随温度升高缓慢降低;相同化学组成和等温条件下,CaEs含量在＜6 GPa区间随压力升高而增加,在＞6 GPa区间随压力升高而降低.单斜辉石定向SiO₂析出物的形成可能涉及多种因素,高压只是其中必要条件之一.榴辉岩质单斜辉石中“石英±角闪石”析出物很可能形成于开放体系,与熔流体活动密切相关,涉及多阶段物质扩散、晶体成核生长、重结晶、退变质反应等复杂作用过程.单斜辉石中定向SiO₂析出物的显微结构特征并非超高压岩石的必要条件,这种特殊显微结构也不能作为证明超高压的充分条件.      

高知县足折岬环斑花岗岩

在日本西南高知县足折岬的第三纪火成杂岩中发现了特殊的环斑花岗岩,这些环斑花岗岩与粗粒正长岩有密切的联系,后者以网状岩墙群侵入到辉长质岩块中。在环斑花岗岩与粗粒正长岩接触边界上,由于中—细粒花岗闪长岩的侵入,粗粒正长岩沿颗粒边缘和裂隙分解成单个晶体或晶体碎片,而这些花岗闪长岩含有环斑花岗岩的基质成分。花岗闪长岩体中的钾长石晶体成其碎片熔蚀成卵形,无规则地分布,而且为薄片斜长石(An15—25)包覆。这些薄片随基质流动构造的形成而加厚. 环斑斜长石呈卵形,直径1～3cm,有时沿基质的流线平行或似平行分布。其内核常为一不具双晶或有双晶的钾长石晶体,而且常伴有石英和斜长石的晶体,而外壳大部分由很多细小的树枝状斜长石和斜长石晶隙间蠕虫状石英组成,它们甚至比基质还细小.在基质极度细小的地方,环斑长石的壳很薄,甚至没有。在同一岩石中,环斑长石和其他卵形体共存,诸如斜长石(被钠长石或钾长石包覆)、石英(被角闪石和斜长石包覆)、角闪石(被角闪石和黑云母的集合体包覆)和黑云母。在岩相学和化学特点上,这些卵形体的内核矿物与粗粒正长岩的矿物相似. 从上面描述可见,该环斑花岗岩的形成是粗粒正长岩发生结晶及随后的花岗闪长岩侵入的结果.粗粒正长岩分离出来的钾长石晶体沉浸到花岗闪长岩岩浆中,其边部遭受侵蚀,形成卵圆状晶体。新结晶的树枝状斜长石被吸附在钾长石周围,然后,树枝状斜长石之间的空隙被石英充填. Hibbard(1981)对其他很多地区环斑花岗岩的成因研究也得出了与岩浆混合作用有关的相似认识.     

伟晶岩中文象结构的成因及其找矿意义

<正>文象结构(graphic texture)是指石英呈一定的外形(如尖棱状、象形文字形等)有规律地镶嵌在钾长石中(图1a),这些石英在正交偏光下同时消光。在显微镜下可见的称为显微文象结构;肉眼可见的称为文象结构。通常认为,文象结构是长石和石英在共结点同时结晶形成的。但基于实验研究的结果,Fenn(1986)提出了文象结构形成的另外一种观点,他认为:在水不饱和且过     

富碱岩长石结构态及其地质意义

采用X射线衍射法研究了16个碱性岩和碱性花岗岩体长石的结构态,获得了122个数据,并对样品进行探针或化学分析。这些富碱岩都以碱性长石为主,有单斜、三斜钾长石,以及由它们和钠长石组成的条纹长石。钾长石分属各种结构态,钠长石有中、低钠长石,钾、钠长石在一些岩体中体现了由高→低结构态有序化演化进程。长右结构态与矿物温度计所获得的资料得到了很好的对照,表明了长石Si/Al有序程度主要受结晶温度的影响。     

北秦岭中生代沙河湾岩体环斑结构特征及有关问题的讨论

该岩体的环斑结构主要发育于边部含巨斑状黑云角闪石英二长岩中 ,环斑长石粒径一般 2cm× 4cm。形态多呈自形、半自形 ,有些为卵形。环斑长石由核部钾长石和多层或单层斜长石外壳两部分构成。钾长石内核呈肉红色 ,一般是由单颗粒组成 ,具卡氏双晶 ,普遍发育规则的条纹结构 ;中心部分钾长石分子含量Or为 95 ,边部为 84。外壳斜长石牌号一般为An2 0± ,为奥长石。内核和外壳中均发育石英、斜长石、黑云母、角闪石等矿物的包裹体 ;包裹体在其边部较多 ,中部较少 ,钾长石斑晶中的石英包裹体呈不规则的凹面状和水滴状。岩石中主要矿物具有 2个世代。这些特征显示 ,沙河湾岩体中的环斑结构与典型的环斑结构是相同的。亦表明典型的环斑结构可以出现于不同的时代和构造环境。由于其形成时代和产出背景不同于典型环斑花岗岩 ,该岩体属典型环斑花岗岩还是一种新的似环斑花岗岩还有待于进一步研究。     

胶东金青顶金矿钾长石化岩相学特征及形成温度的估算

金青顶金矿是胶东东部一个特大型石英脉型金矿床,其探测深度已达约1 340 m,含矿石英脉的围岩主要为黑云二长花岗岩。文中对蚀变围岩中钾长石、斜长石和石英等主要矿物的结构与形态特征进行了详细观察及成因分析,发现热液成因钾长石大量存在于近矿围岩中,且深部钾长石化强度和范围都有加大的趋势,并大量出现由正长石和石英组成的团块状似伟晶岩。关于热液钾长石的形成温度,目前尚无可靠的测试数据。文中运用二长石温度计对蚀变岩中条纹长石平衡温度进行了估算,获得似伟晶岩中条纹长石平衡温度为433~482 ℃,而钾长石化叠加弱黄铁矿化花岗岩中条纹长石平衡温度为362~419 ℃。从似伟晶岩到钾长石化花岗岩、钾长石化叠加黄铁绢英岩化花岗岩中条纹长石平衡温度的渐次降低,反映了成矿流体与围岩反应逐渐弱化的特征。     

钠长石花岗岩中雪球结构形成机理的研究

在某些富锂、氟含稀有金属花岗岩的石英和钾长石斑晶中常见 (半 )雪球结构。雪球结构的产出特征、雪球体中钠长石的电子探针分析结果以及其他间接证据都说明 ,雪球结构是在岩浆结晶分异过程中形成的。对矿物结晶顺序、石英和钠长石的生长速率以及固相线温度的研究表明 ,富锂、氟、钠的花岗质残余岩浆完全具备形成雪球结构的条件。岩浆熔体中较高的Na2 O/K2 O比值和F、H2 O含量在雪球结构的形成过程中起着重要的作用 :F的高含量使岩浆固相线温度降低 ,岩浆得以充分分异演化 ,形成接近端员组分的钾长石和钠长石 ;Na2 O/K2 O比值较大使钠长石首先结晶 ;较高的F和H2 O含量使岩浆粘度降低 ,石英的生长速率相对加快并逐渐包裹钠长石形成雪球结构。     

安徽金寨县沙坪沟钼矿成矿岩体矿物学特征及其成岩成矿意义

安徽金寨县沙坪沟钼矿成矿岩体主要包括二长花岗斑岩、正长斑岩及花岗斑岩,主要矿物组成为石英、钾长石、斜长石、黑云母。本文利用电子探针技术对主要矿物的矿物学特征进行了研究,结果表明,岩石中斜长石为钠长石;钾长石以条纹长石为主;黑云母属于镁质黑云母,富Mg、Al、K,贫Ca,MF值变化于0.65~0.72之间。黑云母化学成分显示本区岩体为I型花岗岩,属于造山带钙碱性花岗岩类,其成岩物质具壳幔混合的特点。黑云母矿物温压计计算结果显示,岩体固结温度约500~580℃,氧逸度变化范围是10-14~10-15.8,其含铁指数对寻找钼矿具较好的指示意义。     

东秦岭钼矿带中皱纹岩的成因及其找矿意义

对东秦岭金堆城钼矿皱纹岩研究表明:它是花岗岩岩浆演化到最晚期的产物,其形成温度约400～500℃;皱纹岩中钾长石的三斜度△=0.24,有序度δ=0.5～0.88,属低正微长石,基本不含有钠长石;皱纹岩中富含 F,P_(HF)有规律的变化引起石英、萤石及钾长石分离结晶形成相平行的条纹,而尚未固结的岩体内部的活动引起条纹褶皱。皱纹岩的形成条件与钼矿成矿条件相似,因而它是钼矿一个良好的找矿标志。     

广西栗木钨锡稀有金属矿床碱长花岗岩的厘定

广西栗木钨锡稀有金属矿床内的花岗岩传统定名为中粒斑状铁白云母花岗岩和钠长石花岗岩.电子探针分析表明,栗木两类花岗岩中的钠长石An值均小于5,95％以上的样品An＜2,均属于碱性长石.显微镜下工作说明,矿物结晶顺序为钠长石→石英+钾长石→黄玉→云母,钠长石是岩浆演化过程中最早结晶的产物,并非钠长石化的结果.在QAP投影中,两类花岗岩落入QA线上的碱长花岗岩区域.故两类花岗岩的确切定名应为中粗粒铁锂云母碱长花岗岩(Ⅰ)和细粒碱长花岗岩(Ⅱ).     

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.     

花岗质岩石中微粒交生体的成因研究

在一些花岗质片麻岩中,经常看到分布在长石颗粒周围的细小矿物集合体——微粒交生体。电子探针分析结果表明,微粒交生体主要由钠长石、钾长石和石英组成,其标准矿物平均成分相当于An-Ab-Or体系的“低限熔点”成分。显然,微粒交生体是花岗质片麻岩经部分熔融产生的“低限熔体”的结晶产物。太古宙高级区一些晚构造钾质花岗岩的矿物,化学特征与微粒交生体类似。这意味着在塑性变形过程中产生和聚集的“低限熔体”最终可以形成大规模钾质花岗岩。     

Graphic intergrowths of feldspars and quartz in some Czechoslovak pegmatites

Graphic intergrowths of alkali feldspar+quartz, and plagioclase+quartz, occur together in pegmatites in the eastern part of the Czechoslovak Moldanubicum. They form zones between the finer-grained wall zone and the central blocky feldspar+quartz core. The normative Or-Pig-Q compositions of the graphic intergrowths and the Or-Ab-An contents of their feldspars show broad variations generally, but have a restricted range within individual pegmatites. At two localities studied in more detail, coexisting feldspars show gradual changes in composition, from the margins up to the innermost graphic pegmatite, compatible with fractional crystallization along the feldspar solidus-solvus intersection in the Or-Ab-An-Q-H₂O system, at different vapour pressures in different pegmatites. Two models are demonstrated for low and high pressure cases. The feldspar compositions from central blocky zones deviate from the magmatic fractional crystallizations paths; this corresponds with the general assumption that they crystallized from supercritical gaseous fluids.In these and in similar pegmatites, coexisting alkali feldspar+quartz and plagioclase + quartz intergrowths are interpreted as the last products of cotectic crystallization from an ultimately fractionated granitic magma. Positive correlation of Ab solid solution and quartz content in the potassic intergrowths suggests that these characteristics may be indicative of the relative pressure and temperature effective during their crystallization, when compared in pegmatites of the same bulk composition.     

The Petrological and Geochemical Evolution of Ediacaran Rare-Metal Bearing A-type Granites from the Jabal Aja Complex, Northern Arabian Shield, Saudi Arabia

New fieldwork, mineralogical and geochemical data and interpretations are presented for the rare-metal bearing A-type granites of the Aja intrusive complex(AIC) in the northern segment of the Arabian Shield. This complex is characterized by discontinuous ring-shaped outcrops cut by later faulting. The A-type rocks of the AIC are late Neoproterozoic post-collisional granites, including alkali feldspar granite, alkaline granite and peralkaline granite. They represent the outer zones of the AIC, surrounding a core of older rocks including monzogranite, syenogranite and granophyre granite. The sharp contacts between A-type granites of the outer zone and the different granitic rocks of the inner zone suggest that the AIC was emplaced as different phases over a time interval, following complete crystallization of earlier batches. The A-type granites represent the late intrusive phases of the AIC, which were emplaced during tectonic extension, as shown by the emplacement of dykes synchronous with the granite emplacement and the presence of cataclastic features. The A-type granites consist of K-feldspars, quartz, albite, amphiboles and sodic pyroxene with a wide variety of accessory minerals, including Fe-Ti oxides, zircon, allanite, fluorite, monazite, titanite, apatite, columbite, xenotime and epidote. They are highly evolved(71.3–75.8 wt% SiO_2) and display the typical geochemical characteristics of post-collisional, within-plate granites. They are rare-metal granites enriched in total alkalis, Nb, Zr, Y, Ga, Ta, REE with low CaO, MgO, Ba, and Sr. Eu-negative anomalies(Eu/Eu* = 0.17–0.37) of the A-type granites reflect extreme magmatic fractionation and perhaps the effects of late fluid-rock interactions. The chemical characteristics indicate that the A-type granites of the AIC represent products of extreme fractional crystallization involving alkali feldspar, quartz and, to a lesser extent, ferromagnesian minerals. The parent magma was derived from the partial melting of a juvenile crustal protolith with a mantle contribution. Accumulation of residual volatile-rich melt and exsolved fluids in the late stage of the magma evolution produced pegmatite and quartz veins that cut the peripheries of the AIC. Post-magmatic alteration related to the final stages of the evolution of the A-type granitic magma, indicated by alterations of sodic amphibole and sodic pyroxene, hematitization and partial albitization.     

Western dome of the Arga-Ynnykh-Khaya granitic intrusion and its white granites

Adamellites, granite porphyries, and white granites of the western dome of the Arga-Ynnykh-Khaya Massif (Yakutiya) have been studied with emphasis on the white granites. The main features of its magmatic crystallization and initial transformation have been obtained by investigating the intensity of transformations using stochastic Markov models. The most widespread transformation is albitization. Two varieties exist. First, and most common, is development of a secondary albite crystal between two primary grains of potassium feldspar. Second, secondary albite has developed between primary quartz grains. Both variants have definite probabilities of occurrence. Less commonly, but with a constant probability, a secondary quartz grain forms as an intergrowth between two grains of potassium feldspar.     

Geochemistry and petrogenesis of post-collisional alkaline and peralkaline granites of the Arabian-Nubian Shield: a case study from the southern tip of Sinai Peninsula,Egypt

The southern Sinai Peninsula, underlain by the northernmost extension of the Arabian-Nubian Shield, exposes post-collisional calc-alkaline and alkaline granites that represent the youngest phase of late Neoproterozoic igneous activity. We report a petrographic, mineralogical and geochemical investigation of post-collisional plutons of alkaline and, in some cases, peralkaline granite. These granites intrude metamorphosed country rocks as well as syn- and post-collisional calc-alkaline granitoids. The alkaline and peralkaline granites of the southern tip of Sinai divide into three subgroups: syenogranite, alkali feldspar granite and riebeckite granite. The rocks of these subgroups essentially consist of alkali feldspar and quartz with variable amounts of plagioclase and mafic minerals. The syenogranite and alkali feldspar granite contain small amounts of calcic amphibole and biotite, often less than 3%, while the riebeckite granite is distinguished by sodic amphibole (5–10%). These plutons have geochemical signatures typical of post-collisional A-type granites and were most likely emplaced during a transition between orogenic and anorogenic settings. The parental mafic magma may be linked to lithospheric delamination and upwelling of asthenospheric mantle material. Differentiation of the underplated basaltic magma with contributions from the juvenile crust eventually yielded the post-collisional alkaline granites. Petrogenetic modelling of the studied granitic suite shows that pure fractional crystallization cannot quantitatively explain chemical variations with the observed suite, with both major oxides and several trace elements displaying trends opposite to those required by the equilibrium phase assemblage. Instead, we show that compositional variation from syenogranite through alkali feldspar granite to riebeckite granite is dominated by mixing between a low-SiO₂ liquid as primitive or more primitive than the lowest-SiO₂ syenogranite and an evolved, high-SiO₂ liquid that might be a high-degree partial melt of lower crust.     

Myrmekite — one hundred years later

Myrmekite, first detected by Michel-Lévy in 1875 and named by Sederholm in 1899, is an intergrowth between vermicular quartz and (sodic) plagioclase situated next to potash feldspar. In felsic plutonic rocks it occurs as: rims bordering granular plagioclase, intergranular blebs set between adjacent microperthite crystals, lobes associated with muscovite in deformed alkali feldspar megacrysts or as bulbous growths at their margins, and rims on plagioclase inclusions held within orthocalse megacrysts. A literature review based largely on papers published in the past quarter century shows that hypotheses for myrmekite genesis fall mainly into five categories: simultaneous or direct crystallization, replacement of potash feldspar by plagioclase, replacement of plagioclase by potash feldspar, solid-state exsolution, and recrystallizing quartz involved with blastic plagioclase.     

同构造花岗岩的一种显微构造标记

在许多同构造花岗岩中，发现一种不同于糜棱基质的微细粒矿物集合体－－微粒交生体，充填于大粒级矿物的三结点和接合缝内，体积分数一般为2％－6％，由微粒石英、斜长石、钾长石、蠕状石等组成，成分上与花岗质岩浆的晚期结晶产物相当。根据其产状、成分、岩相学特征及有关实验资料，认为它是较强应力作用下岩石所含少量残余熔体（质量分数约小于3％－5％）的结晶产物，可作为岩体同构造侵位结晶的显微构造标志。     

华南富锂氟含稀有金属花岗岩的成冈分析

雪球结构的产出特征、钠长石电子探针分析及其它间接证据都说明,雪球结构是在岩浆结晶过程中形成的。雪球结构形成与否主要与岩浆熔体中Ｎａ２Ｏ／Ｋ２Ｏ比值和Ｆ、Ｈ２Ｏ含量有关。较大的Ｎａ２Ｏ／Ｋ２Ｏ比值（〉１）例钠长石首先从熔体中晶出;较高的Ｆ含量使岩浆固相线温度大大降低,有利于岩浆分异演化并形成接近端员组分的钠长石和钾长石同的Ｈ２Ｏ含量有利于石英以较快的速度生长并逐渐包裹钠长石形成雪球结构。自形的α－石英斑晶、接近各自端员组分的甲和工石和钠长石等说明该类花岗岩形成温度较低,众多的地质、地球化学依据都证明了,华南富锂氟含稀有金属花岗岩是从过铝富氟富钠的残余熔体中直接结晶而成的。     

Exsolution in ternary feldspars

The equilibrium precipitate formed by exsolution in alkali feldspar containing calcium in solid solution is a two-phase intergrowth of plagioclase and quartz. It is the kinetics of the precipitation, however, which decides whether or not this reaction will take place. It is shown that lamellar growth of plagioclase and quartz is only accomplished by the advancement of an incoherent boundary originating at an alkali feldspar — alkali feldspar boundary or at an interface between alkali feldspar and plagioclase.