滇西变泥质岩的白云母b_0值及其构造－地层意义

据３３３个白云母ｂ０值，结合区域地层和构造的研究成果，提出：（１）原称昌宁－孟连变质带解体为西盟－勐统变质带（加里东期中压变质带）和澜沧双变质带的高压亚带，两期高压变质作用分别与海西－印支期的洋壳消减和陆－陆碰撞有关；（２）石鼓变质带是三个逆冲岩片的叠置，各岩片内岩石的变质作用各具特色；（３）金沙江洋盆为地块间小洋盆，其消减及嗣后的弧－弧碰撞可能不产生高压变质作用。在此基础上探讨了白云母ｂ０值在造山带研究中的意义。     

Crystal structure refinement of hendricksite,A Zn- and Mn-rich trioctahedral potassium mica: A contribution to the crystal chemistry of zinc-bearing minerals

Summary The crystal structure of hendricksite, a trioctahedral mica of biotite type, characterized by high Zn²⁺ and Mn²⁺ contents has been refined by least square methods. The structural formula is: (K_0.89Na_0.10Ba_0.04)(Mg_1.57Zn_0.54Mn _0.40 ²⁺ Fe _0.25 ²⁺ Al_0.07Ti_0.07Cr_0.01)(Si_2.92Al_1.08)O₁₀ (OH)₂. The space group isC2/m and the cell parameters are:a=5.340(2) Å,b=9.524(2) Å,c=10.235(3) Å, =100.07(2)^o, the cell volume isV=497.98 ų. The final unweightedR=0.072. Average cation-anion distances in polyhedra are: T–O=1.659 Å, M(1)–O=2.093 Å, M(2)–O=2.088 Å, A–O_long=3.316 Å and A–O_short=3.004 Å; A is the alkaline cation. The rotation angle of tetrahedra is =6.7°. The analysis of electron densities, of the dimensions and distorsions of polyhedra shows that Zn²⁺ is exclusively in octahedral sites; there is no order between six-fold coordinated cations. A comparison between the structural features of hendricksite and those of the two main end-members of biotites, phlogopite and annite, is presented.The effect of the strong covalence of Zn–O bonds is particularly visible on the dimensions and orientations of the thermal ellipsoids of octahedral sites M(1) and M(2) which contain zinc. In all the published structures of trioctahedral micas, the ellipsoids of cationic sites are uniaxial positive, elongated parallel toc ^*. In hendricksite, this is observed only for the two zinc-free sites (T and A; in the octahedra M(1) and M(2), which contain zinc, the ellipsoids are approximately uniaxial negative, flattened parallel toa, which is a unique situation.Zinc which habitually favours the tetrahedral coordinations with oxygen, enters the octahedra only, i.e. the chemically anisotropic sites, in hendricksite. The strong polarizability of Zn²⁺ is proposed to explain this behaviour.An examination of the behaviour of Zn²⁺ in other compounds shows that this situation is general, zinc favours chemically anisotropic sites and specially those adjacent to OH or H₂O.

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Affinement de la structure cristalline de la hendricksite, mica trioctaédrique potassique riche en Zn et Mn; une contribution à la connaissance cristallochimique des minéraux zincifères

Résumé On a affiné par moindres carrés la structure de la hendricksite, mica trioctaédrique de type biotite, caractérisé par une teneur élevée en Zn²⁺ et Mn²⁺. La formule structurale de ce mica est: (K_0m89Na_0,10Ba_0,04)(Mg_1,57Zn_0,54Mn _0,40 ²⁺ Fe _0,25 ²⁺ Al_0,07Ti_0,07Cr_0,01)(Si_2,92Al_1,08)O₁₀(OH)₂. Le groupe spatial estC2/m et les paramètres de la maille:a=5,340(2) Å,b=9,254(2) Å,c=10,235(3) Å, =100,07(2)°; le volume de la maille estV=497,98 ų. Le résidu final non-pondéré estR=0,072. Les distances cation-anion moyennes dans les polyèdres sont les suivantes: T–O=1,659 Å, M(1)–O=2,093 Å, M(2)–O=2,088 Å, A–O_long=3,316 Å et A–O_court=3,004 Å où A désigne le cation alcalin. L'angle de rotation tétraédrique, =6,7°, est très semblable à celui de la phlogopite. L'analyse des densités électroniques, des dimensions et distorsions des polyèdres montre que Zn²⁺ est exclusivement en coordinance octaédrique et qu'il n'y a pas d'ordre entre les cations hexacoordonnés. On présente une comparaison des caractères structuraux de la hendricksite avec ceux des deux principaux pôles des biotites, la phlogopite et l'annite.L'effet de la forte covalence de la liaison Zn–O est particulièrement visible sur les dimensions et orientations des ellipsoides d'agitation thermique des deux sites octaédriques, sites zincifères. Dans toutes les structures de micas trioctaédriques publiées, les ellipsoides des sites cationiques sont uniaxes positifs, allongés parallèlement àc ^*, ce qui s' observe effectivement dans les deux sites non-zincifères (T et A) de la hendricksite, par contre, dans les octaèdres M(1) et M(2), qui contiennent le zinc, les ellipsoides sont approximativement uniaxes négatifs, applatis parallèlement àa, ce qui est une situation unique.Le zinc, qui se fixe généralement en sites tétraédriques dans les structures de type oxyde, occupe les sites octaédriques, c'est-à-dire les sites chimiquement anisotropes dans la hendricksite. La forte polarisabilité de Zn²⁺ est proposée pour expliquer ce comportement.Un examen du comportement de Zn²⁺ dans d'autres phases montre que cette situation est tout à fait générale, le zinc privilégiant les sites chimiquement anisotropes et en particulier ceux adjacents à OH où H₂O.

     

Origin of Minette by Mixing of Lamproite and Dacite Magmas in Veliki Majdan, Serbia

Composite dykes consisting of leucominette and dacite as wellas discrete dykes and flows of minette and lamproite composition,occur in the Veliki Majdan area, western Serbia. This area ispart of the Serbian Tertiary magmatic province, which consistsof numerous small occurrences of ultrapotassic igneous rocks.The composite dykes have leucominette margins (up to 150 cmthick) enclosing a central part of dacite up to 100 m in width.Between these two lithologies, a decimetre-sized transitionzone may occur. Petrography, mineral chemistry and bulk-rockgeochemistry, including Sr, Nd and Pb isotopes, provide evidencethat the minettes and leucominettes formed by hybridizationbetween a felsic magma similar in composition to dacite anda mantle-derived lamproitic magma. The leucominettes and minettescontain all phenocryst types (biotite, plagioclase, quartz)present in the dacites, but in partly resorbed and reacted form.The mica displays a great diversity of resorption textures asa result of partial dissolution, incipient melting and phlogopitization,suggesting superheating of the felsic melt during hybridization;the mineral modes and mineral compositions of the leucominettesand minettes resemble those in the lamproites. A model for themodification of lamproite melt towards minette is presentedin which minette is formed by mixing of lamproite and <30%felsic magma. The lack of any significant correlation betweenPb isotopic ratios and some of the ‘mixing-indices’(SiO₂, Zr, Zr/Nb, ¹⁴³Nd/¹⁴⁴Nd_i) recognized in the hybridizationmodel for the Veliki Majdan dykes may be a result of similarityof the Pb-isotopic signature in the two end-members. Highlyphlogopitized biotite xenocrysts in the minettes are ascribedto the retention of volatile components after magma mixing andcrystallization of a new generation of phlogopite from the hybridizedmagma. The magma-mixing model explains the reverse zoning andresorption features of phlogopite macrocrysts commonly recognizedin calcalkaline lamprophyres elsewhere. Therefore, this mixingmechanism may be globally applicable for the origin of minettesassociated with calcalkaline granitic plutonism in post-orogenicsettings. KEY WORDS: Serbia; lamproites; micas; phlogopitization; calcalkaline lamprophyres; superheating; magma mixing     

Adaminaby Group west of Batemans Bay: Deformation and metamorphism of the Narooma accretionary complex,NSW

This study provides new structural data that show that the Adaminaby Group is part of the Narooma accretionary complex and has been overprinted by HT/LP metamorphism associated with Middle Devonian Moruya Suite intrusions. The grade of metamorphism based on Kübler Indices is the same in the Wagonga and Adaminaby Groups at Batemans Bay inferring that these rocks were involved in the same accretionary event. White micas in slates of the Adaminaby Group record apparent K–Ar ages of 384.6 ± 7.9 Ma and 395.8 ± 8.1 Ma. These ages are believed to represent the age of Middle to Upper Devonian Buckenbowra Granodiorite. Kübler Index values indicate lower epizonal (greenschist facies) metamorphic conditions and are not influenced by heating in metamorphic aureoles of the plutons. All b cell lattice parameter values are characteristic of intermediate pressure facies conditions although they are lower in the metamorphic aureole of the Buckenbowra Granodiorite than in the country rock, defining two areas with dissimilar baric conditions. East of the Buckenbowra Granodiorite, b cell lattice parameter values outside the contact aureole (x = 9.033 Å; n = 8) indicate P = 4 kb, and assuming a temperature of 300°C, infer a depth of burial of approximately 15 km for these rocks with a geothermal gradient of 20°C/km. In the metamorphic aureole of the Buckenbowra Granodiorite, b cell lattice parameter values (x = 9.021 Å; n = 41) indicate P = 3.1 kb inferring exhumation of the Adaminaby Group rocks to a depth of approximately 11 km prior to intrusion. A geothermal gradient of 36°C/km operated in the aureole during intrusion. An extensional back-arc environment prevailed in the Adaminaby Group during the Middle to Upper Devonian.     

磁测成果圈定半坡镁铁岩体及推断深部形态

半坡地区地面高精度磁测成果发现正磁异常与镁铁—超镁铁岩体的关系比较密切。随即开展△T异常圈定镁铁—超镁铁岩体范围,推断岩体的大大致形态,为在岩体中寻找岩浆岩型镍铜铂钯矿提供深部岩体形态资料。     

The biotite isograd, Central Pyrenees: a deformation-controlled reaction

The biotite isograd reaction in Cambro–Ordovician pelites from the Garonne dome in the Central Pyrenees involves the production of biotite at the expense of chlorite, and the gradual reduction in the celadonite (Si) content of individual white micas. The mineral assemblages in the biotite-bearing rocks in the Melles area retain abundant evidence of chemical disequilibrium, due to the sluggish nature of major element diffusion within the white micas. The progress of the isograd reaction is strongly controlled by the progressive development of regional Variscan fabrics, and chemical exchange in the white micas in these metamorphic conditions is only possible during active deformation. Chemical resetting of the white micas takes place via the development of compositional zoning in the deforming micas; this probably occurs as a consequence of the introduction of defects and dislocations, causing more efficient diffusion within parts of individual grains. In the absence of deformation, this biotite isograd reaction would take place at significantly higher temperature and be controlled by the relatively high closure temperature of major element diffusion in white micas. Thus, assigning thermal significance to such continuous isograd reactions is impossible without independent constraints: kinetic factors such as deformation may be the dominant influence in many cases, not the thermodynamic controls.     

Twitter and Academic Geography through the Lens of #AAG2018

Twitter has emerged as a global social network of active users who share conversations with one another in an online setting. Academics are one community that has increasingly taken to Twitter as a means of connecting with other scholars, sharing research, and obtaining meaningful feedback. Tweeting has become especially popular during academic conferences where conference attendees use Twitter hashtags to filter conference conversations into a separate dialogue. For geographers, the Annual Meeting of the American Association of Geographers (AAG) represents one such occasion to use Twitter to discuss contemporary developments in geographic research. In this article, we provide an overview of Twitter as well as the ways in which the academic community uses the platform. Following this, we discuss the tweets sent using the hashtag for the 2018 AAG Annual Meeting, #AAG2018. To analyze these tweets, we collected all tweets with this hashtag for a period of four weeks and examined the content using word clouds and sentiment analysis to explore general feelings and trends associated with geography and the AAG Annual Meeting. We conclude with suggestions for future research avenues that could use Twitter data to gauge the pulse of the geographic discipline. Key Words: academic conferences, American Association of Geographers, geography, sentiment analysis, Twitter.     

Mg-Fe云母化学成分的解释和分类(Ⅰ)——基本置换和黑云母平面

 中国东部花岗岩类141个Mg-Fe云母的化学成分将近90%的变化属于八面体层内的类质同象置换,置换矢量Mg ₁Fe⁺²和Fe_-3⁺²(R_Ⅵ⁺³)_-2□_Ⅵ组成了天然黑云母平面,大约80%的变化应当解释为基本置换8Mg ₁Fe⁺²+Fe_-3⁺²(R_Ⅵ⁺³)₂□_Ⅵ.这些是Mg-Fe云母在广泛的自然条件下表现出来的最主要的晶体化学关系。文中还提出了置换矢量的长度、分量和以及电价和三个参数,用以识别矿物化学成分变化的类质同象置换特征。     

Mg-Fe云母化学成分的解释和分类(Ⅱ)——Mg-Fe云母的自然分类

给出云母化学成分比值FM=6(Fe⁺²+Mn)/(Fe²⁺+Mn+Mg),用以表示Mg-Fe云母成分变化的基本趋势。根据291个云母的化学成分相对于FM值的异常变化,提出了天然Mg-Fe云母自然分类的新方案:把FM≌1.20(1.05-1.35)、4.80作为区分金云母、黑云母和铁叶云母的界线;FM>0.60的金云母称为铁金云母;以FM=2.70、3.60把黑云母再细分为富镁黑云母、镁铁黑云母和富铁黑云母。Mg-Fe云母自然分类的本质是各成员云母之间白云母置换方式和限度的潜在差异在不同的自然结晶条件下的反映。     

Thermal evolution of the Lesser Himalaya, central Nepal: Insights from K-white micas compositional variation

The Lesser Himalayan low- to medium-grade metamorphic rocks in central Nepal are rich in K-white micas occurring as porphyroclasts and in matrix defining S₁ and S₂. Porphyroclasts are usually zoned with celadonite-poor cores and celadonite-rich rims. The cores are the relics of igneous or high grade metamorphic muscovites, and the rims were re-equilibrated or overgrown under lower T metamorphic conditions. The matrix K-white micas defining S₁, pre-dating the Main Central Thrust activity, are generally celadonite-rich. They show heterogeneous compositional zoning with celadonite-rich cores and celadonite-poor rims. They were recrystallized at lower T condition prior to the Main Central Thrust activity, most probably prior to the India–Asia collision (pre-Himalayan metamorphism). The matrix K-white micas along S₂, synchronous to the Main Central Thrust activity (Neohimalayan metamorphism), are relatively celadonite-poor and were recrystallized under relatively higher T condition. K-white micas defining S₁ also were partially re-equilibrated during the Neohimalayan metamorphism. The average compositions of recrystallized K-white micas defining both S₁ and S₂ become gradually poor in (Fe + Mg)- and Si-contents and rich in Al- and Ti-contents from south to north showing an increase of metamorphic grade from structurally lower to higher parts in the Lesser Himalaya. This shows that the metamorphism is inverted throughout the inner Lesser Himalaya. The tectono-metamorphic significance of the published K–Ar and ⁴⁰Ar / ³⁹Ar K-white micas ages from the Lesser Himalaya need re-evaluation in the context of observed intrasample compositional variation and zoning, and possible higher closure temperature (500 °C) for K–Ar system.