Fluids in metamorphic rocks

Basic principles for the study of fluid inclusions in metamorphic rocks are reviewed and illustrated. A major problem relates to the number of inclusions, possibly formed on a wide range of P–T conditions, having also suffered, in most cases, extensive changes after initial trapping. The interpretation of fluid inclusion data can only be done by comparison with independent P–T estimates derived from coexisting minerals, but this requires a precise knowledge of the chronology of inclusion formation in respect to their mineral host.

The three essential steps in any fluid inclusion investigation are described: observation, measurements, and interpretation. Observation, with a conventional petrographic microscope, leads to the identification and relative chronology of a limited number of fluid types (same overall composition, eventually changes in fluid density). For the chronology, the notion of GIS (Group of synchronous inclusions) is introduced. It should serve as a systematic basis for the rest of the study. Microthermometry measurements, completed by nondestructive analyses (mostly micro-Raman), specify the composition and density of the different fluid types. The major problem of density variability can be significantly reduced by simple considerations of the shape of density histograms, allowing elimination of a great number of inclusions having suffered late perturbations. Finally, the interpretation is based on the comparison between few isochores, representative of the whole inclusion population, and P–T mineral data. Essential is a clear perception of the relative chronology between the different isochores. When this is possible, as illustrated by the complicated case of the granulites from Central Kola Peninsula, a good interpretation of the fluid inclusion data can be done. If not, fluid inclusions will not tell much about the metamorphic evolution of the rocks in which they occur.     

Omphacite in Californian metamorphic rocks

Omphacite is a common mineral in greenstones, metasediments and related Franciscan rocks of the glaucophane schist facies. It also occurs in late veins cutting amphibolites, glaucophane schists, eclogites, greenstones, and occasionally metagraywackes. It is apparent that this mineral is stable under glaucophane schist facies conditions in rocks of a suitable bulk composition, and is not restricted to the eclogite facies. Association with albite, quartz and lawsonite, and late veining of omphacite veins by aragonite indicates that pressures necessary to form omphacite are reasonably close to those calculated from an ideal solution model.     

Metasomatic zones in metamorphic rocks

Prediction of a unique sequence of metasomatic zones that would develop by intergranular diffusion with local equilibrium is possible only for relatively simple systems, unless extensive thermochemical and kinetic information is available. The complexity of the problem for a given example will depend on what portion of the set of chemical components required to describe the example are ‘diffusing components’, that is, components that move relative to ‘inert markers’. Diffusing components are commonly K-components (Thompson, 1970) for the various local equilibria of a sequence of metasomatic zones, since diffusion tends to impose a monotonie variation of the chemical potentials of these components across the zones. The number of diffusing components may vary from zone to zone in a particular example, as may the number of diffusing components that are K-components. Calculation of the rate of growth of a specific sequence of zones is relatively straightforward only for cases where the zones are primarily due to the variation of the chemical potential of one independent diffusing component. Calculation of the material transfer involved in the growth of a sequence of zones, assuming a single sharp initial contact is meaningful only if ‘inert markers’ or a discontinuity in the otherwise-constant ratio of two components indicate the present location of the initial contact. Examination of some natural calc-silicate diffusion zones suggests that a diffusion-imposed gradient in the chemical potential of calcium is largely responsible for the observed zonation. Metasomatic zones developed at the boundaries of ultramafic bodies, however, are produced by diffusion-imposed chemical potential gradients of several components, notably silica and magnesia, the number varying from zone to zone.     

包含变质岩分类三要素的主要变质岩分类表

变质岩分类的三要素是:变质岩的物质成分(化学成分、矿物成分)、变质岩的组构(结构、构造)和变质岩的成因(变质作用类型和形成变质岩的物理化学条件).由于变质岩的化学成分、矿物成分、组构特征和形成变质岩的地质环境十分复杂,致使至今尚无以变质岩分类三要素为基础的、内容比较完善的分类方案.本文中主要变质岩的分类是以其分类三要素为基础编制的,首次将不同成因的变质岩类并列于同一表中、将鉴定变质岩的主要标志性矿物成分和组构特征列入同一分类表中.该分类对鉴定变质岩石具有可操作性和实用性,分类表中涵盖了自然界主要的变质岩石.     

Porphyroblast-matrix microstructural relationships in deformed metamorphic rocks

Most porphyroblast-matrix microstructural criteria used for inferring relative time relationships between metamorphism and deformation are equivocal. Therefore, all reasonable alternative explanations should be considered, and as many criteria as possible should be applied to a particular microstructure before an interpretation is made. Such microstructures should not be interpreted without adequate consideration of metamorphic implications, with emphasis on assemblages, rather than individual minerals.

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Zusammenfassung Die meisten Gefügekriterien der Beziehung von Porphyroblasten und Matrix, die zur relativen Zeitbestimmung von Metamorphose und Deformation herangezogen werden, sind nicht eindeutig. Es sollten daher alle zusätzlichen Informationen berücksichtigt und alternative Abfolgen geprüft werden, ehe man aus einer Gefügeanalyse eine Altersabfolge aufstellt. Eine Deutung der Gefüge sollte nicht ohne Kenntnis der Metamorphosebedingungen gegeben werden, wobei weniger die Einzelminerale als mehr die Mineralvergesellschaftung die entscheidenden Kriterien liefern können.

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Résumé La plupart des critères microstructuraux matrice-porphyroblastes employés pour établir l'âge relatif entre le métamorphisme et la déformation sont ambigus. C'est pourquoi il faut considérer toutes les explications possibles, et appliquer autant de critères que possible sur une microstructure particulière avant de l'interpréter. L'interprétation de ces microstructures doit tenir compte des événements métamorphiques, et, plus particulièrement, des assemblages plutôt que des minéraux individuels.

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The peristerite gap in low-grade metamorphic rocks

Coexisting Na-plagioclases from greenschists both in the thermal aureole of the Kasugamura Granite, Japan, and in the low-P metamorphic zone of Yap Island, western Pacific were analyzed in great detail; the peristerite solvus was determined for each suite. The asymmetric solvus has steep albite-rich and gentle oligoclase-rich limbs that are similar to those for higher pressure series. The present results together with those from Vermont, New Zealand, and the Sanbagawa belt indicate that the peristerite solvus shifts toward the albite component and higher temperature with increasing pressure. With increasing pressure, albite co-existing with oligoclase (An=100 Ca/Ca+ Na=20) varies in composition from An 8–9 (in Kasugamura), through An 3 (in Yap Island and Vermont), to An 1 (in New Zealand) and An less than 0.5 (in the Sanbagawa belt). The consolute temperatures for the peristerite solvus estimated from available geothermometry are 420° C in Kasugamura, 450–550° C in Vermont and 550°–600° C in the Sanbagawa belt. The variation of plagioclase composition in progressive metamorphic zones is explained by intersection of a plagioclase-forming reaction and the peristerite immiscibility gap in an isobaric T-X _An diagram. The greenschist zone is characterized by albite, the transition zone by occurrence of peristerite pairs and the amphibolite zone by plagioclase of An 20–50.     

超高压岩石中变质脉的研究

超高压岩石中的变质脉体是示踪变质流体活动的重要途径之一。高压-超高压岩石中不仅出露"纯"的石英脉(石英含量>98%),还出露了具有不同复杂矿物组合的"不纯"脉。最近的研究成果表明,对各种类型变质脉的观察和研究有助于深化对大陆碰撞造山带中变质流体/熔体性质以及元素迁移行为的认识。目前主要解决的科学问题包括:(1)确认形成各种类型脉的流体/熔体性质;(2)获得成脉过程中主微量元素(特别是流体不活动性HREE和HFSE)发生迁移的尺度和规模,探讨流体/熔体性质对超高压变质条件下元素地球化学行为的影响;(3)获得各种类型变质脉形成的确切时代。     

Anisotropy of physical properties in metamorphic rocks

The velocity of elastic waves has been measured along the principal fabric directions in 110 Alpine rocks, and these data are analyzed in conjunction with previously measured data on density, porosity, specific heat, and thermal diffusivity for the same rocks. Particular attention is given to the anisotropy in the thermal and elastic properties which are up to three times larger in the direction of lineation than in the direction perpendicular to the foliation. The correlations among the various scalar properties and the average thermal and elastic properties are in general weak and are overshadowed by variations due to mineralogy. However, relative thermal and elastic properties show a significant positive correlation, and for gneisses, schists, ultramafic rocks and carbonate rocks it is meaningful to predict thermal anisotropy on the basis of elastic anisotropy.     

Two-feldspar geothermometry in granulite facies metamorphic rocks

A geothermometric technique based on equilibria between coexisting plagioclase and alkali feldspar was applied to quartzo-feldspathic granulites from Salvador, BA, Brazil. The conditions of metamorphism were determined to be in the range 750 ° C–800 ° C, 4–8 Kb, by comparison with experimental data on the stabilities of sapphirine, phlogopite and other minerals occurring in the associated rocks. Selected feldspar data gives temperatures near, but slightly below, this range. Several variants of the Wood and Banno model, as well as an empirical two-pyroxene geothermometer, were also tested and found to give temperatures which were apparently 50 °–100 ° high. The solubility of Al₂O₃ in orthopyroxene indicates temperatures which are about 200 ° to high, suggesting that Fe in the natural assemblages significantly changes relationships observed experimentally in MgO-Al₂O₃-SiO₂ systems.     

Uranium and boron distributions related to metamorphic microstructure-evidence for metamorphic fluid activity

The distribution of uranium and boron in polymetamorphosed, granulite facies schists and gneisses has been studied using particle track methods. The concentration and distribution of these elements when examined in relation to mineralogy and microstructure provide an insight into: (1) the behaviour of U and B in metamorphism, (2) the activity of a fluid phase in the metamorphic processes and (3) the nature of chemical processes during schistosity development. A low concentration of primary U occurs in micro-inclusions of apatite and zircon (many are metamict) in the granulite facies M₁ assemblage. This assemblage which lacks B, except for zoned sillimanite, has undergone a localized retrograde metamorphism RM₁ characterised by hydrous alteration products containing abundant U and B. The RM₁ metamorphism is attributed to fluids generated during granulite facies dehydration reactions. A schistosity S₂ defined by M₂ fibrolite aggregates overprints the M₁ events. It is associated with (1) intragranular U concentrated in M₂ apatite and titanium bearing minerals and (2) abundant intergranular U within the fibrolite aggregates. High B contents also occur with the fibrolite. The S₂ schistosity appears to develop in a metamorphic environment containing a fluid enriched in U and B.     

Oxygen isotope geothermometers for metamorphic rocks

The Chicago mineral-carbonate oxygen isotope fractionation curves have been combined with mineral-water fractionation data for jadeite, zoisite and rutile and new data for grossular-water to provide a set of self-consistent mineral-pair calibrations. The A coefficients in the equation 1000 In α= A × 10⁶T^-2 of the new mineral-pair fractionations are
Jadeite Zoisite Grossular Rutile
Quartz 1.69 2.00 3.03 5.02
Jadeite 0.31 1.34 3.33
Zoisite 1.03 3.02
Grossular 1.99
The isotopic fractionation properties of natural pyralspite garnet [(Ca, Fe, Mg, Mn)₃Al₂Si₃O₁₂] can be approximated by those of the grossular end-member. Appropriate substitutions also yield coefficients for the solid-solution minerals: sodic pyroxene and epidote, e.g.
A _{quartz-sodic pyroxene}= 2.75 - 1.06X_jd,
A _{quartz-epidote}= 2.00 + 0.75X_ps
where X _Jd and X _Ps are the mole fractions of the jadeite and pistacite components, respectively.
The new data set is particularly suitable for the geothermometry of metamorphic rocks. δ¹⁸O data from minerals of the high-pressure metamorphic rocks of the Sesia Zone of Italy and Cyclades Complex of Greece yield well-constrained mean temperatures of 572 and 478 C, respectively. Type III blueschist metabasalts of the Franciscan Formation of California give mean quartz-garnet temperatures of 354 C.     

Mineralogy of New Caledonian metamorphic rocks

The compositions of metamorphic pyroxenes from blueschists in northern New Caledonia are investigated. Aegerine-augite occurs in siliceous metasediments and aegerine in some low-grade sodic basic schists. Calcic metamorphic pyroxene (omphacite and chloromelanite) appears first in metabasalts in higher grades of the lawsonite zone and is widespread in metamorphosed igneous rocks and quartzofeldspathic gneisses of the epidote zone. Omphacites in basic rocks have higher Mg∶Fe ratios and are less jadeitic than omphacites from adjacent interbedded quartzofeldspathic gneisses. With increasing metamorphic grade pyroxenes become more jadeitic and diopsidic at the expense of their acmite component. Elemental partitioning between coexisting pyroxenes, garnets and amphiboles from in situ regional metamorphic rocks is generally regular, suggesting equilibrium crystallization. Omphacite appears to be a stable phase within blueschist facies over a temperature range of at least 350° to 550° C. The “eclogitic” assemblage almandine-omphacite is stable within the earth's crust in metamorphosed sediments and igneous rocks over a temperature range of 400° to at least 550° C. No estimate of absolute pressures involved in metamorphism in the Ouégoa district can yet be made.     

The Precambrian metamorphic rocks of Ceylon

The Precambrian metamorphic rocks of Ceylon consist of basement gneisses, termed the Vijayan Series; which are overlain by supracrustal metasediments of the Highland Series and Southwest Group. All these rocks have been formed deep in the earths crust under PT conditions of the granulite facies. The apparent differences in metamorphic grade, structure and ultimately topographical expressions can be attributed to original differences in composition.An interpretation of all available geological, structural and geochronological data suggests that the Vijayan basement, quartzofeldspathic gneisses were formed at about 3000 m. y. The Highland Series-Southwest Group geosynclinal sediments were deposited on this basement and were metamorphosed at 2000 m. y. The Highland Series association of quartzite-marble-sillimanite garnet gneiss suggests a limestone-sandstone-shale derivation; while the Southwest Group hypersthene gneiss-metasedimentary sequence indicates a shale-greywacke parentage. The simple notherly trends of this orogen cuts and truncates the more complex Vijayan basement structures.At about 1200 m. y. the Vijayan basement was completely remobilized and at the same time hypersthene gneisses were formed in the Southwest Group. A final lowpressure metamorphic overprint at 650-450 m.y. formed cordierite, wollastonite and andalusite in rocks of appropriate composition in the Southwest Group; only locally affecting the Highland Series. Both the northwest trending Southwest Group and the northerly striking Highland Series can be considered a paired metamorphic belt, where respectively low P/T and intermediate P/T conditions were operative. The Vijayan is a basement complex of wet quartzofeldspathic gneisses in an intricate dome and basin tectonic style.

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Zusammenfassung Die präkambrischen metamorphen Gesteine Ceylons bestehen aus Basis-Gneisen, genannt Vijayan-Serie, die überlagert sind von suprakrustalen Meta-Sedimenten (Paragesteinen), der Highland-Serie (Hochlandserie) und der Südwest-Gruppe. Alle diese Gesteine wurden tief in der Erdkruste unter PT-Bedingungen der Granulitfazies geprägt. Die deutlichen Unterschiede in Metamorphosegrad, Tektonik und letztlich in topographischer Hinsicht können ursprünglichen Unterschieden in der Zusammensetzung zugeordnet werden.Eine Interpretation aller erhältlichen geologischen, tektonischen und geochronologischen Daten weist darauf hin, daß die Vijayan-Liegendgneise ungefähr vor 3 Milliarden Jahren ihre metamorphe Prägung erhielten. Die Geosynklinalsedimente der Hochlandserie und Südwest-Gruppe wurden auf diesem Basement abgelagert und vor ca. 2 Milliarden Jahren metamorphisiert.Die Hochland-Gesteinsvergesellschaftung (Quarzite, Marmore, Sillimanit-Granat-Gneise) legt eine Kalkstein-Sandstein-Tonschiefer-Abkunft nahe, während die Hypersthengneis-Metasedimentabfolge der Südwestgruppe auf Tonund Grauwacke-Herkunft schließen läßt. Die nördlichen Hauptstreichrichtungen dieses einfach gebauten Orogens durchschneiden oder unterbrechen die mehr komplexen Strukturen der Vijayan-Liegendserie.Vor ca. 1,2 Milliarden Jahren wurde die Vijayan-Basisserie völlig remobilisiert, und zur selben Zeit wurden Hypersthengneise in der Südwestgruppe gebildet. Eine abschließende metamorphe Überprägung bei geringen Drucken vor 650 bis 450 Millionen Jahren formte Cordierit, Wollastonit und Andalusit in Gesteinen entsprechender (geeigneter) Zusammensetzung in der Südwestgruppe. Nur lokal wurden auch Gesteine der Hochlandserie davon betroffen.Beide Serien — die Hochlandserie und die Südwestgruppe — können als eine geteilte (gemeinsame) metamorphe Zone mit niedrigen bzw. mittleren P/T-Bedingungen angesehen werden. Die Vijayan-Basisserie ist ein Komplex von nassen Gneisen mit kompliziertem tektonischem Aufbau.

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Résumé Les roches métamorphiques précambriennes de Ceylon consistent en un soussol de gneiss, nommé la «série de Vijayan». Elles sont recouvertes de métasédiments des «series des Highlands» et du «Groupe du Sud-Ouest». Toutes ces roches se sont formées profondement dans la crôute de la terre sous les conditions de pression et de température du faciès granulite. Les differences apparentes dans le degré métamorphique, la structure et finalement dans les expressions topographiques peuvent être attribués aux différences originales de composition.Une interprétation de toutes les données disponibles quelles soient géologiques, structurales et géochronologiques indiquerait que la gisement Vijayan (gneiss quartzofeldspatique) s'est formé il y a environ 3000 M. A. La série des Highlands et le groupe du Sud-Ouest, qui sont des sediments geosynclinaux, furent deposés sur ce gisement et furent métamorphisés il y a 2000 M. A. Les séries des Highlands, association de marbre, quartzite et de gneiss à grenat sillimanite indiquerait un passage du calcaire au grès, puis à l'argile schisteuse tandis que le groupe du Sud-Ouest formé de gneiss à hypersthene à séquence métasedimentaire indique une origine d'argile et grauwacke. Les directions principales vers le nord de cet orogène tronque les structures du gisement plus complexe de Vijayan.Il y a environ 1200 M. A. le gisement du Vijayan fut totalement modifé, et en même temps le gneiss à hypersthène s'est formé dans le groupe du Sud-Ouest. Un metamorphisme final à basse pression il y a 650-450 M. A. forma la cordiérite, la wollastonite et l'andalousite dans les roches de composition appropriée dans le groupe du Sud-Ouest, affectant seulement localement les séries du Highland. Le groupe du Sud-Ouest à direction nord-ouest et la série des Highlands à direction vers le nord peuvent tous les deux être considerés comme une double ceinture métamorphique, où respectivement les conditions de pression basse et intermediaire sont realisés. Le Vijayan est un gisement complexe du gneiss quartzofeldspathique « humide » dans un dôme et une bassine de style tectonique.

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Mineralogy of New Caledonian metamorphic rocks

The chemistry and phase relations of calcic and sodic amphiboles in the Ouégoa blueschists are investigated. The first appearance of sodic amphiboles is controlled by bulkrock chemistry. Sodic amphibole appears first in weakly-metamorphosed pumpellyite metabasalts prior to the crystallization of lawsonite but does not crystallize in pelitic schists until the middle of the lawsonite zone; sodic amphibole continues as an apparently stable phase in rocks of all bulk compositions into, and throughout, the highest-grade rocks in the district. Calcic amphibole is widespread in metabasalts of the lawsonite and epidote zones and also occurs in metasediments of appropriate composition. Coexisting pairs of calcic and sodic amphiboles are common in metabasalts but they have also been found in some metasediments. A grunerite-riebeckite pair is described.Electron-probe analyses of 120 amphiboles from representative rock-types are presented in graphical form. Sodic amphiboles show an increased Mg/(Mg+Fe) ratio with increasing metamorphic grade. Sodic amphiboles in pelitic schists are ferroglaucophane in the lawsonite zone and crossite and glaucophane in the epidote zone. Sodic amphiboles in metabasalts are iron-rich crossites in weakly-metamorphosed rocks and more-magnesian crossites and glaucophanes in the lawsonite and epidote zones. The abrupt increase in Mg/(Mg+Fe) ratio in sodic amphiboles at the epidote isograd is attributed to the crystallization of epidote and almandine which take the place of lawsonite and spessartine of the lawsonite zone. Calcic amphiboles are fibrous actinolites in the lawsonite zone and grade with increasing Al and Na/Ca ratio into prismatic blue-green hornblendes (barroisites) in the upper epidote zone. In calcic amphiboles, increasing metamorphic grade effects the coupled substitution of (Na+Al) for (Ca+Mg) and a small increases in Fe/Mg ratio; octahedrally and tetrahedrally coordinated Al increases in an approximately 11 ratio. Both the calcic and the sodic amphiboles show an increase in A-site occupancy with increasing metamorphic grade. In two-amphibole assemblages Ti, Mn and K are concentrated in the calcic amphibole.The textural and chemical relations between coexisting calcic and sodic amphiboles are discussed. If the calcic and sodic amphiboles are an equilibrium pair then the data collected from the Ouégoa amphiboles gives a picture of a very asymmetric solvus in the system glaucophane-actinolite-hornblende, i.e. steep-sided to glaucophane and with a gentle slope to the calcic amphibole field; there is no indication of any termination of the solvus under the pressure-temperature conditions of crystallization of the Ouégoa schists.     

Mineralogy of New Caledonian metamorphic rocks

Variations in chemistry and related physical properties of sheet silicates in the Ouégoa district with metamorphic grade are investigated. Weakly metamorphosed rocks prior to the crystallization of lawsonite contain phengite (d ₀₀₆=3.317–3.323 Å), chlorite and occasionally paragonite while interstratified basaltic sills contain chlorite, minor phengite and stilpnomelane. Pyrophyllite crystallizes before lawsonite in some metamorphosed acid tuffs and is also stable in the lawsonite zone. Paragonite, phengite and chlorite appear to be stable through the sequence from weakly metamorphosed rocks into high-grade “eclogitic” schists and gneisses. Optical, chemical and some X-ray diffraction data is given for representative sheet silicates. Electron probe analyses of 55 phengites, 21 paragonites, 57 chlorites, 12 vermiculites, 2 stilpnomelanes, and 2 chloritoids are presented in graphical form. All K-micas analysed are consistently phengitic (3.29–3.55 Si^iv ions per formula unit) and show limited solid solution with paragonite (4 to 13% Pa). The K∶Na ratio of the phengite is strongly dependant on the assemblage in which it occurs; the amount of phengite component and its Fe∶Mg ratio depends on bulk-rock composition. Phengites from acid volcanics have the highest Fe∶Mg ratio, highest phengite component and β refractive indices. Phengites from basic volcanics and metasediments of the epidote zone have the lowest Fe∶Mg ratio. Phengites from lawsonite-zone metasediments have intermediate Fe∶Mg ratios. The phengites show a small decrease in phengite component with increasing metamorphic grade. d ₀₀₆ for phengites varied from 3.302 to 3.323 Å but at least in the lawsonite and epidote zones appears to reflect composition and had little systematic variation with metamorphic grade; phengites from very low-grade rocks showed the longest values of d ₀₀₆. Paragonite shows almost no phengite-type substitution and only limited solid solution (4–12%) with muscovite. All paragonites (6) and most phengites (20) which have been examined are 2M₁ polymorphs; one Fe²⁺-phengite appears to be a 1M polymorph. The chemistry of chlorites closely reflects parent-rock chemistry. Chlorites from metasediments have distinctly higher Fe/(Fe+Mg) ratios than chlorites from basic igneous rocks; chlorites from the lawsonite and lawsonite-epidote transitional zone metasediments have the highest Fe/(Fe+Mg) ratios. In metabasalts Fe/(Fe+Mg) ratios appear to reflect individual variations in bulk-rock chemistry and show no direct correlation with metamorphic grade. There is little difference in Al/(Si+Al) ratio between chlorites from sediments and basic igneous rocks although in both lithologies the chlorites from the epidote zone appear to be slightly more aluminous. Fe-rich chlorites of the lawsonite zone metasediments have been altered by a process involving leaching of Fe and Mg and introduction of alkalies to a brown pleochroic Fe-vermiculite. Chemical and physical data for this vermiculite are given. The decrease in Fe/(Fe+Mg) ratio in chlorites and phengites on passing from the lawsonite to the epidote zone can be correlated with the crystallization of Fe-rich epidote and almandine in the epidote zone. Elemental partitioning between coexisting minerals has shown Ti to be partitioned into phengite, while Fe and Mn are strongly partitioned into chlorite. When either stilpnomelane or chloritoid coexists with phengite or chlorite, Fe and Mn are slightly enriched in the stilpnomelane or chloritoid relative to the chlorite.     

Mineralogy of New Caledonian metamorphic rocks

A Cretaceous to low-Tertiary sequence of interbedded pelites, cherts, basic and acidic volcanics and calcareous lenses has been metamorphosed by an Oligocene event. A complete intergradational metamorphic sequence is exposed in the Ouégoa destrict. The following metamorphic zones have been recognised: — (1) lowest-grade rocks consisting of quartz-sericite phyllites and pumpellyite metabasalts (2) lawsonite zone, characterized by the association of lawsonite and albite (3) epidote zone, characterised by epidote-omphacite-sodic hornblendealmandine bearing metabasalts and epidote-albite-almandine-glaucophane bearing metasediments; calcareous metasediments may also carry omphacite. The epidote and lawsonite zones are separated by a narrow belt of transitional rocks. Garnets occur in metasediments throughout the lawsonite zone as rare tiny crystals (<0.03 mm diam.). Garnets first appear in metabasalts in lawsonite-epidote transitional rocks. Garnets are widespread and abundant in epidote-zone metasediments and metabasalts. 45 garnets from rocks representative of all lithologies and metamorphic grades have been analysed with an electron-probe microanalyser. The garnets were consistently zoned. Garnets in lawsonite and low-grade epidote zones show a “bell-type” zoning with cores enriched in Mn relative to Fe and rims enriched in Fe, Mg and frequently Ca. Garnets from high-grade epidote-zone metapelites and metabasalts show, in addition, a shallow oscillatory zoning with complimentary variations in Mn and Fe equivalent to 5 mole- % spessartine and almandine. The Fe-for-Mn substitutional zoning, believed to be caused by a diffusion/saturation effect similar to that of the Rayleigh fractional model (Hollister, 1966), appears to have had superimposed on it the effects of parent-rock chemistry and metamorphic grade which control in a complex manner the composition of the cores and the rims of garnets. Garnets from different rock types and metamorphic grade are compositionally distinct. Garnets from lawsonite-zone rocks, irrespective of parent-rock chemistry appear to be spessartine. Garnets from epidote-zone metaigneous rocks and most metasediments are almandine. Garnets from epidote-zone metasediments with bulk-rock compositions which are manganiferous, or have high oxidation ratios, or both, may be spessartine-rich. Garnets from metabasalts are consistently more pyropic in both core and rim compositions than garnets from pelitic metasediments; the pyrope content of cores and rims of garnets from equivalent rock types and mineral assemblages increases with increasing metamorphic grade. Cores of garnets from epidote-zone pelites are richer in grossular than garnets from lower-grade pelites. The reaction which brings almandine garnet into Ouégoa district blueschist assemblages simultaneously with the replacement of lawsonite by epidote involves components of chlorites and sodic amphiboles and can be represented by the following simplified equation: ferroglaucophane+Fe-rich chlorite+lawsonite → glaucophane+Mg-rich chlorite+epidote+almandine.     

大别山超高压变质带内浅变质岩层再认识

本文依据浅变质岩层所保存的韵律层理、沉积构造、火山角砾一凝灰结构及残存的微古植物组合等,以及硅酸盐、稀土、微量元素分析成果的判别,对浅变质岩层进行了再认识,认定该浅变质岩是一套经受浅变质作用改造的火山碎屑沉积岩层;并就建立其地层序列,区域地层对比进行了论述;对其存在意义和时代归属进行了讨论.