Amphiboles of a Metagabbro--Amphibolite Sequence, Hidaka Metamorphic Belt, Hokkaido

The western part of the Hidaka Metamorphic Belt, Hokkaido, consistsof primary pyroxene gabbro and lesser amounts of olivine gabbrothat have been dynamically metamorphosed to metagabbro—gabbroicamphibolite-amphibolite-epidote amphibolite during uplift andshearing about 23 m.y. ago. Textures and the presence of relic and recrystallized amphiboleand plagioclase in the same rock indicate incomplete reactionand non attainment of equilibrium during recrystallization. EPMA and bulk analyses of 165 amphiboles indicate a continuousoverall compositional range from actinolite to dark green hornblende(with 100 mg/(Mg+Fe²⁺+Fe³⁺Mn) ratios varying from 89.5 to 32.0)marked by increasing Al, Fe, Ti, and Na. A compositional gapis usually present between relic and recrystallized amphibolesin any one rock which becomes more prominent with increasingshearing. In addition to host rock chemical control, amphibole compositionis largely dependent on the An content of coexisting plagioclase.Locally epidote and sphene exert a strong influence on bothamphibole and plagioclase compositions. Amphibole Ti and Mncontents decrease with shearing and Fe enrichment of the hostrocks largely as a result of the incoming of rutile, sphene,and Fe-Ti oxides. Analysis of host rock oxidation ratio andamphibole compositions indicates that the rocks essentiallybehaved as closed systems to oxygen during metamorphism. Al^1V-Al^IV, Al^IV-Fe³⁺, and Al^IV-(Na, K)^A are the main substitutionsin the amphiboles. Within any one rock the recrystallized amphibolesare enriched in Al, Fe, Ti, and Na relative to the relice amphiboles.Increasing metamorphism results in a progressive change of amphiboles(recrystallized) to more Fe and Si (rather than Al) rich compositionsreflecting the trend towards greenschist where Fe-actinolite(+Mg chlorite) would be stable. Differentiation of the amphiboles is within the limits of SiAlreplacement and the compositional limits of the early stagereaction rim and replacement amphiboles in the host olivineand pyroxene metagabbros.     

Composition of hornblendes formed during the hercynian progressive metamorphism of the Aracena metamorphic belt (SW Spain)

This paper attempts to illustrate the chemical variations of metamorphic hornblendes regarding host rocks and prograde variations. Changes related to bulk chemistry (orthoamphibolites) mainly concern Si, Al, Mg, Fe_tot and Ca. The Mg, Fe²⁺ and Fe³⁺ contents of hornblendes are, however, not strictly related to host rook compositions and Mg enrichments are correlated with increasing Fe³⁺ contents in the amphiboles. Thus, variations of oxygen fugacity may control the Mg contents of the Ca amphiboles studied but this does not show clear relations with the prograde metamorphism. The most sensitive but irregular variation related to the metamorphic conditions is the prograde enrichment of the alkalis into the A vacant position and an increase of the (Na+K)_tot/Na+K+Ca ratios of the amphiboles. Increasing Ti and Al^IV contents as well as decreasing Al^VI concentrations are also, but much less evidently, related to increasing T and P. A variation trend from tschermakitic to edenitic hornblendes may be drawn using Shido's end members calculation; this tendency and the relative deficiency of Al^VI contents in the low-grade members suggests that the amphiboles studied were subjected to conditions of a low-pressure metamorphism type. Such a conclusion is in agreement with the occurrence of andalusite-cordierite/sillimanite-cordierite associations in the metapelitic rocks, and the absence of Fe-rich garnet and epidote from the orthoamphibolites of the amphibolite facies at Aracena. Comparisons with Ca amphiboles from other metamorphic areas show, in agreement with various authors, that Abukuma hornblendes are similar to those encountered in high-grade thermal aureoles and tonalitic intrusives but different from the hornblendes of Barrovian metamorphism types.     

Argon retentivity and argon excess in amphiboles from the garbenschists of the Western Tauern Window,Eastern Alps

22 hornblende K-Ar ages and 10 ³⁹Ar-⁴⁰Ar spectra were obtained for hornblende garbenschists from the Western Tauern Window. The post-kinematic amphiboles were produced during the late Alpine prograde metamorphism (6–10 kb and 500–570° C). Two nearly potassiumfree cummingtonites rimming hornblende yield K-Ar ages of 120 Ma, while the 20 tschermakitic hornblendes scatter between 17 and 37 Ma. The reason for this scatter is excess Ar, possibly incorporated into amphiboles during healing of fractures, now traceable by trails of fluid inclusions. Excess Ar is semiquantitatively corrected for by combining cogenetic hornblende and cummingtonite with K-Ar isochrons. It can be quantified in 4 out of 10 hornblendes by ³⁹Ar-⁴⁰Ar stepwise heating experiments. Ages of 18–20 Ma result for corrected hornblendes. The retentivity of ⁴⁰Ar, after correction for excess, shows no correlation with chemistry within the narrow compositional range observed; rather, it shows intriguing correlations with irregularities in Ca/K spectra, pointing to a microstructurally controlled mechanism for Ar loss. This observation leads to a critical evaluation of the closure temperature constant, which apparently depends on an incompletely known number of mineralogical and environmental parameters. In particular those ³⁹Ar-⁴⁰Ar release spectra which yield low temperature steps with younger ages than the plateaus are not interpretable in terms of a synchronous closure. This gives evidence that loss of radiogenic isotopes proceeds by a more complex mechanism than simple volume diffusion through isotropic media.     

Compositions and phase relations of calcic amphiboles in epidote- and clinopyroxene-bearing rocks of the amphibolite and lower granulite facies,central Massachusetts,USA

Calcic amphiboles coexisting with epidotegroup minerals (zoisite, clinozoisite, epidote) and/or clinopyroxene±plagioclase±quartz±garnet occur in amphibolites and calc-silicate rocks that underwent amphibolite to lower granulite-facies metamorphism in the Acadian metamorphic high of central Massachusetts, USA. Across the region, peak metamorphic conditions range from about 580° C and 6.2 kbar to 730° C and 6.3 kbar. The coexistence of most Ca-amphiboles with Fe³⁺-rich epidote-group minerals suggests the presence of Fe³⁺ in most of these amphiboles. An empirical Fe³⁺ estimation for the microprobe analyses is based on two constraints: the Na?Ca content of the M4 sites of Ca-saturated, gravimetrically analyzed hornblendes gives the relation: Ca(M4) ^c =-1.479 Na(M4) ^c +2 (c=corrected). The second constraint is the stoichiometric equation Ca(M4)+Na(M4)+FM=15, where FM is the sum of all cations exclusive of Ca, Na, and K. Solving the two equations simultaneously gives: 20.185=0.479 Ca(M4)+1.479 ΣFM. Starting with the uncorrected values of Ca(M4) ^u and ΣFM(M4) ^u (u = uncorrected) of the all ferrous formula, the normalization factor NF for calculating the corrected cations of the ferric formulas is: 20.185/(0.478 Ca(M4) ^u +1.479 ΣFM ^u ). From the deficient oxygen the Fe³⁺ content which is equal to 2(23-ΣOX) can be calculated. Determinations of Fe³⁺ contents of four hornblende separates by Mössbauer spectroscopy are in agreement with the calculated values. The Ca-amphiboles show systematic changes in composition with increasing grade of metamorphism within the amphibolite and lower granulite-facies zones: increasing edenite and tschermakite substitution, increasing Ti content, and increasing Fe²⁺/(Fe²⁺+Mg) ratio. In addition, the coexisting clinopyroxenes are also characterized by an increase in Fe²⁺/(Fe²⁺+Mg) ratio. In quartz-free rocks with coexisting Ca-amphibole and plagioclase there is an increase in the ratio X _Ab/X _Ed, where X _Ab=Na/(Na+Ca) in plagioclase and X _Ed=Na in the amphibole A-site. These chemical changes in mineral composition together with the disappearance of epidote at the transition to granulite-facies metamorphic conditions are attributed to the continuous reaction: albite+epidote+Fe-Mg hornblende→Fe?Mg clinopyroxene+anorthite+(NaAlSi_-1)^Hbl+H₂O.     

Processes of regional metamorphism and ultrametamorphism and some problems of the geology of the deep-lying sections of folded zones

On the basis of 135 pairs of chemical analyses of coexisting hornblendes and biotites, we have established a relationship between the contents of Al^IV, Al^VI, Fe³⁺, Mg, Ti, Na, and K and the overall iron index in the hornblendes and the depth of granitoid formation. This relationship has been emphasized by the R-method of factor analysis. We have examined the strength and nature of the correlations between the elements in the hornblendes and have considered the types of Isomorphism in the amphiboles according to depth, from the viewpoint of crystal chemistry. A regular increase in the amounts of A^IV in hornblende from <0.8 to > 1.6 formula units; of (Al^VI + Fe³⁺ + Ti) from <0.5 to >1. 0 formula units; of (K + Na) from <0.35 to >0. 64 formula units; and of Group A from <0.24 to >0.51 formula units has been recorded from the near-surface granitoids to the ultra-abyssal types. Biotites In this respect display no adequately clear and reliable information.—Authors.     

Petrology of some Glaucophane Schists and Related Rocks from Taiwan

Detailed laboratory study has been made on pre-Tertiary coarse-grainedglaucophane schist, garnet-epidote amphibolite, and epidoteamphibolite in the eastern slope of the Central Mountain Range,Taiwan. These petrotectonic assemblages are considered to beexotic tectonic blocks emplaced within the feebly metamorphosedin situ graphite and quartzose schists of the Yuli belt. Thinlenses of Mn-rich metamorphosed tuff are intercalated withinthe metabasaltic rocks. Such high MnO (2 wt. per cent) and lowMgO (3–4 wt. per cent) tuffaceous rocks are similar inbulk composition to some volcanic clays collected in deep oceanbasins. They consist of the characteristic assemblage Mn-bearinggarnet (5–7 wt. per cent MnO and 30 volume per cent inthe rock)+muscovite+epidote+hornblende+quartz+ albite+rutile?pyrite. Successive stages of conversion of garnet-epidote amphiboliteto blueschist assemblages were noticed. The most recrystallizedschists display abundant Mn-bearing garnet, zoned amphibole,phengite, zoned epidote, stilpnomelane, chlorite, quartz, minoralbite, magnetite, and sphene. The recrystallization processis nearly isochemical except the glaucophane schists appearto be more oxidized and contain more Na₂O than the relict amphibolites.Intimately associated amphibolites of basaltic composition,in contrast, contain the assemblage hornblende+paragonite+epidote+chlorite+quartz+albite+rutile. Microprobe analyses of the coexisting minerals in glaucophaneschists, garnet-epidote amphibolites and epidote amphibolitesyield the following results: (1) garnets, consisting of almandine,spessartine, and grossular components, are less Mn and Mg-richcompared to those in in situ metabasalts of the Franciscan;(2) rim epidotes of the glaucophane schists are more pistastic(X_Fe=0?27–0?30) than that of the garnet-epidote amphibolite(0?2–0?22) implying higher fO₂ values for the glaucophanization;(3) phengitic micas of the glaucophane schist have less Al₂O₃content (29 wt. per cent) than those of the garnet-epidote amphibolite(32 wt. per cent) whereas micas of epidote amphibolites areparagonites with K/(K+Na) ratio of 0?04; (4) the zoned amphibolesshow glaucophane occurring marginal to cores of calcic amphibole.Sodic amphiboles with Al₂O₃ of 6-? to 10?4 wt. per cent arecrossite-glaucophane whereas all calcic amphiboles analyzedare barroisite-pargasite (Al₂O₃ greater than 10 wt. per cent). The garnet-epidote-rutile bearing glaucophane schist of Taiwanprobably recrystallized at temperatures above 350 ?C (the epidotezone) whereas the lawsonite-sphene glaucophane schists of theFranciscan equilibrated below 350 ?C (the lawsonite zone). TheMn-rich basaltic tuffs and their associated flows appear tohave been metamorphosed at profound depths and at the relativelyhigh temperatures of the epidote amphibolite facies, succeededlater by glaucophane schist facies metamorphism at lower temperatures.     

The Al-Fe(III)epidote miscibility gap in a metamorphic profile through the penninic series of the Tauern window,Austria

The compositional variations in epidotes, Ca₂Al₂(Fe³⁺, Al)-Si₃O₁₂(OH), from a prograde Mesozoic rock series in the Eastern Alps, Austria, are systematically related to metamorphic grade and the oxidation state of the rock. With increasing metamorphic grade the average composition of the zoned epidotes shifts to Fe³⁺-poorer compositions reflecting not only the effect of temperature and total pressure but also the concomitant decrease of the oxidation state of the rocks. Oxidized hematite-bearing assemblages are: 90 mole % Al₂FeEp (greenschists) 70 mole % Al₂FeEp (garnet amphibolites) 58 mole % Al₂FeEp (eclogites); reduced sulfidebearing assemblages are: 42 mole % Al₂FeEp (greenschists) 24 mole % Al₂FeEp (garnet amphibolites) 23 mole % Al₂FeEp (eclogites).A similar compositional evolution of the epidotes as in the spatial sequence of the samples can be observed within the single zoned crystals, reflecting the temporal changes of temperature, total pressure and oxygen fugacity during the prograde crystallization. The Fe³⁺-contents of core and rim decrease with increasing metamorphic grade and decreasing oxidation state. Generally the zoned epidotes consist of a Fe³⁺-rich core (90 to 63 mole % Al₂FeEp) and a Fe³⁺-poorer rim (55 to 23 mole % Al₂FeEp). Core and rim of the epidote crystals are separated by a compositional gap the extension of which is independent of the bulk rock composition, the oxidation state, and the mineralogical composition of the assemblages but becomes smaller with increasing metamorphic grade: 7253 mole % Al₂FeEp (low grade greenschists, 400° C) 6355 mole % Al₂FeEp (higher grade greenschists, 500° C), and 6055 mole % Al₂FeEp (garnet amphibolites, 500–550° C). At the temperature conditions of the highest grade garnet amphibolites and eclogites (550° C) the compositional gap closes at a composition of 58 mole % Al₂FeEp.The data presented thus confirm clearly the existence of an asymmetric miscibility gap in the monoclinic Al-Fe(III)-epidote solid solution series, which, for the first time, has been assumed by Strens (1964, 1965).A model is proposed that describes the prograde compositional evolution of the epidotes studied through the competing mechanisms of growth and diffusional Al-Fe³⁺ exchange and their dependence on metamorphic grade and oxidation state.     

Coarsening kinetics of the exsolution microstructure in alkali feldspar

Actinolite, hornblende and biotite coexisting in greenschist mafic metagreywackes have been analysed with the electron microprobe to obtain information on their chemical relationship during metamorphism. As in some other parts of the world, the two calcic amphiboles coexist in the greenschist facies because of a miscibility gap between them which is observed under conditions of low-pressure regional metamorphism; it is thought that the two amphiboles are in equilibrium, or at least that the actinolite participated in hornblendeforming reactions. Contact metamorphism by granitic intrusives of these metagreywackes has converted them to hornblende hornfelses with the assemblage hornblende, andesine, quartz, biotite±cummingtonite; the hornblendes of the hornfelses are found to have compositions between actinolite and hornblende of the greenschists, and frequently show fine exsolution lamellae of cummingtonite as a result of oversaturation in this component. The distribution of Fe-Mg between hornblende and biotite changes from the greenschist to the hornblende hornfels facies, and the K _D is probably dependent on Al^VI in the hornblende.     

Diffusion in coronas around clinopyroxene: modelling with local equilibrium and steady state,and a non-steady-state modification to account for zoned actinolite-hornblende

Retrograde coronas of Caledonian age, between clinopyroxene and plagioclase in the Jotun Nappe Complex, Norway, illustrate the effects of diffusion kinetics on mineral distributions among layers and on the compositions of hornblende-actinolite. One corona type comprises a symplectite of epidote + quartz adjacent to plagioclase, and a less well-organized intergrowth of amphibole + quartz replacing clinopyroxene. The observed mineral proportions imply an open-system reaction, but the similarity of Al/Si ratios in reactant plagioclase and product symplectite indicates approximate conservation of Al₂O₃ and SiO₂. The largest inferred open-system flux is a loss of CaO, mostly derived from consumption of clinopyroxene. The approximate layer structure, Pl|Ep + Qtz|Hbl + Qtz|Act±Hbl + Qtz|Cpx, is modelled using the theory of steady-state diffusion-controlled growth with local equilibrium. To obtain a solution, it is necessary to use a reactant plagioclase composition which takes into account aluminous (epidote) inclusions. The results indicate that, in terms of Onsager diffusion coefficients L _ii , Ca is more mobile than AL (L _CaCa/L _AlAl3.) (where means greater than or approximately equal to). This behaviour of Ca is comparable with that of Mg in previously studied coronas around olivine. Si is non-diffusing in the present modelling, because of silica saturation. Oxidation of some Fe²⁺ to Fe³⁺ occurs within the corona. Mg diffuses towards its source (clinopyroxene) to maintain local equilibrium. Other coronas consist of two layers, hornblende adjacent to plagioclase and zoned amphibole + quartz adjacent to clinopyroxene. In the zoned layer, actinolitic hornblende forms relict patches, separated from quartz blebs by more aluminous hornblende. A preliminary steady-state, local-equilibrium model of grain-boundary diffusion explains the formation of low-Al and high-Al layers as due to Al immobility. Zoning and replacement are qualitatively explained in terms of evolution of actinolite to more stable aluminous compositions. This is modelled by a non-steady-state modification of the theory, retaining local equilibrium in grain boundaries while relatively steep zoning profiles develop in grain interiors through slow intracrystalline diffusion. Replacement of actinolite by hornblende does not require a change in P–T conditions if actinolite is a kinetically determined, non-equilibrium product. The common preservation of a sharp contact between hornblende and actionolite layers may be explained by ineffectiveness of intracrystalline diffusion: according to the theory, given sufficient grain-boundary Al flux, a metastable actinolite + quartz layer in contact with hornblende may be diffusionally stable and may continue to grow in a steady state.     

Petrology of mafic inclusions from Itinome-gata,Japan

Various types of mafic inclusions up to 30 cm in size occur in lapilli tuff of alkali basalt at Itinome-gata crater, northeastern Japan. They are divided into the following four groups: amphibolite, hornblendite—hornblende gabbro, leucogabbro, and pyroxene gabbro. Also occurring with the mafic inclusions are lherzolite and websterite inclusions and megacrysts of Mg-rich olivine and chromian diopside. New analyses are presented for twenty five representative mafic inclusions, eight clinopyroxenes, six orthopyroxenes, and fifteen brown hornblendes. There are conspicuous chemical differences between the mafic inclusions and lherzolite and websterite inclusions: the former have higher TiO₂, Al₂O₃, total FeO, CaO, Na₂O, and K₂O, and lower MgO than the lherzolites and higher TiO₂, Al₂O₃, total FeO, and alkalis, and lower MgO than the websterites. The petrographic and chemical gradations among these three are not easily recognized. It is indicated that the Moho in this region is a boundary between mafic and ultramafic phases. The mineral assemblages of the mafic inclusions and the compositions of their essential minerals show that all of them recrystallized or crystallized under approximately the same temperature — pressure conditions, within the range of 600–1000° C and 6–9 kb. The following is hypothetically considered. The old and thick tholeiites or high-alumina basalts (may be pre-Silurian) making up the basement of the Japanese Islands had been subjected to the high T/P type metamorphism during Cretaceous time, and changed to amphibolites. In the cataclastic stage, complete melting of the lower part of the amphibolites occured locally and formed a gabbro magma. This gabbro magma moved upward slightly and produced hornblendite, hornblende gabbro and leucogabbro magmas by differentiation under wet conditions and a pyroxene gabbro magma under less wet conditions. Namely, the mafic inclusions are thought to be of fragments of the lower crust.     

Fe-Mg partition and miscibility gap between coexisting calcic amphiboles from the southern Abukuma Plateau,Japan

Lamellar intergrowths of actinolite and hornblende or aluminous actinolite occur in metamorphosed igneous rocks in the Hitachi metamorphic district, southern Abukuma Plateau. Electron microprobe analyses of five pairs are presented. The Fe-Mg partitioning and the miscibility gap are discussed in terms of an Mg-Fe-Al^IV diagram. The Fe-Mg partition coefficients depend on the Al^IV contents in hornblendes in a manner indicating that the pairs are close to equilibrium. Calcic amphibole pairs of high Fe/Mg ratio are richer in Al^IV than those with low ratio. The Al^IV content of the coexisting actinolite first increases with rising temperature, but then decreases as the temperature increases further. On the other hand, the Al^IV content of coexisting hornblende-aluminous actinolite successively decreases with rising temperature.     

Stable isotopes and amphibole chemistry on hydrothermally altered granitoids in the North Chilean Precordillera: a limited role for meteoric water?

Whole rock and mineral stable isotope and microprobe analyses are presented from granitoids of the North Chilean Precordillera. The Cretaceous to Tertiary plutonic rocks contain important ore deposits and frequently display compositional and textural evidence of hydrothermal alteration even in barren rocks. Deuteric alteration includes replacement of biotite and amphibole by chlorite and epidote, sericitization and saussuritization of feldspars, and uralitization of clinopyroxene and/or amphibole. While whole rock compositions are not significantly affected, compositional variations in amphiboles suggest two types of hydrothermal alteration. Hornblende with actinolitic patches and rims and tight compositional trends from hornblende to Mg-rich actinolite indicate increasing oxygen fugacity from magmatic to hydrothermal conditions. Uralitic amphiboles exhibiting irregular Mg-Fe distribution and variable Al content are interpreted as reflecting subsolidus hydration reactions at low temperatures. The δD values of hydrous silicates vary from −63 to −105‰. Most δ¹⁸O values of whole rocks are in the range of 5.7 to 7.7‰ and are considered normal for igneous rocks in the Andes. These δ¹⁸O values also coincide well with the oxygen isotope composition of geochemically similar recent volcanics from the Central Andean Volcanic Zone (δ¹⁸O = 7.0–7.4‰). Only one sample in this study (δ¹⁸O = 3.0‰) appears to be depleted by isotope exchange with light meteoric water at high temperatures. The formation of secondary minerals in all other intrusions is mainly the product of deuteric alteration. This also holds true for the sample from El Abra, the only pluton associated with mineralization. This indicates the dominant role of a magmatic rather than a meteoric fluid in the alteration of the Cretaceous and Tertiary granitoids in northern Chile. Received: 8 July 1998 / Accepted: 15 April 1999     

A new occurrence of garnetiferous skarn rocks in Saudi Arabia: a case study from Bahrah area, Jeddah–Makkah Al Mukaramah highway

Metacarbonate rocks (including marble and skarn deposits) at Bahrah area are confined to a Precambrian island-arc suite made up mostly of massive basalts and volcaniclastics aligned in a NE-trending belt. The marbles are either pure (almost made up of calcite) or contain considerable amounts of tremolite, actinolite, epidote, and diopside. Garnet-bearing rocks at Bahrah area are classified into garnetiferous marble and skarn calc-silicate assemblages that are described here for the first time. The calc-silicates become more abundant when the marble becomes interbedded with foliated metabasalt. Such contact is delineated by an epidote zone of variable thickness. Microscopically, the skarns are enriched in Ca-bearing minerals such as grossular garnet, epidote, titanite, diopside, and augitic salite. There are evidence that calc-silicate skarns were formed due to a thermal effect of a concealed underground shallow granitic intrusion. The basaltic rocks furnished Mg²⁺, Fe²⁺, Ti⁴⁺, and Al³⁺ that were first concentrated in the epidote zone. This was followed by pervasive replacement of epidote by large idiomorphic garnet (grossularite) that attains up to ～1.5 cm wide. It is evident that diopside is earlier than garnet with no replacement fabrics between the two minerals. Two types of titanite (sphene) can be distinguished: The first is secondary in the metabasalt host where titanite develops after titanomagnetite during regional metamorphism (i.e., metamorphic). On the other hand, the second type of titanite is found in the garnet-bearing calc-silicate skarn where it is typically euhedral with no link to any opaque phase and it is believed to be formed due to the event of superimposed thermal metamorphism (i.e., metasomatic). There are several evidence of the thermal metamorphic effect such as distinct granoblastic and annealing textures and K-metasomatism and formation of phlogopite at the expense of tremolite in the marble, in addition to poikiloblastic hornblende in the metabasalt host with distinct recrystallization. Also, there are some evidence of shearing such as brecciation along microshear planes, microfolding, introduction of fine euhedral pyrite, and presence of injected silica postdating crystallization of garnet in the calc-silicates.     

The compositional variation of synthetic sodic amphiboles at high and ultra-high pressures

Sodic amphiboles in high pressure and ultra-high pressure (UHP) metamorphic rocks are complex solid solutions in the system Na₂O–MgO–Al₂O₃–SiO₂–H₂O (NMASH) whose compositions vary with pressure and temperature. We conducted piston-cylinder experiments at 20–30?kbar and 700–800?°C to investigate the stability and compositional variations of sodic amphiboles, based on the reaction glaucophane=2jadeite+talc, by using the starting assemblage of natural glaucophane, talc and quartz, with synthetic jadeite. A close approach to equilibrium was achieved by performing compositional reversals, by evaluating compositional changes with time, and by suppressing the formation of Na-phyllosilicates. STEM observations show that the abundance of wide-chain structures in the synthetic amphiboles is low. An important feature of sodic amphibole in the NMASH system is that the assemblage jadeite–talc?±?quartz does not fix its composition at glaucophane. This is because other amphibole species such as cummingtonite (Cm), nyböite (Nyb), Al–Na-cummingtonite (Al–Na-Cm) and sodium anthophyllite (Na-Anth) are also buffered via the model reactions: 3cummingtonite?+?4quartz?+?4H₂O=7talc, nyböite?+?3quartz=3jadeite?+?talc, 3Al–Na-cummingtonite + 11quartz + 2H₂O=6jadeite + 5talc, and 3 sodium anthophyllite?+?13quartz?+?4H₂O=3 jadeite + 7talc. We observed that at all pressures and temperatures investigated, the compositions of newly grown amphiboles deviate significantly from stoichiometric glaucophane due to varying substitutions of Al^IV for Si, Mg on the M(4) site, and Na on the A-site. The deviation can be described chiefly by two compositional vectors: [Na^AAl^IV]<=>[□^ASi] (edenite) toward nyböite, and [Na^(M4)Al^VI]<=>[Mg^(M4)Mg^VI] toward cummingtonite. The extent of nyböite and cummingtonite substitution increases with temperature and decreases with pressure in the experiments. Similar compositional variations occur in sodic amphiboles from UHP rocks. The experimentally calibrated compositional changes therefore may prove useful for thermobarometric applications.     

Evolution cristallochimique des amphiboles dans la série granitique de Fort-Trinquet (Mauritanie)

The precambrian postorogenic pluton of the Fort-Trinquet area (Northern Mauretania) is composed by a series of granitic rocks in which amphiboles are the characteristic mafic minerals. Twenty six amphiboles have been separated and chemically analysed; the optical properties and the unit-cell data are also given.The crystallographic and chemical differences between these minerals reflect the variations in bulk composition of the host-rocks. In the plutonic suite, two igneous trends have been recognized: 1) a granitic trend (quartz monzonite-adamellite-granite) and 2) a syenitic trend (syenite-quartz bearing syenite-alkali granite). The amphiboles of the first trend belong to the tremolite-hastingsite series; they become richer in hastingsite mole with increasing proportion of quartz and decreasing Mg/(Mg+Fe) ratio in the rocks. The granitoids of the second trend are characterized by the occurrence of two amphiboles: a primary prismatic green-coloured actinolitic hornblende generally surrounded by a dark blue rim of riebeckite composition; the riebeckite may also form some acicular crystals associated to needles of stilpnomelane. These coexisting amphiboles would result from autometasomatic reactions which affected the rocks of the syenitic trend and which gave rise to the late alkali pegmatites where the constituent is a low arfvedsonitic riebeckite. The major substitutions involved in that amphibole transformation are Na^x R³⁺Ca^x R²⁺ and Na^xSiCa^xAl^IV.Comparison with experimental data allows to estimate the physical conditions during the emplacement and the tardimagmatic evolution of this granitoid series.     

Phase equilibria among pumpellyite,lawsonite, epidote and associated minerals in low grade metamorphic rocks

Phase relations of pumpellyite, epidote, lawsonite, CaCO₃, paragonite, actinolite, crossite and iron oxide are analysed on an Al-Ca-Fe³⁺ diagram in which all minerals are projected from quartz, albite or Jadeite, chlorite and fluid. Fe²⁺ and Mg are treated as a single component because variation in Fe²⁺/Mg has little effect on the stability of phases on the diagram. Comparison of assemblages in the Franciscan, Shuksan, Sanbagawa, New Caledonia, Southern Italian, and Otago metamorphic terranes reveals several reactions, useful for construction of a petrogenetic grid:

1. lawsonite+crossite + paragonite = epidote+chlorite + albite + quartz + H₂O
2. lawsonite + crossite = pumpellyite + epidote + chlorite + albite+ quartz + H₂O
3. crossite + pumpellyite + quartz = epidote + actinolite + albite + chlorite + H₂O
4. crossite + epidote + quartz = actinolite + hematite + albite + chlorite + H₂O
5. calcite + epidote + chlorite + quartz = pumpellyite + actinolite + H₂O + CO₂
6. pumpellyite + chlorite + quartz = epidote + actinolite + H₂O

     

Geochemistry of an island-arc plutonic suite: Wadi Dabr intrusive complex, Eastern Desert, Egypt

The Wadi Dabr intrusive complex, west of Mersa-Alam, Eastern Desert, Egypt ranges in composition from gabbro to diorite, quartz diorite and tonalite. The gabbroic rocks include pyroxene-horn blend e gabbro, hornblende gabbro, quartz-hornblende gabbro, metagabbro and amphibolite. Mineral chemistry data for the gabbroic rocks indicate that the composition of clinopyroxenes ranges from diopside to augite and the corresponding magma is equivalent to a volcanic-arc basalt. Plagioclase cores range from An₇₅ to An₃₄ for the gabbroic varieties, except for the metagabbro which has An _11–18. The brown amphiboles are primary phases and classified as calcic amphiboles, which range from tschermakitic hornblende to magnesiohornblende. Green hornblende and actinolite are secondary phases. Hornblende barometry and hornblende-plagioclase themometry for the gabbroic rocks estimate crystallisation conditions of 2–5 kb and 885–716°C.The intrusive rocks cover an extensive silica range (47.86–72.54 wt%) and do not exhibit simple straight-line variation on Harker diagrams for many elements (e.g. TiO₂, Al₂O₃, FeO^*, MgP, CaO, P₂O₅, Cr, Ni, V, Sr, Zr and Y). Most of these elements exhibit two geochemical trends suggesting two magma sources.The gabbroic rocks are relatively enriched in large ion lithophile elements (K, Rb, Sr and Ba) and depleted in high field strength elements (Nb, Zr, Ti and Y) which suggest subduction-related magma. Rare earth element (REE) data demonstrate that the gabbroic rocks have a slight enrichment of light REE [(La/Yb)_N=2.67−3.91] and depletion of heavy REE ((Tb/Yb)_N=1.42−1.47], which suggest the parent magma was of relatively primitive mantle source.The diorites and tonalites are clearly calc-alkaline and have negative anomalies of Nb, Zr, and Y which also suggest subduction-related magma. They are related to continental trondhjemites in terms of Rb---Sr, K---Na---Ca, and to volcanic-arc granites in terms of Rb---and Nb---Y.The Wadi Dabr intrusive complex is analogous to intrusions emplaced in immature ensimatic island-arcs and represents a mixture of mantle (gabbroic rocks) and crustal fusion products (diorites and tonalites) modified by fractional processes.     

Statistical chemistry of calcic amphiboles

Principal components analysis is used to study the chemistry of 639 calcic amphiboles. Eigenvectors representing multiple partial correlation coefficients give various sets of substitutional relationships. The relative significance of each set can be noted by the percent variation of the data it represents. The highest percent variation (36%) is associated with the substitutions $$Si + Mg \rightleftharpoons Al^{IV} + Al^{VI} + Ti + Fe^{3 + } + Fe^{2 + } + Na + K$$ . Other expected substitutions among the ions such as Al^IV + Na ? Si, the positive correlation between Al^IV and Al^VI etc. are shown statistically. The substitution of Al in T ₁ and T ₂ positions imposes an ordering in the M ₁, M ₂ and M ₃ sites. Variability of OH in the amphiboles is found to be significant. There is no definite correlation between OH and Fe³⁺ but OH and Ti are positively correlated. Under certain conditions and provided the concentration of Al^IV does not change significantly, Fe and Mg may be assumed to mix ideally in the amphibole solid solution.     

A petrochemical study of calcic amphiboles from the East Bull Lake anorthosite-gabbro layered complex,District of Algoma,Ontario

Calcic amphiboles are ubiquitous in the East Bull Lake anorthosite-gabbro complex, northeastern Ontario. The mode of occurrence suggests the amphiboles replaced clinopyroxene throughout the stratigraphic levels of the pluton, but they are also prolific in the top younger units. Three types of amphiboles, namely (1) tremolite-actinolite, (2) actinolitic hornblende and (3) hornblende, were identified. The composition of the amphiboles shows that they define a continuous series without any obvious compositional-miscibility gap. The analyses further indicated that the amphiboles evolved from actinolite to hornblende through coupled substitution involving edenite and tschermakite end members. Among the three coexisting amphiboles, chlorine is concentrated in hornblende. This is due to preferential location of hornblende along grain boundaries and/or its favourable structure.The compositional variability of amphiboles is controlled by (1) bulk rock composition and (2) faulting and fracturing. The most Fe- and Al-rich hornblendes are confined generally to top units in the gabbro. These units are rich in Si and Fe, but relatively poor in Mg. The Mg-rich tremolite is restricted to a Si-poor, Mg-rich troctolite unit. Amphiboles in gabbro samples from fault zones also contain relatively Mg-rich calcic amphibole. The fault gabbro is highly oxidized.     

Chemical composition of pargasite and hornblende in low to High grade metamorphic rocks of the Rhodope zone,Xanthi, Greece

Summary The chemistry of amphiboles from schists, quartzofeldspathic gneisses and migmatites ranging in metamorphic grade from greenschist to amphibolite facies has been determined by electron microprobe. Intercalated amphibolites suggest that some of the rocks retrograded from the eclogite stability field; others were never metamorphosed above greenschist facies. Rocks which contain other mineralogical evidence for an original high pressure assemblage have amphiboles with high Na/K, low Fe and relatively low Ti. Other high-grade rocks contain amphibole of broadly pargasitic composition. The pargasites from more Ca-rich bulk compositions have less substitution of Na for Ca in the M4 site than do those from Ca-poor bulk compositions. A lower grade assemblage of amphiboles ranges from hornblende through actinolitic hornblende to actinolite; this is retrograde in the gneisses and migmatites, but may be prograde in the schists. In contrast, the high-grade assemblage shows almost constant high K with variable Na and quite different trends for edenite-type substitution (^IVAl variation with A site occupancy) and variation of^IVAl with Fe/(Fe + Mg) and with Ti.

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Chemische Zusammensetzung von Pargasit und Hornblende in niedrig- bis hoch-gradig metamorphen Gesteinen der Rhodope-Zone, Xanthi, Griechenland

Zusammenfassung Die Chemische Zusammensetzung von Amphibolen aus Schiefern, Quarz-Feldspat-Gneisen, und Migmatiten, deren metamorpher Grad von Grünschiefer- bis zur Amphibolitfazies reicht, wurde mit der Elektronenstrahl-Mikrosonde bestimmt. Zwischengeschaltete Amphibolite legen die Vermutung nahe, daß einige der Gesteine durch retrograde Metamorphose aus dem Eklogit-Stabilitatsfeld hervorgegangen sind; andere jedoch hatten niemals in ihrer Entwicklung einen höheren Grad als den der Grünschie-ferfazies erreicht. Gesteine, die andere mineralogische Hinweise für eine ursprungliche Hochdruck-Paragenese führen, enthalten Amphibole mit hohem Na/K, niedrigen Eisen und relativ niedrigen Ti. Andere high-grade Gesteine enthalten Amphibol von pargasitischer Zusammensetzung. Die Pargasite aus mehr Kalzium-reichen Gesteinen zeigen geringere Substitution von Natrium für Kalzium an den M4 Plätzen als jene aus Kalzium-armen Gesteinen. Eine niedriger-gradige Paragenese von Amphibolen umfaßt Zusammensetzungen von Hornblende über aktinolitische Hornblende bis zu Aktinolit; diese ist in den Gneisen und Migmatiten retrograd, durfte in den Schiefern jedoch prograd sein. Im Gegensatz dazu zeigt die hochgradige Paragenese fast durchwegs konstant hohe Kalium-Gehalte mit variablem Natrium und einen anderen Trend für Edenit-artige Substitution (^IVAl Variation mit Besetzung der A Plätze) und eine Variation von^IVAl mit Fe/(Fe + Mg) und mit Ti.

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With 4 Figures