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1.
ABSTRACT Paragonite-bearing amphibolites occur interbedded with a garbenschist-micaschist sequence in the Austroalpine Schneeberg Complex, southern Tyrol. The mineral assemblage mainly comprises paragonite + Mg-hornblende/tschermakite + quartz + plagioclase + biotite + ankerite + Ti-phase + garnet ± muscovite. Equilibrium P–T conditions for this assemblage are 550–600°C and 8–10 kbar estimated from garnet–amphibole–plagioclase–ilmenite–rutile and Si contents of phengitic muscovites. In the vicinity of amphibole, paragonite is replaced by symplectitic chlorite + plagioclase + margarite +± biotite assemblages. Muscovite in the vicinity of amphibole reacts to form plagioclase + biotite + margarite symplectites. The reaction of white mica + hornblende is the result of decompression during uplift of the Schneeberg Complex. The breakdown of paragonite + hornblende is a water-consuming reaction and therefore it is controlled by the availability of fluid on the retrogressive P–T path. Paragonite + hornblende is a high-temperature equivalent of the common blueschist-assemblage paragonite + glaucophane in Ca-bearing systems and represents restricted P–T conditions just below omphacite stability in a mafic bulk system. While paragonite + glaucophane breakdown to chlorite + albite marks the blueschist/greenschist transition, the paragonite + hornblende breakdown observed in Schneeberg Complex rocks is indicative of a transition from epidote-amphibolite facies to greenschist facies conditions at a flatter P–T gradient of the metamorphic path compared to subduction-zone environments. Ar/Ar dating of paragonite yields an age of 84.5 ± 1 Ma, corroborating an Eoalpine high-pressure metamorphic event within the Austroalpine unit west of the Tauern Window. Eclogites that occur in the Ötztal Crystalline Basement south of the Schneeberg Complex are thought to be associated with this Eoalpine metamorphic event. 相似文献
2.
G. M. Gibson 《Contributions to Mineralogy and Petrology》1979,68(2):171-179
Margarite is both abundant and widespread throughout a sequence of interstratified amphibolite, hornblendite, and metamorphosed anorthosite from the upper Lyvia River, central Fiordland. These rock types comprise part of a metamorphosed layered intrusion. Assemblages recorded from these rocks are the product of two distinct phases of metamorphism. First generation assemblages typically comprise plagioclase (An84–96), hornblende, kyanite, and minor corundum. Clinozoisite and chlorite occur as late stage breakdown products of plagioclase and hornblende. Margarite developed during the second phase of metamorphism.Within the corundum-bearing rocks replacement of corundum or plagioclase by margarite can be observed directly. On the basis of these observations the following reaction is evident: 1 corundum+1 anorthite+1H2O=1 margarite.In other assemblages the formation of margarite can be attributed to the breakdown of kyanite and clinozoisite according to the reaction: 2 kyanite+2 clinozoisite=1 margarite+3 anorthite.Margarite is found, however, to contain appreciable amounts of paragonite solid-solution (up to 28 mol%) and plagioclase produced (second generation) is not pure anorthite but of intermediate compositions (An46–62). The reaction therefore involves the introduction of both soda and silica. Margarite also crystallized independently of clinozoisite according to a reaction of the general form: 5 pargasite+17 kyanite+19 H2O =8 margarite+4 chlorite+7 plagioclase.Application of available experimental data suggests that the margarite formed between 550 and 720 ° C up to a maximum pressure of 9.5 kb. Whereas the involvement of albite component (second generation plagioclase) will tend to lower the temperatures and pressures necessary for the occurrence of margarite, this effect is partially offset by the significant amounts of paragonite end-member held within the margarite. An independent estimate of the metamorphic conditions in metapelites suggests that the introduction of albite lowers equilibration temperatures by about 2 ° C for every 1% albite. 相似文献
3.
White mica from the Liassic black shales and slates in Central Switzerland was analysed by transmission electron microscopy (TEM) and electron microprobe to determine its textural and compositional evolution during very low-grade prograde metamorphism. Samples were studied from the diagenetic zone, anchizone and epizone (T ≈100°–450 °C). Phyllosilicate minerals analysed include illite/smectite (I/S), phengite, muscovite, brammallite, paragonite, margarite and glauconite. Textural evolution primarily is towards larger, more defect-free grains with compositions that approach those of their respective end-members. The smectite-to-illite transformation reduced the amounts of the exchange components SiK?1Al?1, MgSiAl?2, and Fe3+Al?1. These trends continue to a lesser degree in the anchizone and epizone. Correlations between the proportion of smectite in I/S and the composition of I/S indicate that smectite layers may contain a high layer charge. Illite in I/S bears a compositional resemblance to macrocrystalline phengite in some samples, but is different in others. Paragonite first appears in the upper diagenetic zone or lower anchizone as an interlayer-deficient brammallite, and it may be mixed with muscovite on the nanometre scale. Owing to the small calculated structure factor for paragonite-muscovite superstructures, conventional X-ray powder diffraction cannot distinguish between mixed-layer structures and a homogeneous compositionally intermediate solid solutions. However, indirect TEM evidence shows that irregularly shaped domains of Na- and K-rich mica exist below 10 nm. Subsequent coarsening of domains at higher grades produced discrete paragonite grains at the margins of muscovite crystals or in laths parallel to the basal plane of the host muscovite. Margarite appears in the epizone and follows a textural evolution similar to paragonite in that mixtures of margarite, paragonite, and muscovite may initially occur on the nanometre scale. However, no evidence of interlayer-poor margarite has been found. 相似文献
4.
Eclogite boudins occur within an orthogneiss sheet enclosed in a Barrovian metapelite‐dominated volcano‐sedimentary sequence within the Velké Vrbno unit, NE Bohemian Massif. A metamorphic and lithological break defines the base of the eclogite‐bearing orthogneiss nappe, with a structurally lower sequence without eclogite exposed in a tectonic window. The typical assemblage of the structurally upper metapelites is garnet–staurolite–kyanite–biotite–plagioclase–muscovite–quartz–ilmenite ± rutile ± silli‐manite and prograde‐zoned garnet includes chloritoid–chlorite–paragonite–margarite, staurolite–chlorite–paragonite–margarite and kyanite–chlorite–rutile. In pseudosection modelling in the system Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O (NCKFMASH) using THERMOCALC, the prograde path crosses the discontinuous reaction chloritoid + margarite = chlorite + garnet + staurolite + paragonite (with muscovite + quartz + H2O) at 9.5 kbar and 570 °C and the metamorphic peak is reached at 11 kbar and 640 °C. Decompression through about 7 kbar is indicated by sillimanite and biotite growing at the expense of garnet. In the tectonic window, the structurally lower metapelites (garnet–staurolite–biotite–muscovite–quartz ± plagioclase ± sillimanite ± kyanite) and amphibolites (garnet–amphibole–plagioclase ± epidote) indicate a metamorphic peak of 10 kbar at 620 °C and 11 kbar and 610–660 °C, respectively, that is consistent with the other metapelites. The eclogites are composed of garnet, omphacite relicts (jadeite = 33%) within plagioclase–clinopyroxene symplectites, epidote and late amphibole–plagioclase domains. Garnet commonly includes rutile–quartz–epidote ± clinopyroxene (jadeite = 43%) ± magnetite ± amphibole and its growth zoning is compatible in the pseudosection with burial under H2O‐undersaturated conditions to 18 kbar and 680 °C. Plagioclase + amphibole replaces garnet within foliated boudin margins and results in the assemblage epidote–amphibole–plagioclase indicating that decompression occurred under decreasing temperature into garnet‐free epidote–amphibolite facies conditions. The prograde path of eclogites and metapelites up to the metamorphic peak cannot be shared, being along different geothermal gradients, of about 11 and 17 °C km?1, respectively, to metamorphic pressure peaks that are 6–7 kbar apart. The eclogite–orthogneiss sheet docked with metapelites at about 11 kbar and 650 °C, and from this depth the exhumation of the pile is shared. 相似文献
5.
Coexisting white micas and plagioclase were studied by electronmicroprobe (EMP), and transmission and analytical electron microscopy(TEM—AEM) in greenschist- to amphibolite-grade metabauxitesfrom Naxos. The TEM—AEM studies indicate that sub-micronscale (0.01–1.0 µm thick) semicoherent intergrowthsof margarite, paragonite and muscovite are common up to loweramphibolite conditions. If unrecognized, such small-scale micainterlayering can easily lead to incorrect interpretation ofEMP data. Muscovite and paragonite in M2 greenschist-grade Naxosrocks are mainly relics of an earlier high-pressure metamorphism(M1). Owing to the medium-pressure M2 event, margante occursin middle greenschist-grade metabauxites and gradually is replacedby plagioclase + corundum in amphibolite-grade metabauxites.The margarite displays minor IVAl3 VI(Fe3+, Al) Si-3 VI--1 andconsiderable (Na, K) SiCa-1Al-1 substitution, resulting in upto 44 mol% paragonite and 6 mol % muscovite in solution. Thecompositional variation of muscovite is mainly described byVI(Fe2+, Mg) Si VI Al-1VI Al-1 and VI(Fe3+Al-1) exchanges, thelatter becoming dominant at amphibolite grade, Muscovite issignificantly richer in Fe than margarite or paragonite. Ca—Na—Kpartitioning data indicate that margarite commonly has a significantlyhigher Na/(Na+ K+Ca) value than coexisting muscovite or plagioclase.Exceptions are found in several greenschist-grade rocks, inwhich M1-formed mussovite may have failed to equilibrate withM2 margarite. The sluggishness of K-rich micas to recrystallizeand adjust composidonally to changing P-T conditions is alsoreflected in the results of mus-covite-paragonite solvus thermometry.Chemical data for Ca—Na micas from this study and literaturedata indicate that naturally coexisting margarite—paragonitepairs display considerably less mutual solubility than suggestedby experimental work. The variable and irregular Na partitioningbetween margarite and muscovite as observed in many metamorphicrocks could largely be related to opposing effects of pressureon Na solubility in margarite and paragonite and/or non-equilibriumbetween micas. KEY WORDS: Ca—Na—K mica; margarite; metabauxite; Naxos; sub-micron-scale mica interlayering 相似文献
6.
The unmetamorphosed equivalents of the regionally metamorphosedclays and marls that make up the Alpine Liassic black shaleformation consist of illite, irregular mixed-layer illite/montmorillonite,chlorite, kaolinite, quartz, calcite, and dolomite, with accessoryfeldspars and organic material. At higher grade, in the anchizonalslates, pyrophyllite is present and is thought to have formedat the expense of kaolinite; paragonite and a mixed-layer paragonite/muscovitepresumably formed from the mixed-layer illite/montmorillonite.Anchimetamorphic illite is poorer in Fe and Mg than at the diageneticstage, having lost these elements during the formation of chlorite.Detrital feldspar has disappeared. In epimetamorphic phyllites, chloritoid and margarite appearby the reactions pyrophyllite + chlorite = chloritoid + quartz+ H2O and pyrophyllite + calcite ± paragonite = margarite+ quartz + H2O + CO2, respectively. At the epi-mesozone transition,paragonite and chloritoid seem to become incompatible in thepresence of carbonates and yield the following breakdown products:plagioclase, margarite, clinozoisite (and minor zoisite), andbiotite. The maximum distribution of margarite is at the epizone-mesozoneboundary; at higher metamorphic grade margarite is consumedby a continuous reaction producing plagioclase. Most of the observed assemblages in the anchi-and epizone canbe treated in the two subsystems MgO (or FeO)-Na2O–CaO–Al2O3–(KAl3O5–SiO2–H2O–CO2).Chemographic analyses show that the variance of assemblagesdecreases with increasing metamorphic grade. Physical conditions are estimated from calibrated mineral reactionsand other petrographic data. The composition of the fluid phasewas low in XCO2 throughout the metamorphic profile, whereasXCH4 was very high, particularly in the anchizone where aH2Owas probably as low as 0.2. P-T conditions along the metamorphicprofile are 1–2 kb/200–300 °C in the anchizone(Glarus Alps), and 5 kb/500–550 °C at the epi-mesozonetransition (Lukmanier area). Calculated geothermal gradientsdecrease from 50 °C/km in the anchimetamorphic Glarus Alpsto 30 °C/km at the epi-mesozone transition of the Lukmanierarea. 相似文献
7.
E. Hoffer 《Contributions to Mineralogy and Petrology》1978,67(2):209-219
The formation of paragonite at the transition from the low-grade to the medium-grade matamorphism and its breakdown in the presence of quartz in the upper medium grade in common metapelites is investigated.The microprobe work on the white micas from the low and medium-grade rocks yields compositional differences in respect to the celadonite substitutions and the paragonite content. The low-grade white micas are phengites having Si[4] 6.25 to 6.44 and Altot 4.89 to 5.20. The paragonite component in solid solution in the phengites ranges from 11 to 17 mole %. In the transition from the low-grade to the medium-grade metamorphism, concomitant with the breakdown of chlorite, the phengites change to muscovites having Si[4] 6.07 to 6.16 and Altot 5.36 to 5.56. At the same time, the amount of paragonite in solid solution increases up to 22±2 mole % and paragonite makes its first appearance as a separate mineral. The increase of the percentage of paragonite in solid solution in the muscovites is due to the drastical modal decrease of muscovite in the course of the breakdown of chlorite. The formation of paragonite is readily explained by the muscovite-paragonite solvus. Paragonite forms thin lamellae (1–20 m) interlayered with muscovite lamellae (1–40 m). The average composition is Pg88.5Ms7Mar4.5. Paragonite occurs together with staurolite+biotite, kyanite+biotite, cordierite +biotite, and andalusite+biotite. In the presence of quartz, it breaks down in the lower part of the andalusite zone to andalusite and albite-rich plagioclase. At the same time, the amount of paragonite in solid solution in the muscovites decrease to 11–15 mole %. The basal spacings d(002) of the phengites and muscovites investigated show a clear dependence on the Na+ content and the celadonite substitutions. 相似文献
8.
Volker Höck 《Contributions to Mineralogy and Petrology》1974,43(4):261-273
The assemblages phengite-paragonite, phengite-margarite and phengite-paragonitemargarite are very common in metasediments of a N-S profile in the middle sector of the Hohe Tauern. The Si4+-content of phengite shows no regular change with increasing temperature from north to south along the profile. The variations in the d 002 basal spacings of phengite coexisting with paragonite are not only dependent on the Na+ content of phengite but also on the Mg2++Fe2+ content of the micas. Neither the sodium content in phengite nor the potassium content in paragonite shows any dependence on temperature. Chemical analyses of coexisting phengite, paragonite and margarite give the extent of the three-phase-region which is characterized by a small amount of margarite in paragonite (4 Mol%), by a large quantity of Na+ in margarite (28 Mol% paragonite), and limited miscibility between phengite and paragonite. 相似文献
9.
The techniques of electron probe microanalysis and x-ray diffractometry have been utilized in a study of the sillimanite-potassium
feldspar isograd in western Maine. The isograd reaction is theoretically a discontinuous one, calling for the nearly instantaneous
loss of muscovite and crystallization of sillimanite and orthoclase, with a small contribution of albite from the pre-existing
plagioclase. In fact, muscovite coexists with orthoclase, sillimanite, and plagioclase for a distance of at least seven miles
from the isograd (marked by the initial coexistence of orthoclase and sillimanite). In this assemblage, muscovite has an extremely
narrow range of composition, about an average of Ms93.5Pg6.5. A possible explanation for the divariant character of the isograd reaction is that, during dehydration, PH2O slowly increased from initial values less than Ptotal + rock strength, under conditions of low permeability, the actual value of PH2O being controlled by a buffer assemblage and local conditions of P and T. An alternative explanation postulates the flattening
of thermal gradients following the onset of fractional melting.
The isograd reaction is dependent in only a minor way upon the anorthite content of the plagioclase. Below the isograd, a
continuous reaction takes place leading to a diminution in paragonite content of muscovite stable in the presence of quartz.
It is possible that this reaction leads to the nearly ubiquitous normal zoning of the plagioclase.
Changes in the composition of biotite at the isograd are not conspicuous, and can be satisfactorily explained by the release
of Mg, Fe, and Ti impurity from the muscovite, and a continuous reaction between ilmenite, quartz, and muscovite. Garnets
are not abundant and are high in Mn, both facts probably due to the low pressure of metamorphism, The presence of garnet probably
relates to the Mn content of the rock, and seems to be independent of the Mg/Fe ratio of the biotite. The garnets are zoned
with respect to Mn and Mg, but often Mn is enriched and Mg depleted in the marginal zone. The Mg/Fe ratio of the biotite varies
twofold depending on the presence or absence of pyrrhotite. The transition: microcline → orthoclase depends upon the amount
of dissolved albite; the polymorph is orthoclase in the pelitic schists but microcline in the calc-silicate rocks which are
much lower in sodium. The plagioclases are of “low” structural type, although
is slightly greater than many other “low” plagioclases. A correlation of d(002) of muscovite and paragonite solid solution
for the range 0 to 20 % paragonite is given. An appreciable positive volume of mixing for the binary system muscovite-paragonite
is indicated. 相似文献
10.
Staurolite-kyanite schists from the Dalradian of Scotland and Ireland show two types of retrograde alteration. In the Irish examples staurolite and kyanite are replaced by muscovite and paragonite, often with margarite and sometimes chloritoid. Plagioclase in these samples may be albitized. In contrast one Scottish sample shows replacement of both staurolite and plagioclase by muscovite. Retrogression resulted from infiltration of fluid, but while in some samples the large number of retrograde phases internally buffered the fluid composition and cation metasomatism cannot be proven, in others the fluid composition was externally controlled and there was cation metasomatism indicating more extensive infiltration. It is demonstrated by the use of activity diagrams showing the relationships between Al-silicate, white mica and alkali feldspar that wholesale alteration to muscovite was most likely caused by fluids moving down-temperature from a granitic or arkosic source. In contrast, if growth of retrograde albite is actually accompanied by Na-metasomatism then the infiltrating fluids are likely to have been moving up-temperature through pelitic lithologies. Large fluid: rock ratios of around 101 are needed to achieve significant alkali metasomatism except in extreme situations of large temperature changes and highly concentrated fluids. 相似文献
11.
Paragonite occurs in pelitic schists and amphibole schists of the Ötztal Alps (Tyrol); its occurence in amphibole schists is controlled by the Na2O/K2O, in pelitic schists by the local fabric. Some analysis of the bulk chemistry of the paragonite bearing rocks and one microprobe analysis of paragonite are given. Paragonite is stable together with muscovite, together with quartz it is unstable. Its breakdown can be described by the reaction: paragonite+quartz kyanite+albite+H2O. The age of this breakdown is discussed by comparison of experimental, field and radiometric data, it is thought to be of Variscian age. 相似文献
12.
C. V. GUIDOTTI F. P. SASSI R. SASSI J. G. BLENCOE 《Journal of Metamorphic Geology》1994,12(6):779-788
Chemical data for 139 natural paragonite-muscovite (Pg-Ms) pairs illustrate the effects of ferromagnesian components on the P-T-X topology of the Pg-Ms solvus. The pairs were selected on the basis of: reasonably accurate knowledge of the P-T conditions of formation; evidence for close approach to equilibrium at peak metamorphic conditions; exclusion of pairs in which paragonite contains more than 5 mol% margarite; and exclusion of pairs from polymetamorphic rocks that contain more than one set of cogenetic Pg-Ms pairs. Graphical analysis reveals considerable scatter in the data; nevertheless, it is evident that the muscovite limb of the solvus shifts markedly toward end-member muscovite with increasing pressure from approximately 7 kbar to 21 kbar. This shift is attributed to a pressure-induced increase of the ferromagnesian content of muscovite, which increases the size of the XII alkali site - to the effect that K is more readily accommodated than Na. The data also suggest that the paragonite limb of the solvus migrates slightly toward end-member paragonite with increasing pressure. Broadening of the Pg-Ms solvus with increasing pressure reflects increasingly nonideal Na-K mixing as the phengite content of muscovite increases. Due to the wide scatter of data for Pg-phengitic-Ms pairs, it is concluded that, at the present time, Pg-Ms solvus thermometry is only viable for quasibinary Pg-Ms pairs. 相似文献
13.
Niranjan D. Chatterjee 《Contributions to Mineralogy and Petrology》1970,27(3):244-257
The mineral paragonite, NaAl2[AlSi3O10 (OH)]2, has been synthesized on its own composition starting from a variety of different materials. Indexed powder data and refined cell parameters are given for both the 1M and 2M1 polymorphs obtained. The upper stability limit of paragonite is marked by its breakdown to albite + corundum + vapour. The univariant equilibria pertaining to this reaction have been established by reversing the reaction at six different pressures, the equilibrium curve running through the following
intervals: 1 kb: 530°–550° C 2 kb: 555°–575° C 3 kb: 580°–600° C 5kb: 625°–640° C 6 kb: 620°–650° C 7 kb: 650°–670° C.Comparison with the upper stability limit of muscovite (Velde, 1966) shows that paragonite has a notably lower thermal stability thus explaining the field observation that paragonite is absent in many higher grade metamorphic rocks in which muscovite is still stable.The enthalpy and entropy of the paragonite breakdown reaction have been estimated. Since intermediate albites of varying structural states are in equilibrium with paragonite, corundum and H2O along the univariant equilibrium curve, two sets of data pertaining to the entropy of paragonite (S
298
0
) as well as the enthalpy ( H
f,298
0
) and Gibbs free energy ( G
f,298
0
) of its formation were computed, assuming (1) high albite and (2) low albite as the equilibrium phase. The values are: (1) (2) S
298
0
67.8±3.9 cal deg–1 gfw–1 63.7±3.9 cal deg–1 gfw–1
H
f,298
0
–1417.9±2.7 kcal gfw–1 –1420.2±2.6 kcal gfw–1
G
f,298
0
–1327.4±4.0 kcal gfw–1 –1328.5±4.0 kcal gfw–1.Adapted from a part of the author's Habilitationsschrift accepted by the Ruhr University, Bochum (Chatterjee, 1968). 相似文献
14.
Preiswerkite and Na-(Mg,Fe)-margarite are two unusual micas very rare in Nature. They have been observed together in two
eclogite occurrences (La Compointrie, France; Liset, Norway) as retrogression products in coronae or symplectites around kyanite.
The chemical compositions and some physical properties of these micas are presented. The possible solid solutions and the
conditions of stability are discussed. The preiswerkites display slight solid solution towards phengitic muscovite and Na-phlogopite.
On the other hand, there is negligible solid solution towards more aluminous compositions; AlIV ≤ 4 appears to be a composition limit for natural (K,Na)-micas. The margarites have an unusual Na-(Mg,Fe)-rich composition.
They can be considered as a solid solution of about 2/3 mol% of margarite and 1/3 mol% of the theoretical end-member Na2(Mg,Fe)1AlVI
4[Si4AlIV
4]O20(OH)4 (“Mica L”), with a possible substitution towards paragonite. The Marg2/3 Mica L1/3 composition (i.e. NaCa2(Mg,Fe)0.5 AlVI
6 [Si6AlIV
6]O30(OH)6) might represent a particularly stable crystallographic configuration and could be considered as a true end-member. Many
“sodian” margarites described in the literature are, in fact, complex solid solutions between margarite, paragonite and Marg2/3 Mica L1/3. The rarity of these micas is not related to extreme or unusual P-T conditions. They seem to be related to unusual chemical compositions, appearing in H2O-saturated Na-Al-rich Si-poor systems, principally, if not only, at greenschist- or amphibolite-facies P-T conditions. Moreover, they are subject to crystallographic constraints whereby the high proportion of Al-tetrahedra create
considerable distortion which prevents the entry of K into the interlayer site, thus necessitating Na (preiswerkite or ephesite)
or Ca (margarite or clintonite) instead.
Received: 21 April 1998 / Accepted: 25 January 1999 相似文献
15.
David M. Jenkins 《Contributions to Mineralogy and Petrology》1984,88(4):332-339
Thermodynamic calculations have shown that the dP/dT slope of the reaction 4 margarite+3 quartz5 kyanite +2 zoisite+3 H2O as determined by Storre and Nitsch (1974) is too steep. This reaction has been reinvestigated using synthetic margarite, zoisite, kyanite, and natural quartz in the starting mixtures and using infrared spectroscopy to examine the run products. The experimentally determined dP/dT slope ranges between –2.2 and –17 bars/ K, which is in excellent agreement with predictions based on thermodynamics. An internally consistent set of univariant curves could be fitted to the experimental reversals for the above reaction and for the reactions margarite+ quartz anorthite+kyanite+H2O and 2 zoisite+kyanite +quartz 4 anorthite+H2O investigated by Nitsch et al. (1981) and Goldsmith (1981), respectively. Addition of up to 40 mol % of the component NaAl2(Si3Al) ·O10(OH)2 (paragonite) to margarite will increase the stability of the margarite solid solution plus quartz by 2–3 kbar without significantly affecting the dP/dT slope, making the paragenesis margarite plus quartz a good geobarometer. 相似文献
16.
The trioctahedral mica ephesite, Na(LiAl2) [Al2Si2O10] (OH)2, has a large
-T stability field in the quaternary system NaAlSiO4-LiAlSiO4-Al2O3-H2O. At temperatures below 400–500° C it coexists with diaspore, while at higher temperatures it occurs with corundum, until it decomposes to nepheline +eucryptite+corundum+H2O at 600–800° C (Fig. 1). Nature faithfully reflects these phase relations; ephesite is found to coexist with diaspore or corundum in silicadeficient metamorphosed rocks or in hydrothermally altered nepheline-syenite pegmatite.Thermodynamic analysis of phase relations of ephesite in the silica saturated portion of the quinary system NaAlSiO4-LiAlSiO4-Al2O3-SiO2-H2O shows that the assemblage quartz+ephesite is always metastable with respect to paragonite+spodumene or paragonite+petalite at temperatures down to approximately 300° C (Fig. 3). At lower temperatures, a number of other phases like bikitaite, cookeite, Na-montmorillonite, and analcime are stabilized. Stability and compatibility relations involving these phases are presently not amenable to thermodynamic treatment due to lack of suitable data. Nevertheless, the absence of the assemblage quartz+ephesite in nature seems to vindicate our conclusion that it is metastable down to at least 300° C.The frequently encountered assemblage quartzspodumene (or petalite)-microcline-albite of some lithium pegmatites contains muscovite (±lepidolite), rather than paragonite. The absence of paragonite in such rocks is best explained by the inherent metastability of the phase-pair paragonite+microcline with respect to muscovite+albite. The pegmatite bulk compositions plot in the four-phase field spodumene (petalite)-microcline-muscovite-albite, cutting out paragonite from the observed assemblage Thus, absence of paragonite-spodumene or paragonitepetalite in nature reflects lack of suitable bulk compositions in rocks. 相似文献
17.
Giovanna Giorgetti Peter Tropper Eric J. Essene Donald R. Peacor 《Contributions to Mineralogy and Petrology》2000,138(4):326-336
Coexisting muscovite and paragonite have been observed in an eclogite from the Sesia–Lanzo Zone (Western Alps, Italy). The
P-T conditions of this eclogite reached 570–650 °C and 19–21 kbar and the rocks show several stages of mineral growth during
their retrograde path, ranging from the subsequent lower-P eclogite facies to the blueschist facies and then the greenschist facies. Muscovite and paragonite are very common in these
rocks and show two texturally different occurrences indicating equilibrium and non-equilibrium states between them. In one
mode of occurrence they coexist in equilibrium in the lower-P eclogite facies. In the same rock muscovite ± albite also replaced paragonite during a greenschist-facies overprint, as evidenced
by unique across – (001) layer boundaries. The chemical compositions of the lower-P eclogite-facies micas plot astride the muscovite – paragonite solvus, whereas the compositions of the greenschist-facies
micas lie outside the solvus and indicate disequilibrium. The TEM observations of the textural relations of the greenschist-facies
micas imply structural coherency between paragonite and muscovite along the layers, but there is a sharp discontinuity in
the composition of the octahedral and tetrahedral sheets across the phase boundary. We propose that muscovite formed through
a dissolution and recrystallization process, since no gradual variations toward the muscovite – paragonite interfaces occur
and no intermediate, homogeneous Na-K phase has been observed. Because a solid-state diffusion mechanism is highly unlikely
at these low temperatures (300–500 °C), especially with respect to octahedral and tetrahedral sites, it is assumed that H2O plays an important role in this process. The across-layer boundaries are inferred to be characteristic of such non-equilibrium
replacement processes. The characterization of these intergrowths is crucial to avoiding erroneous assumptions regarding composition
and therefore about the state of equilibrium between both micas, which in turn may lead to misinterpretations of thermometric
results.
Received: 3 February 1999 / Accepted: 19 October 1999 相似文献
18.
A. F. Cooper 《Contributions to Mineralogy and Petrology》1980,75(2):153-164
Chromian kyanites with a maximum content of 2.88 wt.% Cr2O3 occur in metachert and amphibolite from the Southern Alps, New Zealand. The presence of the whiteschist assemblage kyanite-talc, together with kyanite-zoisite assemblages in calc silicate bands imply high pressure metamorphism, with climactic conditions of approximately 10 kb at 650°–700° C. Mylonitization caused by a change to oblique-slip movements on the Alpine Fault is succeeded by retrograde alteration of kyanite-bearing assemblages. Kyanite is pseudomorphed by Cr-margarite-fuchsite-Cr-zoisite assemblages in metachert and by less chromian margarite and zoisite in amphibolite. Contemporaneously hornblende and phlogopite break down to chlorite. Subsequently the metachert pseudomorphs are mantled by muscovite and those in amphibolite by anorthite and chromite. The breakdown of margarite and zoisite to anorthite implies decompression under a low thermal gradient, compatible with almost isothermal uplift on the Alpine Fault. Late stage retrograde products include fibrous kyanite (probably forming by recrystallization of primary alluminosilicate) and scapolite (possibly orginating through interaction of Cl-bearing fluids in a geothermal system).In the Southern Alps there is a significant uplift following the Cretaceous Rangitata Orogeny, probably in the order of 11–15 km. However, the bulk of the uplift, approximately 25 km, took place in the past 10 m.y. during Kaikoura Orogenic uplift on the Alpine Fault. It is during this latest and continuing phase of uplift that the sequence of kyanite alteration reactions occurred. 相似文献
19.
New experimental data are presented at stability conditions of paragenesis in the system K2O-CaO-Al2O3-SiO2-H2O. These results are used to estimate the pressure temperature conditions under which minute inclusions, mostly consisting of zoisite/clinozoisite and muscovite, have crystallized in calcic plagioclases from metatonalites and metadiorites (Hohe Tauern, Austria). In the pressure region 1.5–8 kb the following reactions were observed: zoisite+muscovite+quartz=anorthite+potash feldspar+water (1) grossularite+muscovite+quartz=anorthite+potash feldspar+water (2) zoisite+quartz=anorthite+grossularite+water (3) natural plagioclase with its inclusions (zoisite/clinozoisite and muscovite) (4) =more basic plagioclase without inclusions.In order to determine the curves of reaction (1), (2) and (3), runs were made in hydrothermal bombs using synthetic phases crystallized from gels as starting materials. The reaction curves (1), (2) and (3) intersect at an invariant point at 7.25±0.5 kb and 685±20° C. In runs to define the reaction (4), it could be demonstrated that the inclusion minerals zoisite/ clinozoisite and muscovite became instable at slightly lower temperatures than those occurring in reaction (1). These facts illustrate that the reaction curve (1), found in the pure system, gives possible information about the pressure temperature conditions during the formation of the inclusions. 相似文献
20.
F. J. MARTINEZ J. RECHE M. L. ARBOLEYA M. JULIVERT 《Journal of Metamorphic Geology》2004,22(9):777-792
Andalusite porphyroblasts are totally pseudomorphosed by margarite–paragonite aggregates in aluminous pelites containing the peak mineral assemblage andalusite, chlorite, chloritoid, margarite, paragonite, quartz ± garnet, in a NW Iberia contact area. Equilibria at low P–T are investigated using new KFMASH and (mainly) MnCNKFMASH grids constructed with Thermocalc 3.21. P–T and T–X pseudosections with phase modal volume isopleths are constructed for compositions relatively richer and poorer in andalusite to model the assemblages in an andalusite‐bearing rock that contains a thin andalusite‐rich band (ARB) during retrogression. Their compositions, prior to retrogression, are used in the modelling, and have been retrieved by restoring the pseudomorph‐forming elements into the current‐depleted matrix, except for Al2O3 which is assumed to be immobile. Compositional differences between the thin band and the rest of the rock have not resulted in differences in andalusite porphyroblast retrogression. The absence of chloritoid resorbtion implies either a pressure increase at constant reacting‐system composition, or that its composition changed during retrogression at constant pressure, by becoming enriched in the progressively replaced andalusite porphyroblasts. T–X pseudosections at 1 kbar model this latter process using as end‐members in X, first, the restored original rock and ARB compositions, and, then the same process, taking into account the change in composition of both as retrogression proceeded. The MnNCKFMASH pseudosections of rocks with different Al contents facilitate making further deductions on the rock‐composition control of the resulting assemblages upon retrogression. Andalusite eventually disappears in relatively Al‐poor rocks, resulting, as in this study, in a rock formed by chloritoid–chlorite as the only FM minerals, plus margarite–paragonite pseudomorphs of andalusite. In rocks richer in Al, chlorite would progressively disappear and a kyanite/andalusite–chloritoid assemblage would eventually be stable at retrograde conditions. The Al‐silicate, stable during retrogression in Al‐rich rocks, indicates pressure conditions and hence the tectonic context under which retrogression took place. 相似文献