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1.
Microprobe analyses are performed on micas (biotite, muscovite and phlogopite) and chlorite from 1.9–1.8 Ga acid K- or Na-rich metavolcanics, cordierit-emica schists and manganiferous rocks from the Hästefält area in central Sweden. The results indicate that Fe-rich biotites and muscovites containing 10 to 25% celadonite and/or pyrophyllite are common in the K- and Na-rich metavolcanics. In the cordierite-mica schists the biotites are Mg-rich and the muscovites contain less than 10% celadonite and/or pyrophyllite. The predominant mica in the manganiferous rocks are phlogopite and less frequent rather pure muscovite. The chlorites show a wide range in composition, but principally those occurring in the K- and Na-rich metavolcanics are brunsvigite and diabantite and those in the cordierite-mica schists and the manganiferous rocks are mainly sheridanite and clinochlore. The chlorites of the manganiferous rocks show enrichment in Mn compared to those in other rock types. In general the compositional variations in the micas and less commonly chlorites are strongly controlled by rock type and fluid chemistry, particularly with respect to the ratio of FeO/(FeO+MgO). Estimates of maximum prograde metamorphic temperature, based on phyllosilicates and co-existing cordierite and garnets, indicate a value of up to 500° C.  相似文献   

2.
The Menderes Massif comprises an inner crystalline core with gneissic rocks and an outer surrounding schist belt with predominantly metasedimentary rocks. Both units have a complex metamorphic history including a late Alpine overprint. Temperatures inferred from oxygen isotope compositions of coexisting minerals increase from 420 to 600°C from the rim to the center. More positive '18O values in all minerals from the schist belt may reflect a higher abundance of sedimentary precursor material, whereas biotites and muscovites in core and rim are indistinguishable in hydrogen isotope composition. 'D values of muscovites range from -35 to -60‰, whereas 'D values of biotites range from -65 to -125‰, indicating normal values for muscovite but anomalously negative values for some biotites. For muscovite the trend can be interpreted in terms of increasing loss of water with rising metamorphic temperature. For biotite the 'D values decrease with increasing H2O content and decreasing Na2O+K2O content, which provides evidence for alteration processes or exchange of K and Na with water from interlayers of biotite forming hydro-biotite. The data suggest isotopic resetting of pre-Alpine characteristics during Alpine metamorphism. The hydrogen isotope composition of biotite was later disturbed, probably during extensional neotectonic movements in this region, as this allowed infiltration of and exchange with D-depleted meteoric water; however, the muscovites retained its Alpine characteristics.  相似文献   

3.
Phase relations of biotite and stilpnomelane in the greenschist facies   总被引：1，自引：0，他引：1  
Phase relations of biotite and stilpnomelane and associated silicate minerals have been studied in rocks of the greenschist facies, chiefly from Otago, New Zealand and western Vermont, but also from Scotland, Minnesota-Michigan iron range, and northwest Washington. That stilpnomelane in the greenschicht facies crystallizes initially with nearly all iron in the ferrous state is indicated by chemical analyses, high p-T experiments, and phase relationships. Alteration of stilpnomelane after metamorphism not only oxidizes iron but leaches potassium; corrections for both effects must be made in using analyses of brown stilpnomelane in studies of phase relations. Two discontinuous reactions which produce biotite at the biotite isograd have been identified:
  1. muscovite+stilpnomelane+actinolite→ biotite+chlorite+epidote
  2. chlorite+microcline→ biotite+muscovite. Biotite produced by the first of these reactions has a limited range of variation in Fe/Mg. As grade advances within the biotite zone more magnesian and ferruginous biotites become stable in consequence of the two continuous reactions:
  3. muscovite+actinolite+chlorite→ biotite (Mg-rich)+epidote
  4. muscovite+stilpnomelane→ biotite (Fe-rich)+chlorite.
Stilpnomelane is stable in muscovite-free rocks throughout the biotite zone, and even up to the grade at which hornblende becomes stable. Phengitic muscovite is stable throughout the biotite zone in New Zealand and thus apparently does not contribute to the formation of biotite until a higher grade is reached.  相似文献   

4.
Two types of biotite isograd are defined in the low-grade metamorphism of the Wazuka area, a Ryoke metamorphic terrain in the Kii Peninsula, Japan. The first, BI1, is defined by the reaction of chlorite+K-feldspar= biotite+muscovite+quartz+H2O that took place in psammitic rocks, and the second, BI2, by the continuous reaction between muscovite, chlorite, biotite and quartz in pelitic rocks. The Fe/Mg ratios of the host rocks do not significantly affect the reactions. From the paragenesis of pelitic and psammitic metamorphic rocks, the following mineral zones were established for this low-pressure regional metamorphic terrain: chlorite, transitional, chlorite-biotite, biotite, and sillimanite. The celadonite content of muscovite solid solution in pelitic rocks decreases systematically with the grade of metamorphism from 38% in the chlorite zone to 11% in the biotite zone. Low pressure does not prohibit muscovite from showing the progressive change of composition, if only rocks with appropriate paragenesis are chosen. A qualitative phase diagram of the AKF system relevant to biotite formation suggests that the higher the pressure of metamorphism, the higher the celadonite content of muscovite at BI1, which is confirmed by comparing the muscovites from the Barrovian and Ryoke metamorphism.  相似文献   

5.
The Reading Prong is part of the Precambrian basement whichis discontinuously exposed in the north-central Appalachians.Precambrian rocks in the northeastern Prong consist of diversegneisses which were plastically deformed, metmorphosed to thehornblende granulite facies, and intruded by syntectonic granitesaround 1150 m.y. ago. Sillimanite-garnet-biotite-quartz-two feldspar paragneisses(locally with hornblende or cordierite and without sillimanite)are one of the predominate lithologies in this area. Micro-probeand wet chemical analyses show biotites from these assemblageshave variable Fe/Mg mol. ratios (3.5–0.3) and containappreciable Ti and A1. The biotites are not zoned with respectto Fe or Mg (except for a systematic decrease in the Fe/Mg ratioin rims contiguous with garnet), however Ti is typically depletedin rim compositions with a reciprocal increase of octahedralA1. Garnets are principally solutions of almandine and pyrope(Fe/Mg mol. ratio, 13.1–1.2) with minor grossularite andspessartine components. They are compositionally zoned, withFe concentrated in rims and Mg in cores (particularly wherethey are contiguous with biotite). The analyzed garnets arehomogeneous with respect to Ca and Mn. The most iron-rich biotitesand garnets are found associated with hornblende. The partitioning of Fe and Mg between the interiors of coexistinggarnet and biotite is uniform within each locality sampled.Although the partitioning is variable on a regional scale, thisappears to reflect the non-ideality of the distribution (a resultof the large and variable octahedral Ti and A1 concentrationsin biotite) and not any major, systematic variations in progrademetamorphic conditions. Low temperature alteration of these prograde assemblages issporadically present in paragneisses west of the Hudson Riverand includes associations of: phengitic muscovite, epidote,clinozoisite, chlorite, and secondary calcite. Isotopic agedata (Sutter & Dallmeyer, 1972) suggest slow uplift of thissegment of the Reading Prong, and it appears that these alterationfeatures developed as a result of prolonged cooling followingthe 1150 m.y. metamorphism. In an attempt to re-equilibrateto the lower temperature conditions during uplift, localizedexchange of Fe and Mg occurred between contiguous garnet andbiotite. Alteration is more widespread in the easternmost paragneisseswhere textural, chemical, and isotopic data suggest these secondaryfeatures are, in part, a result of retrograde metamorphism duringone or more Paleozoic orogenic events recorded in the surroundingterrain.  相似文献   

6.
Diffusion parameters for hydrogen diffusion in epidote-group minerals and micas have been measured under hydrothermal conditions, or calculated from existing experimental data, for bulk hydrogen isotope exchange experiments between hydrous minerals and water. Activation energies in the range 14 to 31 kcals/g-atom H are comparable to those derived by application of kinetic theory to experimental hydrogen isotope exchange data, and to those for oxygen diffusion in minerals under hydrothermal conditions. Diffusion of hydrogen in epidote is about four orders of magnitude faster than in muscovite, and about two orders of magnitude faster than in zoisite. Hydrogen diffusion in micas is about five orders of magnitude faster than oxygen diffusion, and hydrogen transport occurs dominantly parallel to the layers rather than parallel to the c-axis as for oxygen.Rapid hydrogen transport in minerals may proceed by hydrolysis of Si-O and Al-O bonds, followed by exchange of hydrolyzed oxygens with slower-diffusing (OH) or H2O. Water appears to be essential for stable isotope exchange between minerals in slowly cooling metamorphic rocks.Stable isotope data for regional metamorphic mineral assemblages suggests that water is usually present in small amounts during cooling of prograde regional metamorphic systems, and estimated closure temperatures for cessation of stable isotope exchange are often more comparable to those calculated from diffusion data than to likely temperatures of metamorphism.Alpine deformation of the Hercynian Monte Rose Granite (Frey et al. 1976) permitted access of water and initiated stable isotope exchange amongst coexisting minerals. The diffusional behaviour of species in relict Hercynian muscovites is consistent with available experimental diffusion data.  相似文献   

7.
Some 150 white K-micas from the Central Alps were analysed for their polymorph and phengite content. Pre-Alpine white K-micas and those belonging to the Meso-Alpine Lepontine Metamorphic “High” show exclusively the 2M1 polymorph. The 3T structural form, on the other hand, has been found in one third of the white K-micas formed during the Alpine regional metamorphism. In most cases this trigonal structure coexists with varying amounts of the 2M1 form. The 3T distribution pattern suggests that this polymorph originated during the Eo-Alpine high-pressure/“low temperature” metamorphism. Provided this interpretation is correct, the sporadic occurrence of this polymorph within the Meso-Alpine staurolite zone may be used as a tracer for the Eo-Alpine metamorphism. The following improved correlation between the (060, 331) reflections of 2M1 white K-micas and the RM-content (= 2Fe2O3+FeO+MgO in molar proportions), based on 24 micas from granitoid rocks, is presented: d(060, 331)= 1.498+0.082 RM. The phengite content of Alpine white K-micas belonging to the assemblage muscovite-biotite-K-feldspar-quartz was estimated from RM values or derived from chemical analyses and was found to be clearly related to metamorphic grade. Phengite-rich micas were formed during the Eo-Alpine high-P/“ low-T” metamorphism while aluminous muscovite was found within the Meso-Alpine thermal high of the Lepontine gneiss area. White K-micas from areas which underwent both the Eo-Alpine and the Meso-Alpine metamorphism display variable phengite contents. Although these micas show Tertiary Rb-Sr and K-Ar ages, the variable phengite content presumably reflects conditions during the Eo-Alpine high-P/“low-T” metamorphism. This interpretation implies that the cations occupying the interlayer positions are more easily equilibrated than those in octahedral and tetrahedral structural sites. A compilation of 3T white K-mica occurrences described in the literature is given in the appendix.  相似文献   

8.
Nine samples from the Monte Rosa Granite have been investigated by microscopic, X-ray, wet chemical, electron microprobe, stable isotope and Rb-Sr and K-Ar methods. Two mineral assemblages have been distinguished by optical methods and dated as Permian and mid-Tertiary by means of Rb-Sr age determinations. The Permian assemblage comprises quartz, orthoclase, oligoclase, biotite, and muscovite whereas the Alpine assemblage comprises quartz, microcline, albite+epidote or oligoclase, biotite, and phengite. Disequilibrium between the Permian and Alpine mineral assemblages is documented by the following facts: (i) Two texturally distinguishable generations of white K-mica are 2 M muscovite (Si=3.1–3.2) and 2 M or 3 T phengite (Si=3.3–3.4). Five muscovites show Permian Rb-Sr ages and oxygen isotope fractionations indicating temperatures between 520 and 560 ° C; however, K-Ar ages are mixed or rejuvenated. Phengite always shows mid-Tertiary Rb-Sr ages, (ii) Two biotite generations can be recognized, although textural evidence is often ambiguous. Three out of four texturally old biotites show mid-Tertiary Rb-Sr cooling ages while the oxygen isotopic fractionations point to Permian, mixed or Alpine temperatures, (iii) Comparison of radiogenic and stable isotope relations indicates that the radiogenic isotopes in the interlayer positions of the micas were mobilized during Alpine time without recrystallization, that is, without breaking Al-O or Si-O bonds. High Ti contents in young muscovites and biotites also indicate that the octahedral (and tetrahedral) sites remained undisturbed during rejuvenation. (iv) Isotopic reversals in the order of O18 enrichment between K-feldspar and albite exist. Arguments for equilibrium during Permian time are meagre because of Alpine overprinting effects. Texturally old muscovites show high temperatures and Permian Rb-Sr ages in concordancy with Rb-Sr whole rock ages. For the tectonically least affected samples, excellent concordance between quartz-muscovite and quartz-biotite Permian temperatures implies oxygen isotope equilibrium in Permian time which was undisturbed during Alpine metamorphism. Arguments for equilibrium during the mid-Tertiary metamorphism are as follows: (i) Mid-Tertiary Rb-Sr mineral isochrons of up to six minerals exist, (ii) Oxygen isotope temperatures of coexisting Alpine phengites and biotites are concordant.The major factor for the adjustment of the Permian assemblages to Alpine conditions was the degree of Alpine tectonic overprinting rather than the maximum temperatures reached during the mid-Tertiary Alpine metamorphism. The lack of exchange with externally introduced fluid phases in the samples least affected by tectonism indicates that the Monte Rosa Granite stewed in its own juices. This seems to be the major cause for the persistence of Permian ages and corresponding temperatures.  相似文献   

9.
In 61 pairs of coexisting biotites and muscovites from the Central Alps total Al scatters considerably, but in both series a gradual increase is noticed with increasing metamorphic grade. The ratio Al Mu tot /Al Bi tot remains virtually constant (1.61 average for greenschist facies, 1.57 for amphibolite facies). Tetrahedral Al varies little in biotites and increases in muscovites-phengites with rising metamorphic grade; accordingly the ratio Al Mu IV /Al Bi IV increases slightly with grade. Far the best control of metamorphism is evidenced by octahedral Al. In the muscovite series, and still more pronounced in the biotite series, AlVI increases with increasing metamorphic grade. Consequently 1 $$K_D = \frac{{Al_{Mu}^{VI} }}{{Al_{Bl}^{VI} }}$$ decreases from 14 to 3. A map (Fig. 6) representing the regional distribution of the KD values locates a 100 km long and 23 km broad central zone with low KD. The outline of this central core almost coincides with the isograds anorthite-diopside-calcite and labradorite-pyroxene-hornblende of the Tertiary regional metamorphism; with some deviations this core also agrees with the zone in which phenomena of partial anatexis are observed. The KD values of micas from anateotic pegmatites agree with those of associated gneisses and schists. The study demonstrates that in the course of progressive regional metamorphism equilibrium has been approached to an unexpected extent and that the two micas coexisted in a strict sense.  相似文献   

10.
RAMSAY  C. R. 《Journal of Petrology》1973,14(3):467-488
Archaean meta-sediments near Yellowknife, Canada, exhibit alow-pressure facies series and broad metamorphic zonation rounda central pluton. Meta-pelites and meta-greywackes from thebiotite zone have been studied using analyses of 59 mineralsand 14 rocks. Mineral compositions were controlled by both host-rockcomposition and metamorphic grade. Increased grade commonly imposed on the minerals a progressivecompositional maturation. This involved progressive compositionalchange (especially in meta-greywacke biotites) and/or narrowingof compositional range (particularly in muscovites). Specificeffects of increased grade are as follows. Biotites in meta-greywackesexhibit increased Mg/Fe and Na/K but decreased (Na+K) content.Biotites in meta-pelites change little except for increasedAlIV/Si. The muscovites display decreasing maximum Si contents,increased AlVI at the expense of Fe and Mg, and increased Na/K.The chlorites show only a slight general increase in Mg/Fe. Bulk compositional control is manifested in various ways. Insome instances minerals from subtly different rock types areperceptibly distinct; in others the pro-grade maturation trendsof minerals in these rock types are different. Thus Si is moreabundant in meta-greywacke biotites than in meta-pelite biotitesand (Na+Ba+K) content of muscovites is greater in meta-greywackes. The observed features are used to deduce mechanisms of controlby grade and bulk composition and to discuss general circumstancesunder which they operate.  相似文献   

11.
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   

12.
Overprinting of white micas from high pressure, low to medium temperature (M 1) metamorphic assemblages in pelitic schists on Naxos during subsequent thermal dome (M 2) metamorphism ranges from minor in the southeast of the island to complete recrystallization in the amphibolite facies rocks near the migmatites in the centre of the dome. The original (M 1) minerals are phengites (Si4+=6.7–7.0) and the overprinting minerals are muscovites (Si4+=6.0–6.45). 40Ar/39Ar step heating analyses of white mica separates from rocks in the area where phengite and muscovite occur together yield complex age spectra, characterized by low apparent ages in the first and the last stages of gas release and high apparent ages in between. These upward-convex age spectra are shown to be caused by mixing of two generations of micas, each of which has a different age spectrum and argon release pattern. Seemingly good plateaus in some age spectra from white micas of the area must be interpreted as providing meaningless intermediate ages. Further, the upward-convex age spectra have been used to trace the isotopic signature of phengites toward increasing M 2 metamorphic grade, and suggest that as long as phengites can be observed in the rocks upward-convex age spectra occur. On Naxos, crystallization of muscovite at the expense of phengite appears to be the main mechanism of resetting argon isotopic ages in white micas. However, there is also good evidence for argon loss by volume diffusion from phengites. Simple diffusion calculations suggest that the M 2 metamorphism was caused by a shortlived heat source.Now at Department of Geology, University of Alberta, Edmonton T6G 2E3, Canada  相似文献   

13.
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   

14.
Résumé On a étudié statistiquement les variations de (OH+F) dans 392 micas (muscovites, biotites, micas lithiques). Dans les muscovites et les micas lithiques alumineux l'écart du nombre de (OH+F) à la valeur théorique apparait dû à des erreurs de dosage de H2O+. Dans les micas lithiques ferreux on observe une moyenne de 4,62 (OH+F)/maille. Cet excès en (OH+F), exprimé en oxonium H3O+, correspond au déficit en cations XII. Dans les micas ferromagnésiens (biotites, phlogopites) on observe un déficit en (OH+F) (x = 3,59). On rend compte de ce déficit en supposant que Fe3+ présent dans la structure est dû à une oxydation in situ par déprotonation.Les conséquences quant aux erreurs introduites sur le nombre de cations lors du calcul de la formule structurale sont discutées.
Statistical study of (OH+F) content of muscovites, biotites, and lithium micas
Summary A statistical study of the variations of (OH+F) content of 392 micas (muscovites, biotites, lithium micas) has been undertaken. In muscovites and lithium-aluminium micas, the fluctuation of (OH+F) content can be explain by analytical errors in determination of H2O+ and F. In ferrous lithium micas the mean content of (OH+F) is 4.62 at. per unit cell. Converted to oxonium H3O+, this excess is equal to the K+ deficiency. Biotites and phlogopites are hydroxyl deficient (x = 3,59). This can be explained if it is assumed that all Fe3+ present is due to auto-oxydation of Fe2+ with loss of H+ to maintain charge neutrality.A discussion of the cationic variations introduced in the calculation of the structural formulae is given.

Avec 5 figures  相似文献   

15.
The Lesser Himalayan low- to medium-grade metamorphic rocks in central Nepal are rich in K-white micas occurring as porphyroclasts and in matrix defining S1 and S2. Porphyroclasts are usually zoned with celadonite-poor cores and celadonite-rich rims. The cores are the relics of igneous or high grade metamorphic muscovites, and the rims were re-equilibrated or overgrown under lower T metamorphic conditions. The matrix K-white micas defining S1, pre-dating the Main Central Thrust activity, are generally celadonite-rich. They show heterogeneous compositional zoning with celadonite-rich cores and celadonite-poor rims. They were recrystallized at lower T condition prior to the Main Central Thrust activity, most probably prior to the India–Asia collision (pre-Himalayan metamorphism). The matrix K-white micas along S2, synchronous to the Main Central Thrust activity (Neohimalayan metamorphism), are relatively celadonite-poor and were recrystallized under relatively higher T condition. K-white micas defining S1 also were partially re-equilibrated during the Neohimalayan metamorphism. The average compositions of recrystallized K-white micas defining both S1 and S2 become gradually poor in (Fe + Mg)- and Si-contents and rich in Al- and Ti-contents from south to north showing an increase of metamorphic grade from structurally lower to higher parts in the Lesser Himalaya. This shows that the metamorphism is inverted throughout the inner Lesser Himalaya. The tectono-metamorphic significance of the published K–Ar and 40Ar / 39Ar K-white micas ages from the Lesser Himalaya need re-evaluation in the context of observed intrasample compositional variation and zoning, and possible higher closure temperature (500 °C) for K–Ar system.  相似文献   

16.
The Crossite Content of Ca-Amphibole as a Guide to Pressure of Metamorphism   总被引：10，自引：0，他引：10  
A correlation between the crossite component (NaM4) in Ca-amphiboleand pressure of metamorphism has long been recognized (Shido& Miyashiro, 1959), but only recently has the reaction beenidentified which buffers this aspect of amphibole composition(Brown, 1974): Ca-amphibole+iron oxide+albite+chloriteI+H2O (±stilp,qtz) = crossite+epidote (±muscovite, qtz). The exact stoichiometry of the reaction depends on compositionalvariables in the minerals, especially Fe2+/Mg and Fe3+/Al. Ca-amphiboleshould have fixed NaM4, at any given T and P, where it coexistswith iron oxide, albite, and chlorite. Comparison of Ca-amphibole composition with mineral assemblage,in rocks from Otago, N.Z., and elsewhere, supports this hypothesis.In any terrane NaM4 is nearly constant at a particular metamorphicgrade where amphibole exists in the buffering assemblage, butvaries widely outside of this assemblage. Variations in Fe2+/Mgand Fe3+/Al in the amphibole have relatively little effect onNaM4, but in high pressure amphiboles NaM4 varies inverselywith Aliv. Ca-amphiboles from high pressure areas have substantially moreNaM4 (Otago, 0.6 of 2.0) than those from lower pressure areas(Sierra contact aureoles, 0.1). These relations suggest thatin the buffering assemblage, the NaM4 content of Ca-amphiboleshould be a useful relative barometer for low to medium grademetamorphic rocks.  相似文献   

17.
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.  相似文献   

18.
Complete chemical analyses, structural formulae and trace elementdata are presented for 9 hornblendes and 12 biotites from thecharnockitic and associated rock types of Kondapalli (AndhraPradesh, India), some 200 miles north of the type area nearMadras. The chemical characteristics of the hornblendes are:very high [Al]4, nearly constant Ca, very high and nearly constanttotal alkalis (Na2O > K2O in the hornblendes of ultrabasiccharnockites and Na2O < K2O in those of basic charnockites),very high F and Cl, low total water, and Ni in amount greaterthan Co. The biotites are characterized by high Ti and F, lowMn, total absence of Ca, exceptionally high amounts of Cl, lowtotal water, and have Ni > Co; the biotites from differentgroups of host rocks have different Al and Fe3+ contents. Thevariations in composition of these minerals are interpretedas due to the change in bulk rock chemistry and mineralogy andnot to the change in temperatures of crystallization. The element distributional relationships in hornblende-biotite,hornblende-orthopyroxene, hornblende-clinopyroxene, biotite-orthopyroxene,and biotite-clinopyroxene pairs from the Kondapalli charnockitesare evaluated, with particular attention to the partition ofFe2+, Mg, Mn, Ti, Cr, Ni, Co, and V, and comparisons are madewith similar data available for corresponding mineral pairs.Highly consistent and uniform distribution coefficients forFe2+ and Mg (and a small range of distribution coefficientsfor many of the trace elements) are obtained for most of thedifferent sets of mineral pairs from Kondapalli, suggestingthat equilibrium distribution of the elements between the phaseshad been attained.  相似文献   

19.
The compositions of biotite and muscovite were examined in terms of the paragenesis and the metamorphic grade in low- to medium-grade pelitic rocks of the Ryoke metamorphism in the Yanai district, southwest Japan. The biotite and muscovite that coexist with K-feldspar have a higher K component in an A'KF diagram than those in rocks lacking K-feldspar. This fact reflects an increase in the K2O content in muscovite, but in biotite it reflects an increase of not only the K2O content but also of the octahedral vacancy.
At higher metamorphic grade beyond the cordierite isograd, where cordierite coexists with neither chlorite nor K-feldspar, the biotite shows an increase in illite, K Aliv □xii−1 Si−1, and Tschermak components, Alvi Aliv R+−1 Si−1, where □xii and R+ denote the interlayer vacancy and (Fe+Mg+Mn), respectively. A reaction to define the cordierite isograd is proposed by treating this chemical change as being responsible for the first appearance of cordierite, i.e. K,Al-poor biotite+phengitic muscovite=K,Al-rich biotite+cordierite+quartz+water .By treating this as a key reaction in medium-grade metamorphism, a set of reaction in a progressive metamorphism is established for the Ryoke metamorphism, a typical low-pressure type metamorphism. Some textures in one of the high-grade areas, the K-feldspar-cordierite zone, suggest that a further two prograde reactions have taken place, i.e. andalusite+biotite+quartz=cordierite+K-feldspar+water
and   andalusite=sillimanite.quartz=cordierite+K-feldspar+water
This implies that this zone probably has a P–T  path involving isobaric heating.  相似文献   

20.
Micas from magmatic and metamorphic rocks differ from one another in chemical composition and in trace element content. The chemical composition of micas is discussed in relation to their occurrence, paragenesis and sequence of crystallization. On the basis of previous studies of the relationship between the physical properties and the chemical composition of 34 chemically analysed micas, reliable physical methods have been established which permit identification of different mica varieties in the same rock. Structural formulae and trace element content of micas from basic and granitic rocks, as well as from skarns, schists, ortho- and paragneisses are discussed. The relationship between the components of the tetrahedral and octahedral layers and of the interlayer are illustrated as ratios. Poorly differentiated, hybrid and metasomatic rocks often contain more than one variety of mica. Some prophyritic basalts and lamprophyres contain an early phlogopite which is paragenetically related to pyroxene phenocrysts and late biotite which occurs in the groundmass and in the fractures as a result of the crystallization of residual magma. The biotitemuscovite assemblage was observed in granodiorites, quartz-monzonites, schists and gneisses. In the albite-K-feldspar granites, muscovite predominates and the biotite is usually altered. The chemical composition of micas from metamorphic rocks depends on the grade of metamorphism and on the nature of associated minerals. The biotite from paragneisses contains considerable quantities of octahedral alumina. Pre-metamorphic micas show variable deficiencies of the (OH, F) group. The micas are useful minerals in determining the degree of differentiation and subsequent alteration of igneous rocks. The present study was carried out on the basis of 34 recent complete chemical analyses andca 100 X-ray fluorescence analyses. Dedicated to Professor Dr.Carl W. Correns on the occasion of his 70th birthday.  相似文献   

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