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1.
Abstract. Pink piemontite-spessartine-bearing and grey-green spessartine-bearing manganiferous quartzose schists derived from siliceous pelagites, and green quartzofeldspathic schists, are described from the greenschist facies of the Haast Schist terrane, near Arrow Junction, western Otago. Electron microprobe data are reported for sphene, spessartine-rich garnet, manganoan epidote, piemontite, tourmaline, phengitic muscovite, chlorite, albite, haematite, rutile, manganoan calcite and chalcopyrite. Metamorphism occurred at about 6.4kbar, 400°C. Xco2 was above the quartz-rutile-calcite-sphene buffer (Xco2± 0.02) throughout the recorded metamorphic history of the piemontite schists. It dropped from above to below this critical buffering value in a spessartine-rich schist and it was close to or below the buffering value in the quartzofeldspathic schists. Production of piemontite required high fO2, believed to be inherited from MnOx in the parent pelagite. Substantial loss of O2 (e.g. minimum of 0.19% by weight in one rock) during diagenesis and/or metamorphism is inferred. In the grey-green schists this inhibited piemontite formation. Slight loss of O2 and Ca2+ accompanied minor late-stage replacement of piemontite by second generation spessartine. Observed zoning and mineral replacements indicate rise of temperature, drop in pressure, or invasion by solutions of lower fO2 and XCO2 equilibrated with surrounding schists. The detailed chemistry of the minerals studied correlates with available Mn and with bulk-rock (Fe3+ x 100)/(Fe2++ Fe3+). The oxidation ratio ranges from 24 in average green quartzofeldspathic schist, through 78 in average grey-green manganiferous quartzose schist, to almost 100 in some piemontite-bearing schists. As Fe2+ gives way to Fe3+, Mg/Fe ratios tend to rise in chlorite, phengite, tourmaline, spessartine, and calcite, Mn increases and Ti decreases in haematite, Mn increases in spessartine and calcite, and Fe increases in rutile. Available divalent cations are depleted relative to Al; chlorite is more aluminous, and phengite more paragonitic than in typical Haast schists.  相似文献   

2.
Equilibria between different valence states of Fe and Mn have been studied in a microcline-plagioclase-quartz gneiss which locally contains ferromagnesian minerals unusually high in Mn+3 and Fe+3 and low in Fe+2. The compositions of coexistent minerals have been determined by chemical and microprobe analyses. The minerals in some layers were formed under highly-oxidizing conditions, as indicated by extremely low Fe+2/Fe+3 ratios in the silicates, by the presence of hematite, and by the occurrence of piemontite, which requires Mn+3 for its formation. The minerals in other layers were formed under less-oxidizing conditions, as indicated by the fact that epidote, rather than piemontite, crystallized with Mn-rich garnet and by the presence of biotite rather than phlogopite. In the less-oxidized layers Mn+3 appears to be absent. The differences in oxidation of Fe and Mn occur between adjacent layers and probably reflect sedimentary differences preserved despite the metamorphism.Iron and manganese with different valences are sharply partitioned between the coexisting phases. In highly-oxidized layers, muscovite contains more iron (as Fe+3) than coexistent phlogopite; in piemontite most of the manganese is Mn+3, while in coexistent garnet most of the manganese is Mn+2. In less-oxidized layers, epidote contains no Mn+3 and contains less Mn+2 than coexistent garnet, biotite, or amphibole. Analytical data, crystal-chemical arguments, and characteristics of Fe and Mn L-spectra indicate that in coexistent garnet and piemontite, Fe+2, Fe+3, Mn+2, and Mn+3 are present, in spite of the fact that trivalent manganese strongly oxidizes divalent iron in aqueous solution under normal conditions.Contribution No. 1468.  相似文献   

3.
Despite having been a target for volcanic-hosted massive sulfide (VHMS) deposits since the 1960s, few resources have been defined in the Archean Yilgarn Craton of Western Australia. Exploration challenges associated with regolith and deep cover exacerbate the already-difficult task of exploring for small, deformed deposits in stratigraphically complex, metamorphosed volcanic terranes. We present results of drill-core logging, petrography, whole-rock geochemistry and portable X-ray Fluorescence data from the King Zn deposit, to help refine mineralogical and geochemical halos associated with VHMS mineralisation in amphibolite-facies greenstone sequences of the Yilgarn Craton. The King Zn deposit (2.15?Mt at 3.47?wt% Zn) occurs as a 1–7 m-thick stratiform lens dominated by iron sulfides, in an overturned, metamorphosed volcanic rock-dominated sequence located ～140?km east of Kalgoorlie. The local stratigraphy is characterised by garnet-amphibolite and strongly banded intermediate to felsic schists, with rare horizons of graphitic schist and talc schist. Massive sulfide mineralisation is characterised by stratiform pyrite–pyrrhotite–sphalerite at the contact between quartz–muscovite schists (‘the footwall dacite’), and banded quartz–biotite and amphibole?±?garnet schists of the stratigraphic hanging-wall. A zone of pyrite–(sphalerite) and pyrrhotite–pyrite–(chalcopyrite) veining extends throughout the stratigraphic footwall. Footwall garnet-amphibolites are of sub-alkaline basaltic affinity, with a central zone dominated by chlorite?±?magnetite interpreted to represent the Cu-bearing feeder zone. SiO2, CaO, Fe2O3T, MgO and Cu concentrations are highly variable, reflecting quartz–epidote?±?chlorite?±?magnetite?±?sulfide alteration. Hydrothermal alteration in stratigraphically overlying intermediate to felsic rocks is characterised by a mineral assemblage of quartz–muscovite?±?chlorite?±?albite?±?carbonate. Cordierite and anthophyllite are locally significant and indicative of zones of Mg-metasomatism prior to metamorphism. Increases in SiO2, Fe2O3T, pathfinder elements (e.g. As, Sb, Tl), and depletions of Na2O, CaO, Sr and MgO occur in quartz–muscovite schists approaching massive sulfide mineralisation. Within all strata (including the immediate hanging-wall), the following pathfinder elements are strongly correlated with Zn: Ag, As, Au, Bi, Cd, Eu/Eu*, Hg, In, Ni, Pb, Sb, Se and Tl. These geochemical halos resemble less metamorphosed VHMS deposits across the Yilgarn Craton and suggest that although metamorphism leads to element mobility and mineral segregation at the thin-section scale, assay samples of ～20?cm length are sufficient to vector to mineralisation in amphibolite facies greenstone belts. Recognition of minerals such as Mg-chlorite, muscovite, cordierite, anthophyllite, biotite/phlogopite, and abundant garnet are significant, in addition to Al-rich phases (i.e. kyanite, sillimanite, andalusite and/or staurolite) not identified at King. Chemographic diagrams may be used to identify and distinguish different alteration trends, along with several alteration indices (e.g. Alteration Index, Carbonate–Chlorite–Pyrite Index, Silicification Index) and the abundance of normative corundum and quartz.  相似文献   

4.
《International Geology Review》2012,54(11):1067-1077
Mineral chemistry and typomorphic characteristics are used to monitor the physicochemical evolution of late-magmatic to postmagmatic alteration processes that resulted in the development of a radioactive and albite-enriched microgranite stock. The mineral paragenesis indicates that postmagmatic fluids were enriched in Nb, Zn, Mn, U., Th, Zr, and Y, in addition to Rb, Li, and F Manganocolumbite with extremely high Nb/(Nb+Ta) (0.99), Mn/(Mn+Fe) (0.82), and zircon with high Zr/(Zr+Hf) (0.97) indicate crystallization under alkaline, relatively high-temperature conditions (>425° C). The close association of manganocolumbite, Nb-Mn-Zn- rich ilmenite (with 1.2 to 14.5 wt% ZnO), spessartine garnet (with 68.2-89.4 mol% spessartine), zircon, xenotime, zinnwaldite mica (up to 5.98 wt% F), and fluorite indicates the strong affinity of the elements of Nb, Y., Zr, Mn, and Zn for stable complexing by K+, Na+, Li+, and F? rich supercritical fluids during the course of extraction and transportation.

The enrichment of the interacting fluid in U and Th is depicted by the presence of up to 1.6% UO2 in manganocolumbite and Hf-bearing zircon, and up to 10.5% ThO2 in monazite, in addition to locally abundant thorite and uranophane. It is suggested that the uranium mineralization, mainly as fracture fillings, formed during the waning stage of hydrothermal activity.  相似文献   

5.
 Cation partitioning data for coexisting muscovite and biotite are shown to be useful indicators of relative interlayer bond length/strength in these minerals. These data therefore provide a useful crystal-chemical perspective on relative mass-transfer kinetics of radiogenic isotopes, and account for the observation that biotite is generally less retentive of 40Ar and 87Sr than coexisting muscovite. Partitioning behavior of trace elements underscores three reasons why overall interlayer bonding in biotite is weaker than in muscovite. First, the preferences of large (Rb, Cs)+ in biotite and of small La3+ and Na+ in muscovite indicate a relatively spacious interlayer volume in biotite (suggesting a longer mean K−O bond). Second, the preference of interlayer vacancies in biotite (with some/all possibly H2O/H3O+-filled) suggests that its adjacent 2:1 sheets are connected by fewer interlayer bonds per unit cell than those of muscovite. Third, the relative exclusion of large Ba2+ from biotite despite its large interlayer sites is attributed to O−H bonds pointing into the interlayer cavity sub-normal to (001); (K+, Ba2+)-H+ repulsion thereby induced by the bare proton both destabilizes Ba2+ and weakens K−O bonds. In contrast, muscovite offers a more favorable electrostatic environment for Ba2+ substitution since its O−H bonds are directed into the vacant M 1 octahedral site sub-parallel to (001). This hypothesis is supported by the observation that progressive F(OH)−1 exchange enhances Ba2+ partitioning into biotite/phlogopite relative to coexisting muscovite. These crystal-chemical differences between biotite and muscovite are mirrored in calculated values of “ionic porosity”, Z i , defined here as the percentage of their interlayer unit-cell volume not occupied by ions. A monitor of ionic packing density and geometry, Z i is inversely correlated with K−O bond strength, which appears to be the rate-determining “kinetic common denominator” for a variety of processes affecting micas – including those responsible for loss of radiogenic isotopes in biotite and muscovite. Accordingly, the relatively longer/weaker K−O bonds of biotite are envisioned as being more easily stretched (during volume diffusion) or broken (during recrystallization or retrograde alteration). This in turn accounts for common observations of enhanced radiogenic Ar/Sr loss and younger 40Ar/39Ar and Rb/Sr ages in natural biotite (high Z i ) relative to coexisting muscovite (lower Z i ). Significantly, this pattern may arise irrespective of isotopic loss mechanism (diffusion or recrystallization, etc.), and it follows that any age discordance observed between muscovite and biotite cannot be ascribed uniquely to one mechanism or the other without appropriate field, petrographic, and petrologic constraints. Extension of this partitioning/porosity-based synthesis leads to prediction of corollary age-retentivity-composition effects among chemically diverse trioctahedral and dioctahedral micas, which are best field tested in terranes that cooled slowly under dry, static conditions. Pressure effects on argon retention are also inferred from the porosity model. Received: 9 February 1995 / Accepted: 8 September 1995  相似文献   

6.
Loss of metals from pelites during regional metamorphism   总被引：3，自引：0，他引：3  
In aluminous metapelites the ratio H2O+/K2O decreases with increasing metamorphic grade and degree of reaction. This ratio is a very practical indicator for the progress of the mineral reconstitution during progressive metamorphism. With decreasing values of the ratio H2O+/ K2O the Cu concentration and the following element ratios also decrease either continuously or in stepwise fashion: Tl/K2O, Ba/K2O, Pb/K2O, Bi/K2O, Hg/K2O, Sr/Na2O, Zn/(Fe2++Mg), Cd/(Fe2++Mg); Rb/K2O remains approximately constant. In the aluminous metapelites of the Damara Orogen in Namibia the following losses occur between the biotite isograd and anatexis: 61% Cu, 20% Tl, 34% Ba, 59% Pb, 86% Bi, 46% Hg, 30% Sr, 25% Zn, 31% Cd. Thus the potential of regional metamorphism to form hydrothermal deposits in the low grade environment should not be neglected.  相似文献   

7.
Prograde mineral assemblages and compositions have been predicted for pelitic schist in the 10 component system Na2O–K2O–CaO–MnO–FeO–MgO–Al2O3–SiO2–CO2–H2O for three cases of prograde metamorphism and fluid-rock interaction: (1) increasing temperature (T) at constant pressure (P) and constant pore fluid volume (1%) without infiltration (no-infiltration case); (2) increasing T at constant P accompanied by sufficient fluid infiltration that fluid composition is at all times constant (large-flux case); and (3) increasing T at constantP accompanied by a timeintegrated fluid flux f 104 cm3 cm 2 (intermediate-flux case). Stable mineral assemblages and compositions were calculated by solving a system of non-linear equations that specify mass balance and chemical equilibrium between minerals and fluid. The model pelitic system includes quartz, muscovite, plagioclasc, chlorite, ankerite, siderite, biotite, garnet, staurolite, andalusite, kyanite, sillimanite, K-feldspar, and a coexisting, binary H2O–CO2 fluid. Specifically, prograde thermal metamorphism was modelled for Shaw's (1956) average low-grade pelite and for a moderate range of bulk rock compositions at P=3, 5, and 7 kb and initial fluids with Xco 2 o =0.02–0.40. The model predicts a carbonate-bearing mineral assemblage for average pelite under chlorite zone conditions composed of quartz, muscovite, albite, chlorite, ankerite, and siderite. The mineral assemblages predicted for the noinfiltration case are unlike those typically observed in regional metamorphic terranes. Simulations of metamorphism for the large-flux and intermediate-flux cases, however, reproduce the sequence of mineral assemblages observed in normal Barrovian regional metamorphic terranes. These results suggest that regional metamorphism of pelitic schists is typically associated with infiltration of significant quantities of aqueous fluid.  相似文献   

8.
Problems related to the formation of chloritoid in metapelites, associated with lawsonite-glaucophane bearing metabasalts, in the quartzitephyllite series of western Crete (Greece) are discussed. It is supposed that chloritoid was formed, during prograde metamorphism, according to a gliding-equilibrium reaction of the type (Fe,Mg)-carpholite1+chlorite1 (Fe,Mg)-carpholite1 2+(Fe,Mg)-chloritoid1 2 +chlorite1→2+quartz+H2O ? (Fe,Mg)-chloritoid2+chlorite2+quartz+H2O. This view is stipulated by the occurrence of ferrocarpholite-chloritoid schists in the southeastern part of central Crete. The assemblage chloritoid+ lawsonite recently recognized in western Crete provides evidence that the formation of chloritoid started well within the stability field of lawsonite.  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.
Viridine containing the highest amounts of Mn2O3 detected thus far (up to 20.5 mol % “Mn2SiO5”) coexists in a metasedimentary hornfels with spessartine, Mn-phlogopite (mangan-ophyllite), Mn-phengite (alurgite), hematite, quartz and probably some primary braunite. In layers poorer in viridine spessartine is absent but piemontite appears as an additional phase. Microprobe analyses of all these phases are presented which indicate very strong fractionation of Mg and Mn in coexisting phlogopite and garnet, and of Fe and Mn in coexisting hematite and braunite. Sericitic aggregates consisting of phengitic muscovite and braunite are interpreted as retrograde alteration products of viridine, but might partly be pinitic alterations of a former Mg-rich cordierite. Due to the occurrence of the assemblage spessartine-viridine-quartz Mn-cordierite cannot have been a stable phase prior to retrograde alterations. In general the stability field of viridine is extended towards higher temperatures as compared to that of pure andalusite, Al2SiO5. Due to the coexistence of phlogopite and muscovite (phengite) the temperature of contact metamorphism cannot have exceeded some 550°–650° C depending on the prevailing water pressure.  相似文献   

11.
The distribution and dynamics of water molecules and monovalent cations (Li+, Na+, K+, Cs+, and H3O+) on muscovite surfaces were investigated by molecular dynamics (MD) simulations. The direct comparison of calculated X-ray reflectivity profiles and electron density profiles with experiments revealed the precise structure at the aqueous monovalent electrolyte solutions/muscovite interface. To explain the experimentally observed electron density profiles for the CsCl solution-muscovite interface, the co-adsorption of Cs+ and Cl− ion pairs would be necessary. Two types of inner-sphere complexes and one type of outer-sphere complex were observed for hydrated Li+ ions near the muscovite surface. For Na+, K+, Cs+, and H3O+ ions, the inner-sphere complexes were stable on the muscovite surface. The density oscillation of water molecules was observed to approximately 1.5 nm from the muscovite surface. The number of peaks and the locations for the density of water oxygen atoms were almost similar among the water molecules coordinated to Li+, Na+, K+, and H3O+ ions adsorbed on the muscovite surfaces. The water molecules around Cs+ ions that were adsorbed to muscovite surfaces seemed to avoid coordinating with Cs+ ions on the surface, and the density of water oxygen near the muscovite surface decreased relative to that in a bulk state. There was no significant difference in self-diffusion, viscosity, retention time, and reorientation time of water molecules among different cations adsorbed to muscovite surfaces. These translational and rotational motions of water molecules located at less than 1 nm from the muscovite surfaces were slower than those in a bulk state. A significant difference was observed for the exchange times of water molecules around monovalent cations. The exchange time of water molecules was long around Li+ ions and decreased with an increase in the ionic radius.  相似文献   

12.
Staurolite–cordierite assemblages are common in mica schists of the Aston and Hospitalet gneiss domes of the central Axial Zone, Pyrenees (France, Andorra). Within a 200 m wide zone, staurolite, cordierite and andalusite porphyroblasts contain inclusion trails that preserve the same stage of development of a crenulation cleavage, strongly suggesting that all three phases are contemporaneous. Their syntectonic growth occurred during a short period at the beginning of the formation of the dominant schistosity (S2) of the domes. Staurolite and cordierite touching each other further indicates an equilibrium relationship. Whole‐rock analyses show that some staurolite–cordierite schists are depleted in K2O compared to post‐Archean shales (PAAS) and amphibolite facies pelites. Analysis of the st‐crd paragenesis in K‐poor schists without muscovite using KFMASH and MnNCKFMASH petrogentic grids, pseudosections and AFM compatibility diagrams predicts stable conditions at pressures of ~3.5 kbar at 575 °C. For metapelites with intermediate XMg values (0.7 >  XMg >0.48) a ‘muscovite‐out window’ exists from 550–650 °C at 3.5 kbar in the KFMASH system. Conventional thermobarometry (GB‐GASP, AvT‐AvP) and petrogenetic grids show an isobaric P–T path to peak temperatures of ~650 °C, supported by the presence of sillimanite‐K‐feldspar gneiss and migmatites. LP‐HT metamorphism in the Aston dome is related to early Carboniferous (c. 339 Ma) granitic intrusions into the dome core. As metamorphism is directly linked with the formation of the main S2 schistosity, the temporal relations demonstrated in this study conflict with previous studies which constrained LP‐HT metamorphism and the development of flat‐lying schistosity to the late Carboniferous (315–305 Ma) – at least in the eastern Axial Zone.  相似文献   

13.
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   

14.
The effects of H2O, CO2, CH4 and HF on partial melting of a model phlogopite harzburgite mantle are considered with regard to the production of ultrapotassic magmas. Fluorine has a polymerising effect in H2O-poor conditions, but in the presence of abundant H2O where HF rather than F is dominant, the overall effect is depolymerisation. Methane also dissolves by forming (OH)– groups, and so has a depolymerising effect. Group I ultrapotassic rocks (lamproites) probably originate from primary magmas with SiO2 contents ranging from around 40 wt% to at least 52 wt%. This range can be explained by differing depths of origin from a similar source with a similar reduced H2O-CH4-HF volatile mixture. The formation of silica-rich initial melts from a model phlogopite harzburgite is assisted by the presence of CH4 and HF. Dissociation of less than 0.1 wt% H2O, driven by H2 loss, is sufficient to cause oxidation during emplacement to observed oxidation states. Silica-poor ultrapotassic rocks could be produced at higher pressures in a reduced environment, or in an oxidised environment with high CO2/(CO2 + H2O) ratios.Group II (African Rift) potassic rocks may originate in H2O-poor conditions in which fluorine will maintain a large phlogopite phase field, so that initial melts will be magnesian and silica-undersaturated.  相似文献   

15.
Garnet-bearing mineral assemblages are commonly observed in pelitic schists regionally metamorphosed to upper greenschist and amphibolite facies conditions. Modelling of thermodynamic data for minerals in the system Na2O–K2O–FeO–MgO–Al2O3–SiO2–H2O, however, predicts that garnet should be observed only in rocks of a narrow range of very high Fe/Mg bulk compositions. Traditionally, the nearly ubiquitous presence of garnet in medium- to high-grade pelitic schists is attributed qualitatively to the stabilizing effect of MnO, based on the observed strong partitioning of MnO into garnet relative to other minerals. In order to quantify the dependence of garnet stability on whole-rock MnO content, we have calculated mineral stabilities for pelitic rocks in the system MnO–Na2O–K2O–FeO–MgO–Al2O3–SiO2–H2O for a moderate range of MnO contents from a set of non-linear equations that specify mass balance and chemical equilibrium among minerals and fluid. The model pelitic system includes quartz, muscovite. albite, pyrophyllite, chlorite, chloritoid, biotite, garnet, staurolite, cordierite, andalusite, kyanite. sillimanite, K-feldspar and H2O fluid. In the MnO-free system, garnet is restricted to high Fe/Mg bulk compositions, and commonly observed mineral assemblages such as garnet–chlorite and garnet–kyanite are not predicted at any pressure and temperature. In bulk compositions with XMn= Mn/(Fe + Mg + Mn) > 0.01, however, the predicted garnet-bearing mineral assemblages are the same as the sequence of prograde mineral assemblages typically observed in regional metamorphic terranes. Temperatures predicted for the first appearance of garnet in model pelitic schist are also strongly dependent on whole-rock MnO content. The small MnO contents of normal pelitic schists (XMn= 0.01–0.04) are both sufficient and necessary to account for the observed stability of garnet.  相似文献   

16.
In quartz-free Fe, Na-poor and high-Mg, Al schists from the Bundelkhand craton in north central India, corundum porphyroblasts in finely interleaved phlogopite–chlorite aggregates with rare clinozoisite are mantled by fine-grained (<3 μm) intergrowths of outer collars (>200 μm wide) of phengitic-muscovite and chlorite (phlogopite + corundum + H2O → phengitic-muscovite + chlorite), and <100 μm wide inner collars of margarite–muscovite (corundum + clinozoisite + phengitic muscovite → margarite + muscovite + H2O). Wide-beam electron probe microanalyses indicate Mg in the bi-layered corona increases from corundum outwards, with a complementary decrease in Al and K. Si and Ca increase and then decrease to matrix values. The sharp chemical gradients across the highly structured bi-layered corona are inferred to suggest that the corona-forming reactions were promoted by local grain-boundary-controlled thermodynamic instability as opposed to element transport by advective diffusion. The P–T convergence of KMASH reactions and NCKMASH pseudosection phase relations computed using micro-domain compositions indicate the chlorite–phengitic muscovite outer collar formed at 18–20 kbar and ca. 630°C. The NCKMASH margarite–muscovite inner collar yielded lower metamorphic P–T conditions of 11 ± 3 kbar, ca. 630°C. U–Th–Pb chemical dating of metamorphic monazite and LA-ICPMS U–Pb isotope dating of re-equilibrated zircon yield ca. 2.78 Ga ages, which are interpreted to date corona formation and Neoarchean high-P metamorphism in the Bundelkhand craton, hitherto unknown in the Indian Precambrian. (220)  相似文献   

17.
A calcite-marble containing gem-quality ruby is exposed in the Hunza Valley, northwestern part of the Karakoram mountains, Pakistan zone of Kashmir. The marble forms concordant intercalations within sillimanite- and garnet-bearing biotite-plagioclase gneisses and mica schists. The metamorphic sequence is cut by discordant aplite and pegmatite dikes. The following mineral assemblages are recognized in the marble:1) Calcite+corundum+phlogopite±margarite±sheridanite±Al-rich pargasite±anorthite (An 96.7),2) calcite+spinel±corundum+phlogopite+sheridanite.Microprobe analyses are given for the essential minerals including corundum (ruby) and three different colour varieties of spinel. On the basis of recent experimental data, especially in the system CaO - Al2O3 - SiO2 - H2O -CO2 (and related subsystems), we assume that, during the regional metamorphism, temperatures of about 600 – 620°C and a water vapour pressure of about 6 kb were realized in part of the Hunza area. The gas phase must have contained roughly 20 mole-% of CO2. Thus the total fluid pressure may have reached about 7 kb. Presumably, temperatures increased in northwest direction, perhaps up to about 700°C. The estimated P-T conditions are consistent with a geothermal gradient of about 25°C/km.  相似文献   

18.
The terrane in the Panamint Mountains, California, was regionallymetamorphosed under low-pressure conditions and subsequentlyunderwent retrograde metamorphism. Prograde metamorphic isogradsthat mark the stability of tremolite + calcite, diopside, andsillimanite indicate a westward increase in grade. The studywas undertaken to determine the effects of the addition of Caon the types of assemblages that may occur in pelitic schists,to contribute to the understanding of the stability limits inP – T – aH2O – XFe of the pelitic assemblagechlorite + muscovite + quartz, and to estimate the change inenvironment from prograde to retrograde metamorphism. Peliticassemblages are characterized by andalusite + biotite + stauroliteand andalusite + biotite + cordierite. Within a small changein grade, chlorite breaks down over nearly the entire rangein Mg/(Mg + Fe) to biotite + aluminous mineral. Chlorite withMg/(Mg + Fe) = 0.55 is stable to the highest grade, and thegeneralized terminal reaction is chlorite + muscovite + quartz= andalusite + biotite + cordierite + H2O. Calcic schists arecharacterized by the assemblage epidote + muscovite + quartz+ chlorite + actinolite + biotite + calcite + plagioclase atlow grades and by epidote + muscovite + quartz + garnet + hornblende+ biotite + calcite + plagioclase at high grades. Epidote doesnot coexist with any AFM phase that is more aluminous than garnetor chlorite. Lithostatic pressure ranged from 2.3 kb to 3.0kb. During prograde-metamorphism temperatures ranged from lessthan 400° to nearly 700°C, and XH2O (assuming PH2O +PCO3 = Ptotal) is estimated to be 0.25 in siliceous dolomite,0.8 in pelitic schist, and 1.0 in calcic schist. Temperatureduring retrograde metamorphism was 450° ± 50°C,and all fluid were H2O-rich. A flux of H2O-rich fluid duringfolding is believed to have caused retrograde metamorphism.The petrogenetic grid of Albee (1965b) is modified to positionthe (A, Cd) invariant point relative to the aluminosilicatetriple point, which allows the comparison of facies series thatinvolve different chloritoid-reactions.  相似文献   

19.
Isochemical contact metamorphism was observed in 3 localities based on 184 chemical analyses of rock samples and the significance tested using simple statistical techniques. The intrusions included gabbro, granosyenite, and granitic type rocks being intruded into respective schists, clays and shales, and schists and gneisses. Conductive heat transfer appeared to be the most important heat transfer mechanism in isochemical contact metamorphism. During isochemical metamorphism both H2O and CO2 were rather mobile and impoverished in the metamorphic rocks.  相似文献   

20.
Metapelite is one of the predominant rock types in the high-pressure–ultrahigh-pressure(HP–UHP) metamorphic belt of western Tianshan, NW China; however, the spatial and temporal variations of this belt during metamorphism are poorly understood. In this study, we present comparative petrological studies and 40Ar/39 Ar geochronology of HP and UHP pelitic schist exposed along the Habutengsu valley. The schist mainly comprises quartz, white mica, garnet, albite and bluish amphibole. In the Mn O–Na2O–Ca O–K2O–Fe O–Mg O–Al2O3–Si O2–H2O(Mn NCKFMASH) system, P–T pseudosections were constructed using THERMOCALC 333 for two representative pelitic schists. The results demonstrate that there was a break in the peak metamorphic pressures in the Habutengsu area. The northern schist has experienced UHP metamorphism, consistent with the presence of coesite in the same section, while the southern one formed at lower pressures that stabilized the quartz. This result supports the previous finding of a metamorphic gradient through the HP–UHP metamorphic belt of the Chinese western Tianshan by the authors. Additionally, phengite in the northern schist was modelled as having a Si content of 3.55–3.70(a.p.f.u.) at the peak stage, a value much higher than that of oriented matrix phengite(Si content 3.32–3.38 a.p.f.u.). This indicates that the phengite flakes in the UHP schist were subjected to recrystallization during exhumation, which is consistent with the presence of phengite aggregates surrounding garnet porphyroblast. The 40Ar/39 Ar age spectra of white mica(dominantly phengite) from the two schists exhibit similar plateau ages of ca. 315 Ma, which is interpreted as the timing of a tectonometamorphic event that occurred during the exhumation of the HP–UHP metamorphic belt of the Chinese western Tianshan.  相似文献   

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