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1.
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
2.
Abstract In the Twin Lakes area, central Sierra Nevada, California, most contact metamorphosed marbles contain calcite + dolomite + forsterite ± diopside ± phlogopite ± tremolite, and most calc-silicate hornfelses contain calcite + diopside + wollastonite + quartz ± anorthite ± K-feldspar ± grossular ± titanite. Mineral-fluid equilibria involving calcite + dolomite + tremolite + diopside + forsterite in two marble samples and wollastonite + anorthite + quartz + grossular in three hornfels samples record P± 3 kbar and T± 630° C. Various isobaric univariant assemblages record CO2-H2O fluid compositions of χCO2= 0.61–0.74 in the marbles and χCO2= 0.11 in the hornfelses. Assuming a siliceous dolomitic limestone protolith consisting of dolomite + quartz ° Calcite ± K-feldspar ± muscovite ± rutile, all plausible prograde reaction pathways were deduced for marble and hornfels on isobaric T-XCO2 diagrams in the model system K2O-CaO-MgO-Al2O3-SiO2-H2O-CO2. Progress of the prograde reactions was estimated from measured modes and mass-balance calculations. Time-integrated fluxes of reactive fluid which infiltrated samples were computed for a temperature gradient of 150 °C/km along the fluid flow path, calculated fluid compositions, and estimated reaction progress using the mass-continuity equation. Marbles and hornfelses record values in the range 0.1–3.6 × 104 cm3/cm2 and 4.8–12.9 × 104 cm3/cm2, respectively. For an estimated duration of metamorphism of 105 years, average in situ metamorphic rock permeabilities, calculated from Darcy's Law, are 0.1–8 × 10?6 D in the marbles and 10–27 × 10?6 D in the hornfelses. Reactive metamorphic fluids flowed up-temperature, and were preferentially channellized in hornfelses relative to the marbles. These results appear to give a general characterization of hydrothermal activity during contact metamorphism of small pendants and screens (dimensions ± 1 km or less) associated with emplacement of the Sierra Nevada batholith. 相似文献
3.
Metamorphism, Fluid Flow and Partial Melting in Pelitic Rocks from the Onawa Contact Aureole, Central Maine, USA 总被引:3,自引:2,他引:1
Field, petrologic and geochemical data were used to characterizefluid infiltration and partial melting during metamorphism ofpelitic rocks in the contact aureole of the Onawa pluton, centralMaine, USA. Mineral assemblages delineate five metamorphic zoneswithin the contact aureole: chlorite zone, andalusitecordierite(ac) zone, alkali feldspar zone, sillimanite zone andleucocratic-vein (lv) zone. The sequence of observedmineral assemblages and mineralfluid reactions calculatedby mass balance is similar to those observed in other contactaureoles. Pressure of contact metamorphism is 3 kbar, on thebasis of optimum geothermobarometry calculations. Metamorphictemperatures vary from 500C in the andalusitecordieritezone to 65OC in the leucocratic-vein zone. Data from fieldobservations, mineral textures, observed reaction stoichiometry,geothermometry and major-element geochemistry suggest that theleucocratic veins of the l-v zone represent crystallized, partialmelts. Two overall calculated mineral reactions are responsiblefor vein formation: which can be modeled as combinations of two NKFMTASH meltingreactions: Progress of (M1) and (M2) was measured in eight samples, andreaction (M1) is the dominant melt-forming reaction in all samples.Partial melting (and vein formation) was therefore driven byinfiltration of the l-v zone by H2O-rich fluids. Calculatedtime-integrated fluid fluxes for l-v zone samples range from09 104 to 31 104 mol/cm2, and flow was in the directionof increasing temperature. KEY WORDS: pelites; contact metamorphism; fluid infiltration; partial melting; Onawa Pluton; Maine; USA
*Corresponding author. Telephone:(516) 6328192. Fax (516)6328240 e-mail: gsymmes{at}ccmail.sunysb.edu 相似文献
4.
The biotite isograd in pelitic schists of the Waterville Formationinvolved reaction of muscovite + ankerite + rutile + pyrite+graphite + siderite or calcite to form biotite + plagioclase+ ilmenite. There was no single reaction in all pelites; eachrock experienced a unique reaction depending on the mineralogyand proportions of minerals in the chlorite-zone equivalentfrom which it evolved. Quartz, chlorite, and pyrrhotite werereactants in some rocks and products in others. All inferredbiotite-forming reactions involved decarbonation and desulfidation;some were dehydration reactions and others were hydration reactions.P-T conditions at the biotite isograd were near 3500 bars and400 °C. C-O-H-S fluids in equilibrium with the pelitic rockswere close to binary CO2-H2O mixtures with XCO2 = 0.020.04.During the biotite-forming reaction, pelitic rocks (a) decreasedby 25 percent in volume, (b) performed (411)kcal/liter P-V work on their surroundings, (c) absorbed 3885kcal/liter heat from their surroundings, and (d) were infiltratedby at least 0.92.2 rock volumes H2O fluid. The biotite isograd sharply marks the limit of a decarbonationfront that passed through the terrane during regional metamorphism.Decarbonation converted meta-shales with 610 per centcarbonate to carbonate-free pelitic schists. One essential causeof the decarbonation event was pervasive infiltration of theterrane by at least 12 rock volumes H2O fluid early inthe metamorphic event under P-T conditions of the biotite isograd.Average shale contains 413 per cent siderite, ankerite,and/or calcite, but average pelitic schist is devoid of carbonateminerals. If the Waterville Formation serves as a general modelfor the metamorphism of pelitic rocks, it is likely that worldwidemany pelitic schists developed by decarbonation of shale caused,in part, by pervasive infiltration of metamorphic terranes byseveral rock volumes of aqueous fluid during an early stageof the metamorphic event. 相似文献
5.
Contact-mctamorphic assemblages in ophicarbonate from the Bergellaureole correspond either to model isobaric invariant T-XCO2points [Atg-Cal-Di-Tr-Fo (6 samples) and Atg-Cal-Tr-Fo-Dol (2)]or to isobaric univariant T-XCO2, curves [Tr-Cal-Di-Atg (18),Tr-Dol-Atg-Cal (1), Atg-Cal-Fo-Di (1), and Atg-Cal-Tr-Fo (1)].Calcite-dolomite thermometry and mineral-fluid equilibria inthe invariant assemblages record T=440540C at P=35kbar. Equilibrium metamorphic fluids were very H2O rich withX CO2,=00010027. In the invariant assemblagesTr + Fo were produced by prograde decarbonation-dehydrationreactions. In contrast, measured modes and reaction texturesin samples with univariant assemblages indicate thai Tr wasproduced by carbonation reactions. The apparent paradox of simultaneousdecarbonation reactions in the model isobaric invariant assemblagesand carbonation reactions in univariant assemblages is resolvedby local mineral-fluid equilibrium and fluid flow through ophicarbohatesin the direction of decreasing temperature as the aureole heated.Time-integrated flux (q) was computed from measured reactionprogress in 28 samples for models of both horizontal and verticaldown-temperature flow. Results are similar, with q decreasingrapidly from (0251) 105 cm3 fluid/cm2 rock1317 km from the intrusion to 006105cm3/cm2 at 1840 km. The decrease in q ismore consistent with vertical than horizontal flow. Variationsin time-integrated flux of more than an order of magnitude arerecorded by samples from the same outcrop. The absence of carbonatein adjacent metaperidotite indicates that flow was confinedto the ophicarbonate. Channelized, spatially heterogeneous,vertical flow can be explained by the brecciation and strongvertical foliation of the ophicarbonate relative to surroundingmassive metaperidotite. Generation of metamorphicfluids by decarbonation-dehydrationreactions within the ophicarbonates explains larger averageflux 12 km from the intrusion compared with more distalpoints. KEY WORDS: Bergell; contact metamorphism; fluid flow; ophicarbonate
*Telephone: (410) 516-8121. Fax: (410) 516-7933 相似文献
6.
FERRY JOHN M.; RUMBLE DOUGLAS III; WING BOSWELL A.; PENNISTON-DORLAND SARAH C. 《Journal of Petrology》2005,46(8):1725-1746
Progress () of the infiltration-driven reaction, 4olivine +5CO2 + H2O = talc + 5magnesite, that occurred during Barrovianregional metamorphism, varies at the cm-scale by a factor of3·5 within an 3 m3 volume of rock. Mineral and stableisotope compositions record that XCO2, 18Ofluid, and 13Cfluidwere uniform within error of measurement in the same rock volume.The conventional interpretation of small-scale variations in in terms of channelized fluid flow cannot explain the uniformityin fluid composition. Small-scale variations in resulted insteadbecause (a) reactant olivine was a solid solution, (b) initiallythere were small-scale variations in the amount and compositionof olivine, and (c) fluid composition was completely homogenizedover the same scale by diffusiondispersion during infiltrationand subsequent reaction. Assuming isochemical reaction, spatialvariations in image variations in the (Mg + Fe)/Si of the parentrock rather than the geometry of metamorphic fluid flow. Ifinfiltration-driven reactions involve minerals fixed in composition,on the other hand, spatial variations in do directly imagefluid flow paths. The geometry of fluid flow can never be determinedfrom geochemical tracers over a distance smaller than the oneover which fluid composition is completely homogenized by diffusiondispersion. KEY WORDS: Alpine Barrovian metamorphism; diffusion; metamorphic fluid composition; metamorphic fluid flow; reaction progress 相似文献
7.
Formation of Wollastonite by Chemically Reactive Fluid Flow During Contact Metamorphism, Mt. Morrison Pendant, Sierra Nevada, California, USA 总被引:2,自引:1,他引:2
Quartzcalcite sandstones experienced the reaction calcite+ quartz = wollastonite + CO2 during prograde contact metamorphismat P = 1500 bars and T = 560°C. Rocks were in equilibriumduring reaction with a CO2H2O fluid with XCO2 = 0·14.The transition from calcite-bearing, wollastonite-free to wollastonite-bearing,calcite-free rocks across the wollastonite isograd is only severalmillimeters wide. The wollastonite-forming reaction was drivenby infiltration of quartzcalcite sandstone by chemicallyreactive H2O-rich fluids, and the distribution of wollastonitedirectly images the flow paths of reactive fluids during metamorphism.The mapped distribution of wollastonite and modeling of an O-isotopeprofile across a lithologic contact indicate that the principaldirection of flow was layer-parallel, directed upward, withany cross-layer component of flow <0·1% of the layer-parallelcomponent. Fluid flow was channeled at a scale of 1100m by pre-metamorphic dikes, thrust and strike-slip faults, foldhinges, bedding, and stratigraphic contacts. Limits on the amountof fluid, based on minimum and maximum estimates for the displacementof the wollastonite reaction front from the fluid source, are(0·71·9) x 105 cm3 fluid/cm2 rock. Thesharpness of the wollastonite isograd, the consistency of mineralthermobarometry, the uniform measured 18O16O fractionationsbetween quartz and calcite, and model calculations all arguefor a close approach to local mineralfluid equilibriumduring the wollastonite-forming reaction. KEY WORDS: contact metamorphism, fluid flow, wollastonite, oxygen isotopes, reaction front 相似文献
8.
The Middle Oxfordian of the eastern Paris Basin constitutes a remarkable example of the growth and demise of a carbonate platform. Fischer plots, sedimentary and diagenetic features allow the identification of four depositional cycles (S5 to S8) in the Transversarium Zone; they are inserted in a lower frequency cycle of increased/decreased accommodation space (SoIII). The long‐term period of accommodation creation occurred during the older S5 and S6 cycles, the maximum accommodation zone being located in the lower part of the S6 cycle. This high accommodation period was tectonically controlled and was coeval with local distensive activity of a Hercynian fault. A major minimum accommodation zone exists during the S8 cycle. At that time, the platform was isolated and presented both a windward and a leeward margin. The growth of the platform was favoured by a warm and arid climate, oligotrophic conditions and reduced siliciclastic input during a highstand in relative sea‐level. These palaeoenvironmental features favoured the proliferation of phototrophic organisms producing carbonate material. The death of the platform was generated by a reduction in the carbonate production surface during a lowstand in relative sea‐level and by the appearance of mesotrophic conditions induced by the increase in siliciclastic inputs at the beginning of a period with a cooler and more humid climate. In the eastern Paris Basin, during the Middle Oxfordian, the parasequences are ordered and present ‘greenhouse’ characteristics. In contrast, at the beginning of the S8 cycle, the randomness in the thickness of contiguous parasequences increased. Decreased carbonate production during the lowstand caused by a transition from photozoan to heterozoan benthic communities certainly favoured this randomness and the appearance of catch‐down parasequences. 相似文献
9.
Fluid infiltration and regional metamorphism of the Waits River Formation, north-east Vermont, USA 总被引:2,自引:0,他引:2
Abstract The Siluro-Devonian Waits River Formation of north-east Vermont was deformed, intruded by plutons and regionally metamorphosed during the Devonian Acadian Orogeny. Five metamorphic zones were mapped based on the mineralogy of carbonate rocks. From low to high grade, these are: (1) ankerite-albite, (2) ankerite-oligoclase, (3) biotite, (4) amphibole and (5) diopside zones. Pressure was near 4.5kbar and temperature varied from c. 450° C in the ankerite-albite zone to c. 525° C in the diopside zone. Fluid composition for all metamorphic zones was estimated from mineral equilibria. Average calculated χco2[= CO2/(CO2+ H2O)] of fluid in equilibrium with the marls increases with increasing grade from 0.05 in the ankerite-oligoclase zone, to 0.25 in the biotite zone and to 0.44 in the amphibole zone. In the diopside zone, χCO2 decreases to 0.06. Model prograde metamorphic reactions were derived from measured modes, mineral chemistry, and whole-rock chemistry. Prograde reactions involved decarbonation with an evolved volatile mixture of χCO2 > 0.50. The χCO2 of fluid in equilibrium with rocks from all zones, however, was generally <0.40. This difference attests to the infiltration of a reactive H2O-rich fluid during metamorphism. Metamorphosed carbonate rocks from the formation suggests that both heat flow and pervasive infiltration of a reactive H2O-rich fluid drove mineral reactions during metamorphism. Average time-integrated volume fluxes (cm3 fluid/cm2 rock), calculated from the standard equation for coupled fluid flow and reaction in porous media, are (1) ankerite-oligoclase zone: c. 1 × 104; (2) biotite zone: c. 3 × 104; (3) amphibole zone: c. 10 × 104; and diopside zone: c. 60 × 104. The increase in calculated flux with increasing grade is at least in part the result of internal production of volatiles from prograde reactions in pelitic schists and metacarbonate rocks within the Waits River Formation. The mapped pattern of time-integrated fluxes indicates that the Strafford-Willoughby Arch and the numerous igneous intrusions in the field area focused fluid flow during metamorphism. Many rock specimens in the diopside zone experienced extreme alkali depletion and also record low χCO2. Metamorphic fluids in equilibrium with diopside zone rocks may therefore represent a mixture of acid, H2O-rich fluids given off by the crystallizing magmas, and CO2-H2O fluids produced by devolatilization reactions in the host marls. Higher fluxes and different fluid compositions recorded near the plutons suggest that pluton-driven hydrothermal cells were local highs in the larger regional metamorphic hydrothermal system. 相似文献
10.
Regional Metamorphism of the Waits River Formation, Eastern Vermont: Delineation of a New Type of Giant Metamorphic Hydrothermal System 总被引:4,自引:0,他引:4
Metamorphic isograds and time-integrated fluid fluxes were mappedover the 1500 km2 exposure of the Waits River Formation, easternVermont, south of latitude 4430'N. Isograds based on the appearanceof oligoclase, biotite, and amphibole in metacarbonate rocksdefine elongated metamorphic highs centered on the axes of twolarge antiforms. The highest-grade isograd based on the appearanceof diopside is closely associated spatially with synmetamorphicgranitic plutons. Pressure, calculated from mineral equilibria,was fairly uniform in the area, 7 1.5 kb; calculated temperatureincreases from {small tilde} 480C at the lowest grades in thearea to {small tilde} 575C in the diopside zone. CalculatedXco2f equilibrium metamorphic fluid increases from <0-03at the lowest grades to 0.2 in the amphibole zone and decreasesto 0.07 in the diopside zone. Time-integrated fluid fluxesincrease with increasing metamorphic grade, with the followingmean values for each metamorphic zone (in cm3/cm2): ankerite-oligoclasezone, 1 x 104; biotite zone, 7 x 104; amphibole zone, 2 x 105;diopside zone, 7 x 105. The mapped pattern of time-integrated fluxes delineates twolarge deep-seated ({small tilde} 25-km depth) regional metamorphichydrothermal systems, each centered on one of the major antiforms.Fluid flowed subhorizontally perpendicular to the axis of theantiforms from their low-temperature flanks to their hot axialregions and drove prograde decarbonation reactions as they went.Along the axes of the antiforms fluid flow was further focusedaround synmetamorphic granitic intrusions. In the hot axialregion fluid changed direction and flowed subvertically outof the metamorphic terrane, precipitating quartz veins. Estimatesof the total recharge, based on progress of prograde decarbonationreactions, nearly match estimates of the total discharge, basedon measured quartz vein abundance, (2-10) x 1012 cm3 fluid percm system measured parallel to the axes of the antiforms. Withinthe axial regions fluids had lower XCO2 and rocks record greatertime-integrated fluxes close to the intrusions than at positionsmore than {small tilde} 5 km from them. The differences in bothfluid composition and time-integrated flux can be explainedby mixing close to the intrusions of regional metamorphic fluidsof XCO2/ with fluids from another source with XCO2{small tilde}0 in the approximate volume ratio of 1:2. 相似文献