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1.
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=440–540C at P=3•5kbar. Equilibrium metamorphic fluids were very H2O rich withX CO2,=0•001–0•027. 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 (0•2–5•1) 105 cm3 fluid/cm2 rock1•3–1•7 km from the intrusion to 0–0•6105cm3/cm2 at 1•8–4•0 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 1–2 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
2.
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
Quartz–calcite 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 CO2–H2O 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 quartz–calcite 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 1–100m 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(0·7–1·9) x 105 cm3 fluid/cm2 rock. Thesharpness of the wollastonite isograd, the consistency of mineralthermobarometry, the uniform measured 18O–16O fractionationsbetween quartz and calcite, and model calculations all arguefor a close approach to local mineral–fluid equilibriumduring the wollastonite-forming reaction. KEY WORDS: contact metamorphism, fluid flow, wollastonite, oxygen isotopes, reaction front 相似文献
3.
Contact metamorphism of siliceous dolomite in the southern partof the metamorphic aureole of the Alta stock (Utah, USA) producedthe prograde isograd sequence: talc (Tc), tremolite (Tr), forsterite(Fo), and periclase (Per). Calcite (Cc)–dolomite (Do)geothermometry and phase equilibria define a general progradeT–X(CO2) path of decreasing X(CO2) with rising temperaturefor the dolomite. High-variance assemblages typify the aureole.Per + Cc and Fo + Cc + Do characterize the inner aureole (Perand Fo zones), and Tr + Do + Cc and Tc + Do + Cc are widespreadin the outer aureole (Tr and Tc zones). Low-variance assemblagesare rare and the thickness of reaction zones (coexisting reactantand product minerals) at the isogradic reaction fronts are narrow(tens of metres or less). The mineral assemblages, calculatedprogress of isograd reactions, and the prograde T–X(CO2)path all indicate that massive dolomite was infiltrated by significantfluxes of water-rich fluids during prograde metamorphism, andthat the fluid flow was down-temperature and laterally awayfrom the igneous contact. Fluid infiltration continued throughat least the initial retrograde cooling of the periclase zone.Down-T fluid flow is also consistent with the results of Cc–Dogeothermometry and patterns of 18O depletion in this area. Theclose spatial association of reacted and unreacted chert nodulesin both the tremolite and talc zones plus the formation of tremoliteby two reactions indicate that the outer aureole varied in X(CO2),and imply that fluid flow in the outer aureole was heterogeneous.The occurrence of dolomite-rich and periclase (brucite)-absent,high- 相似文献
4.
Analytical and numerical solutions to the differential equationsfor the conduction of heat with heat production or with fluidflow have been used to evaluate the role of volatiles in thethermal history of regional metamorphic terranes. The maximumthermal effect from pervasive, single-pass, regional volatileflow may be predicted from a steady-state solution given byBredehoeft & Papadopoulos (1965). For fluid velocity vF(m/s) and connected porosity , combinations of volatile fluxvF (m3 of fluid/m2s) and transport distance L(m) such that vLis greater than 3?6?10–7 should produce regional temperatureincreases due to fluid flow, if the flow persists for l05–106a (depending on the transport distance L). The absolute valueof the temperature increase due to volatile flow will be greaterin regions with higher ambient geothermal gradients. For L=20km, a volatile flux of 1?8 ? 10–11 (m3 of fluid/m2s) orgreater is required to achieve a temperature effect. Few geologicprocesses release volatiles at this rate for extended periodsof time, so regional thermal effects from the single-pass, pervasiveflow of volatiles are unlikely. A new analytical solution forthe steady state temperature distribution between idealizedparallel channels of fluid flow is presented along with theresults of two-dimensional numerical models of channelized fluidflow. Both approaches show that little temperature increaseis expected near channels of fluid flow relative to the rocksbetween the channels, unless the channels exceed 100 m in widthor unless the fluid fluxes are very large and transient. A possiblethermal effect of volatile flow in metamorphic terranes is theproduction of metamorphic hot spots due to focusing of volatilesinto widely spaced channels or conduits exceeding 1 km in width.Given a sufficient fluid flux (exceeding 10–10 m3 of fluid/m2s),thermal gradients of over 100K from center to edge may be producedin such channels during relatively short time intervals (105–106a). 相似文献
5.
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 diffusion–dispersion 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 diffusion–dispersion. KEY WORDS: Alpine Barrovian metamorphism; diffusion; metamorphic fluid composition; metamorphic fluid flow; reaction progress 相似文献
6.
Pelitic schists and quartzites in the Picuris Range of northernNew Mexico exhibit mineral 18O and D compositions that indicaterock-buffered isotopic exchange during metamorphism at uniformphysical conditions of T 530C and P 4 kbar. Phase assemblagesand major-element compositions among silicates and oxides areuniform within stratigraphic units, but they change abruptlyacross lithologic contacts, yielding distinctive mineral Mg/Fe2+ratios and inferred f(O2) values. Mineral compositions reflectthe pre-metamorphic compositions of individual rock units. O-and H-isotopic compositions of quartz and muscovite are alsodiscontinuous across lithologic boundaries, showing intra-layerhomogeneity and bulk-rock isotopic compositions retained fromsedimentary protoliths. Uniform 18 OQu-Ms values indicate isotopicequilibrium at peak metamorphic conditions. Sharp discontinuitiesin mineral and fluid isotopic compositions reflect limited isotopicexchange between units. The isotopic system in these units wasprobably one of rock-buffered exchange, in which the sedimentarycompositions of individual rock units exerted the dominant controlon mineral and fluid isotopic composition over short distancesin a relatively closed metamorphic system. Fluid migration duringprogressive metamorphic devolatilization in this simple systemwas probably non-pervasive, and it was probably influenced bycontrasting rock permeability. Consequently, our study suggeststhat pervasive exchange between interlayered units may be uncommonin regionally metamorphosed terrains that show weak initialgeochemical gradients. In contrast, the chemical and isotopichomogenization that attends pervasive fluid flow and high fluidfluxes may be restricted to settings characterized by extremegeochemical gradients, such as interlayered silicates and carbonates,or terrains that host plutonic hydrothermal systems. KEY WORDS: fluids; metamorphism; stable isotopes; New Mexico
*Corresponding author. E-mail: jgoodgc{at}sun.cis.smu.edu.. Telephone (214) 768–4140. Fax (214) 768–2701 相似文献
7.
Talc-, Magnesite- and Ti-Clinohumite-Bearing Ultrahigh-Pressure Meta-Mafic and Ultramafic Complex in the Dabie Mountains, China 总被引：18，自引：4，他引：18
The Bixiling mafic-ultramafic metamorphic complex is a 1•5km2 tectonic block within biotite gneiss in the southern Dabieultrahigh-pressure terrane, central China. The complex consistsof banded eclogites that contain thin layers of garnet-bearingcumulate ultramafic rock. Except for common eclogitic phases(garnet, omphacite, kyanite, phengite, zoisite and rutilc),banded eclogites contain additional talc and abundant coesiteinclusions in omphacite, zoisite, kyanite and garnet. Some metaultramaficrocks contain magnesite and Ti-clinohumite. Both eclogites andmeta-ultramafic rocks have undergone multi-stage metamorphism.Eclogite facies metamorphisrn occurred at 610–700C andP>27 kbar, whereas amphibolite facies retrograde metamorphismis characterized by symplectites of plagioclase and hornblendeafter omphacite and replacement of tremolite after talc at P<6–15kbar and T <600C. The meta-ultramafic assemblages such asolivine + enstatite + diopside + garnet and Ti-clinohumite +diopside + enstatite + garnet + magnesite olivine formed at700–800C and 47–67 kbar. Investigation of the phaserelations for the system CaO-MgO-SiO2-H2O-CO2 and the experimentallydetermined stabilities of talc, magnesite and Ti-clinohumiteindicate that (1) UHP talc assemblages are restricted to Mg-Algabbro composition and cannot be an important water-bearingphase in the ultramafic mantle, and (2) Ti-clinohumite and magnesiteare stable H2O-bearing and CO2-bearing phases at depths >100km. The mafic-ultramafic cumulates were initially emplaced atcrustal levels, then subducted to great depths during the Triassiccollision of the Sine-Korean and Yangtze cratons. KEY WORDS: eclogite; magnesite; meta-ultramafics; talc; ultrahigh-P metamorphism
*Corresponding author 相似文献
8.
Chemistry of Kornerupine and Associated Minerals, a Wet Chemical, Ion Microprobe, and X-Ray Study Emphasizing Li, Be, B and F Contents 总被引：5，自引：2，他引：3
GREW E. S.; CHERNOSKY J. V.; WERDING G.; ABRAHAM K.; MARQUEZ N.; HINTHORNE J. R. 《Journal of Petrology》1990,31(5):1025-1070
Kornerupine and associated minerals in 31 samples of high-graderocks relatively rich in Al and Mg were analysed by wet chemistry,ion microprobe mass analyser, electron microprobe and X-raypowder diffraction. For 11 samples of kornerupine and threesamples of biotite (F only) analysed by both wet chemical andion microprobe methods, the best agreement was obtained forB2O3, whereas the ion microprobe Li2O values were systematicallysomewhat higher than the wet chemical values. The wet chemicalmethods give Li2O=0–0?19 wt.%; BeO=0–0?032 wt.%;B2O3=0–4?01 wt.%; and F=0?07–0?77 wt.% in kornerupine,whereas ion microprobe analyses on other kornerupines give valuesup to 0?35 wt.% Li2O, O066 wt.% BeO, and 4?72 wt.% B2O3. Thesum B+Al+Fe3++Cr is close to 6?9 atoms per 22 (O, OH, F) or21?5 (O) in kornerupine. In general, Li/Fe ratios decrease as follows: kornerupine ?sapphirinebiotite> Crd (Na<0?03 per 18 oxygens)>tourmaline, garnet,orthopyroxene. However, for cordierite with Na>004, Li/Fedecreases as follows: cordierite>kornerupine. Sapphirineand sillimanite are the only associated minerals to incorporatesignificant boron (0?1–0?85 wt.% B2O3) and then only whenthe single site for B in kornerupine is approaching capacity.Sillimanite B2O3 contents increase regularly with kornerupineF. Fractionation of fluorine increases as follows: kornerupine<biotite<tourmaline,and Kkrn-BtD=(F/OH)Krn/(F/(OH)Bt (assuming ideal anion composition)increases with biotite Ti. Kornerupine B2O3 content is a measureof B2O3 activity in associated metamorphic fluid, whereas sillimaniteB2O3 content increases with temperature, exceeding 0?4 wt.%whenT=900?C at very low water activities. New data on 11 kornerupines and literature data indicate thatthe unit cell parameters a, c, and V decrease with increasingB content and b, c, and V increase with increasing Fe3+ content.In Fe3+-poor kornerupines, b increases with Mg and with (Mg+ Fe2+) but the effect of Mg on b via the substitution VIMg+IVSi=VIAl+IVAloverwhelms the effect of Fe2+=Mg substitution. 相似文献
9.
Metamorphism in the Adirondacks: II. The Role of Fluids 总被引：6，自引：2，他引：4
VALLEY JOHN W.; BOHLEN STEVEN R.; ESSENE ERIC J.; LAMB WILL 《Journal of Petrology》1990,31(3):555-596
Quantitative estimates of metamorphic fluid speciation, stableisotopic analyses, and studies of fluid inclusions all documentthe local complexity of fluids in the deep crustal rocks exposedin the Adirondack Mountains, NY. Estimates of the activity ofH2O in the granulite facies are substantially lower than inthe amphibolite facies gneisses. The onset of low water activitiesin semi-pelitic gneisses generally correlates with migmatitictextures in the uppermost amphibolite facies, suggesting thatpartial melts absorbed H2O at the peak of metamorphism. In granulitefacies marbles and calc-silicates, conditions varied from extremelyundersaturated in H2O-CO2 fluid to fluid saturated, and H2Oand CO2 show sharp gradients within single outcrops. Low valuesof fO2 and fH2O, or of fCO2, and fH2O indicate fluid-absentconditions for some orthogneisses and marbles, which are inferredto have been ‘dry’ rocks before and during granulitefacies recrystallization. Wollastonite is preserved from earlycontact metamorphism and serves as an index mineral for fluid-absentconditions in granulites where H2O is low. Values offO2 rangefrom near the hematite + magnetite buffer in metamorphosed ironformation to substantially below the quartz + magnetite + fayalitebuffer in some orthogneisses. The anorthosite suite is moreoxidized than some associated granitic gneisses. Halogens (Fand Cl) substitute extensively for OH in micas and amphiboles,extending their stability, although F2, Cl2, HCl, and HF areminor components in any fluid. Oxybiotite-type exchanges involvingO for OH are also important, extending the stability of biotite.Stable isotopic ratios of O and C demonstrate that premetamorphicwhole-rock compositions are commonly preserved whereas mineralcompositions generally reflect equilibration at the peak ofmetamorphism. The Marcy Anorthosite Massif was intruded as ahigh 18O magma. The combination of mineral equilibria, stable isotope data,and fluid inclusions is used to identify and to distinguishamong pre-orogenic contact metamorphic/hydrothermal events,peak metamorphic events, and retrograde/postmetamorphic events.Polymetamorphism is documented at skarn zones adjacent to anorthosite,where large volumes of hydrothermal fluid were channeled duringearly, shallow contact metamorphism and where conditions werefluid poor during subsequent regional metamorphism. Peak metamorphicevents are inferred to have been caused primarily by magmaticprocesses of intrusion and anatexis. Partial melting has causedlow values of H2O in many rocks, but in other cases low valuesof H2O are recorded in orthogneisses derived from H2O-poor magmas.Isotopic studies show that maximum fluid/rock ratios were <0?land possibly 0?0 for infiltrating fluids at the peak of metamorphismin many localities. No evidence of pervasive, regional infiltrationby a fluid at the peak of metamorphism has been substantiatedin the Adirondacks. Fluid inclusions containing high-densityCO2 or CO2 + H2O represent conditions from after the peak ofmetamorphism and document isobaric cooling, in agreement withestimates from garnet zoning. Fine-scale retrograde veins arecommon and are associated with high-density CO2-rich fluid inclusions. 相似文献
10.
Transition from the Zeolite to Prehnite-Pumpellyite Facies in the Karmutsen Metabasites, Vancouver Island, British Columbia 总被引：1，自引：0，他引：1
The upper Triassic Karmutsen metabasites from northeast VancouverIsland, B.C., are thermally metamorphosed by the intrusion ofthe Coast Range Batholith. The amygdaloidal metabasites developedin the outer portion of the contact aureole show a progressivemetamorphism from zeolite to prehnite-pumpellyite facies. Thesize of an equilibrium domain is extremely small for these metabasites,and the individual amygdule assemblages are assumed to be inequilibrium. Two major calcite-free assemblages (+chlorite+quartz)are characteristic: (i) laumontite+pumpellyite+epidote in thezeolite facies and (ii) prehnite+pumpellyite+epidote in theprehnite-pumpellyite facies. The assemblages and compositionsof Ca-Al silicates are chemographically and theoretically interpretedon the basis of the predicted P-T grid for the model basalticsystem, CaO-MgO-A12O3-Fe2O3-SiO2-H2O. The results indicate:(1) local equilibrium has been approached in mineral assemblagesand compositions; (2) the XFe3+ values in the coexisting Ca-Alsilicates decrease from epidote, through pumpellyite to prehnite;(3) with increasing metamorphic grade, the Fe3+ contents ofepidotes in reaction assemblages decrease in the zeolite facies,then increase in the prehnite-pumpellyite facies rocks. Suchvariations in the assemblages and mineral compositions are controlledby a sequence of continuous and discontinuous reactions, andallow delineation of T-XFe3+ relations at constant pressure.The transition from the zeolite to prehnite-pumpellyite faciesof the Karmutsen metabasites is defined by a discontinuous reaction:0·18 laumontite+pumpellyite+0·15 quartz = 1·31prehnite+ 0·78 epidote+0·2 chlorite+ 1·72H2O, where the XFe3+ values of prehnite, pumpellyite and epidoteare 0·03, 0·10 and 0·18, respectively.These values together with available thermodynamic data andour preliminary experimental data are used to calculate theP-T condition for the discontinuous reaction as P = 1·1±0·5 kb and T = 190±30°C. The effectsof pressure on the upper stability of the zeolite facies assemblagesare discussed utilizing T-XFe3+ diagrams. The stability of thelaumontite-bearing assemblages for the zeolite facies metamorphismof basaltic rocks may be defined by either continuous or discontinuousreactions depending on the imposed metamorphic field gradient.Hence, the zeolite and prehnite-pumpellyite facies transitionboundary is multivariant. 相似文献
11.
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
12.
Metamorphism of Calcic Pelitic Schists, Strafford Dome, Vermont: Compositional Zoning and Reaction History 总被引：3，自引：2，他引：3
Four assemblages from calcic pelitic schists from South Strafford,Vermont, have been studied in detail to determine the relationshipbetween reaction history and compositional zoning of minerals.The lowest-grade assemblage is garnet + biotite + chlorite +plagioclase + epidote + quartz + muscovite + graphite + fluid.Along a path of isobaric heating, the net reaction is Chl +Ms + Ep + Gr = Grt + Bt + Pl + fluid. Garnet grows with decreasingFe/(Fe + Mg) and XSpa, (from 0•2 to 0•05), XGra staysnearly constant between 0•20 and 0•25, and plagioclasegrows with XAn increasing from peristerite to 0•2–0•5. The subsequent evolution depends on whether chlorite or epidotereacts out first. If chlorite is removed from the assemblagefirst, the net reaction along an isobaric heating path becomesGrt + Ms + Ep + Qtz + Gr = Bt + Pl + fluid. XAn of plagioclaseincreases to 0•20–0•70, depending on the bulk-rockcomposition and changes in pressure and temperature. If epidoteis removed first, the assemblage becomes a simple pelite andthe net reaction becomes Chl + Pl + Ms + Qtz = Grt + Bt + H2O.Plagioclase is consumed to provide Ca for growing garnet, andXAn, Fe/(Fe + Mg) of garnet, XGra, and XSpa all decrease. Afterboth chlorite and epidote are removed, continued heating upto the metamorphic peak of {small tilde}600C produces littleprogress of the reaction Grt + Ms = Bt + Pl; and XAn increases. The four assemblages have been numerically modeled using theGibbs method starting with measured compositions. The modelssuccessfully predict the observed compositional zoning and trendsof mineral growth and consumption along the computed P–Tpaths. The models also predict the compositional mineral zoningthat would have resulted from other P–T paths.
* Present address: Department of Geology, University of Alabama, Tuscaloosa, Alabama 35487 相似文献
13.
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
14.
Activity diagrams in the system KAlSi3O8-NaAlSi3O8-Al2SiO5-SiO2-H2O-HClhave been calculated in terms of aK+/aH+ and aN+/aH+ from existingexperimental data. They show the effect of temperature, pressure,and aH2O on the stability fields of the alkali feldspars, micas,and aluminium silicate. These activity diagrams are useful in revealing the bufferingcapacity of mineral assemblages and the chemical potential gradientsestablished by changes in T, P, aH2O, and mineral assemblage.An analysis of mineral paragenesis in terms of these diagramssuggests that mosaic equilibrium, allowing limited metasomatismand internal buffering of chemical potentials, best describemetamorphic systems. Thus the dehydration reaction: muscovite+quartz=K-feldspar+Al2SiO5+H2O which is most important in closed systems, probably fails todescribe in detail the mechanism of natural muscovite decomposition.Rather the decomposition of muscovite is more likely representedby ionic reactions. The replacement of muscovite by feldspar: muscovite+6 SiO2+2 K+=3 K-feldspar+2 H+ muscovite+6 SiO2+3 Na+=3 Albite+K++2 H+ is favored at high temperature and low pressure, and may accountfor the crystallization of some feldspars in metamorphic rocks.The reaction involving aluminium silicate replacement of muscovite: 2 muscovite+2 H+=3 Al2SiO5+3 SiO2+3 H2O+2 K+ is favored at high temperature and pressure and low aH2O, andcould contribute to the development of the aluminium silicates.It is concluded that both activity diagrams and AKNa projectionsshould be used together to more completely evaluate mineralparagenesis in terms of mosaic equilibria. 相似文献
15.
The Stability of Andradite, Hedenbergite, and Related Minerals in the System Ca--Fe--Si--O--H 总被引：3，自引：0，他引：3
Phase relations for the bulk compositions 3CaO·2FeOx·3SiO2+excessH2O and CaO·FeOx·2SiO2+excess H2O were determinedusing conventional hydrothermal techniques with solid phaseoxygen buffers to control fO2. Andradite, Ca3Fe3+2Si3O12, synthesized above 550 °C hasan average unit cell edge, ao, of 12.055±0.001 Å,and an index of refraction, n, of 1.887±0.003. Belowthis temperature, ao increases whereas n decreases, indicatingthe formation of a member of the andradite-hydroandradite solidsolution. At 2000 bars Pfluid andradite is stable above an fO2of 1015 bar at 800 °C and 10-32 bar at 400 °C. At lowerfO2 andradite+fluid gives way at successively lower temperaturesto the condensed assemblages magnetite+wollastonite, kirschsteinite(CaFe2+SiO4)+ wollastonite and kirschsteinite+xonotlite (Ca6Si6O17(OH)2). Synthetic hedenbergite, CaFe2+Si2O6, has average unit cell dimensionsof ao = 9.857± 0.004 Å, bo = 9.033±0.002Å, co = 5.254±0.002 Å and ß = 104.82°±0.03°,and refractive indices of n = 1.731±0.003 and n = 1.755±0.005.At 2000 bars Pfiuid, hedenbergite is stable below an fO2 of10-13 bar at 800 °C and 10-28 bar at 400 °C. Above thesefO2 values, hedenbergite+O2 breaks down to andradite+magnetite+quartz. The mineral pair andradite +hedenbergite thus limit the fO2range possible for their joint formation under equilibrium conditions. The hydration of wollastonite to xonotlite occurs at much lowertemperatures than previous experimental work indicated. A tentativehigh temperature limit for this reaction is set at 185°±15°C and 5000±25 bars and 210°±15 °Cand 2000±20 bars. Inasmuch as the growth of xonotlitefrom wollastonite + H2O was never accomplished, this high temperaturelimit does not represent an equilibrium univariant curve. Nine phases were encountered in the study of andradite and hedenbergite.They are andradite, hedenbergite, magnetite, wollastonite, kirschsteinite,xonotlite, quartz, ilvaite, and vapor (fluid). An invariantpoint analysis using the method of Schreinemakers shows thetopologic relations of the reactions involved. The resultinggrid can be used to interpret natural occurrences. 相似文献
16.
Petrographic and microprobe investigations of calc-alkaline (CA) rocks from the High Cascade Range (i.e., Mt. St. Helens, Mt. Jefferson, Crater Lake and Mt. Shasta) of western North America show that crystal clots represent primary igneous phase assemblages and are not products of amphibole reactions with melt. For each eruptive complex, crystal clots display diverse modal proportions even within a single eruptive unit. Nevertheless, in all cases the crystal-clot minerals are also represented in the rock as phenocrysts or microphenocrysts. Basalts contain clots of ol+plag+mgt, ol+mgt, cpx+ plag+mgt, cpx+mgt and plag+mgt; andesites, clots of cpx+mgt, opx+mgt, cpx+opx+plag+mgt, cpx+plag+mgt, opx+plag+mgt and plag±mgt; and dacites, clots of opx+mgt, cpx+opx+plag+ mgt, opx+plag+mgt, amph+plag+mgt±ilm, amph+mgt±ilm and plag±mgt. The bulk compositions of most of these clot assemblages could not have been derived from amphibole percursors. Although some amphiboles in dacitic rocks display a breakdown reaction of amph=plag+cpx+opx +mag, these mineral clusters, unlike those of clots, typically have a relict amphibole crystal outline and a fine-grained metamorphic texture. Plagioclase grains in the mineral clusters lack oscillatory zoning which is typical of crystal clot plagioclase grains. The euhedral to subhedral shapes of most clot minerals and the oscillatory zoning present in most clot plagioclase grains are not likely to have formed from the breakdown of amphibole. Crystal clots are also observed in Hawaiian and ocean floor basalts, although amphibole fractionation has not been proposed for those lavas. Magnetite fractionation may be the controlling process limiting iron enrichment in CA magmas rather than amphibole fractionation. Textural evidence indicates that magnetite is an early-forming phase in CA magmas. V, which is concentrated in magnetite, shows a strong decrease with increasing silica in many CA rocks, supporting a magnetite fractionation model.Hawaii Institute of Geophysics Contrib. No. 969 相似文献
17.
The Genesis of Zoned Skarns in the Sierra Nevada, California 总被引：1，自引：0，他引：1
Zoned skarns occur at plutonic-metamorphic contacts, in veinscutting marble, and at contacts between marble and interlayeredamphibolite and biotite-rich rocks. For P = 2 kb, fluid inclusionsand P-T-XCO2 stability relations of calc-silicate assemblagessuggest T< 650 °C and a H2O-rich fluid (XCO2 < 0.1).Small-scale, Ca-rich endoskarns are common near exoskarns. Massbalance calculations suggest that: (a) the formation of exoskarnrequires the influx of solute in an aqueous solution from uncontaminatedmagma in addition to material derived from the endoskarn, (b)some ‘limestone assimilation’ is required to formendoskarns, and (c) skarn formation was essentially a constant-volumeprocess. Applying chromatographic theory, compositional profilesof garnet and pyroxene across zoned skarns suggest that infiltrationmetasomatism was an important process, although diffusion metasomatismappears to have produced local compositional gradients at theinfiltration ‘fronts’. Fluid flow calculations showthat thick exoskarns could readily form by intergranular infiltration of aqueous solutions. Reciprocal diffusional exchangeis suggested as a dominant mechanism in the formation of zonedskarns formed at contacts between interlayered metamorphic lithologies. 相似文献
18.
This study explores the origin and geochemical evolution ofapatite, monazite, and xenotime along two metamorphic traverses.The first, from the Kigluaik Mountains, Seward Peninsula, Alaska,consists of a localized (85 cm) orthopyroxene–clinopyroxene-bearingdehydration zone. The second consists of orthopyroxene ±clinopyroxene-bearing granulite facies metabasite layers interlayeredwith metapelites over a 3–4 km traverse, along the ValStrona, Ivrea–Verbano Zone, Northern Italy (IVZ). In bothdehydration zones small Th- and U-poor inclusions of monaziteand/or xenotime occur in the apatite. These inclusions are metasomaticallyinduced and nucleated within the apatite via the coupled substitutionsNa+ + (Y + REE)3+ = 2 Ca2+ and Si4+ + (Y + REE)3+ = P5+ + Ca2+.These are not present in apatite from the original amphibolitefacies gneiss. Apatite, in both dehydration zones, also showsa relative increase in both F and Cl compared with apatite fromthe amphibolite facies zone. Granulite facies metabasites inthe IVZ also contain isolated monazite grains, which range fromuniform to complexly zoned in Th the (13–30·1 mol% ThSiO4). These are the product of breakdown and subsequentmobilization of the lanthanides and actinides from monazite-(Ce)in the metapelite layers into the metabasite layers at the startof granulite facies metamorphism. KEY WORDS: apatite; monazite; xenotime; KCl–NaCl brines; metasomatism; phosphate minerals; charnockite–enderbite; granulite facies metamorphism 相似文献
19.
Metamorphic Evolution of Neoproterozoic Manganese Formations and their Country Rocks at Otjosondu, Namibia 总被引：2，自引：0，他引：2
Manganiferous chemical sediments of Neoproterozoic age in Namibiawere subjected to high-T–low-P metamorphism during theDamara Orogeny and display unique phase assemblages. The manganeseformations are embedded in iron formations and siliciclasticcountry rocks. This sequence is petrographically subdividedinto restricted lithotypes which bear specific mineral assemblagesand compositions depending on their protolith type. In puremanganese ores the critical assemblage braunite + haematite+ jacobsite + rhodonite is frequently developed, whereas interlayeredimpure silicate ores bear various proportions of spessartine,Mn3+-bearing andradite–calderite and andradite garnets,rhodonite, manganoan aegirine–augite, aegirine, Ba–K–Na-feldspars,barite and rare kinoshitalite. Petrological constraints derivedfrom country rock lithologies indicate peak metamorphic conditionsof 660–700C at estimated pressures of 35–45 kbar.Numerous Ba-rich pegmatitic veins restricted to the ore horizonstestify to the production of partial melts from siliciclasticstrata within the manganese formations. They are correlatedwith peak pressure conditions between 5 and 6 kbar, accompanyingthe main deformation event and pre-dating the thermal peak.An early H2O-rich generation of fluid inclusions is interpretedas a manifestation of prograde dehydration reactions in theore horizons. This caused hydraulic fracturing of the ores and,subsequently, triggered the formation of partial melts whichintruded the fracture planes in situ. Peak metamorphism thenoccurred under strainfree conditions allowing equilibrium recrystallizationof all minerals to develop. Phase relationships of manganeseoxides and silicates modelled in the system Mn–Fe–Si–Oreveal variable chemical compositions of braunites, jacobsitesand haematites depending on their paragenesis. They indicatevery restricted oxygen reservoirs within specific strata ofthe manganese ores and eliminate a prominent mass exchange evenon a small scale. This is supported by 18O analyses of silicateassemblages which further exclude mass transfer between manganeseores and country rocks, and indicate preservation of the exchangeequilibria during cooling. The uplift path of the sequence canbe constrained using different decrepitation patterns of H2Ofluid inclusions and a syn-to late-metamorphic CO2-rich fluidinclusion population, which indicate high geothermal gradientsof 70C/km and more. The P–T–D evolution of thishigh-T–low-P metamorphic belt conforms with the palaeotectonicsetting of the study area at the southernmost part of the CongoCraton, representing the continental buttress colliding withthe Kalahari Craton during the Pan-African orogeny. KEY WORDS: manganiferous sediments; Damara Orogeny; Namibia; metamorphism; oxygen isotopes; fluid inclusions
*Corresponding author. Present address Institut fr Geowissenschaften und Lithosphrenforschung, Senckenbergstrasse 3, D-35390 Giessen, Germany 相似文献
20.
Electron microprobe analyses are presented for new-formed mineralsfrom a small exposure of semi-schistose Taveyanne Formationof the pumpellyite-actinolite facies near Lo?che, Valais. Comparisonsare drawn with minerals of other low-grade metamorphic areas,especially in southern New Zealand. Sphene shows considerablesubstitution of Ca(Al,Fe)SiO4(OH) for CaTiSiO5. Epidotes aresharply divided into early pistacitic (Ps = 0.28–0.37)and later clinozoisitic varieties (Ps = 0.11–0.19). Pumpellyitesrange from pumpellyite-(Fe) to pumpellyite-(Al) and are generallyless Fe-rich than those of zeolite and prehnite-pumpellyitefacies. Pumpellyite inclusions in albitized plagioclase areparticularly low in Mg. Actinolites are low in A12O3, TiO2,and Na2O, essentially identical compositions being nucleatedon detrital augite, hornblende, and in the matrix. Phengitesare also extremely low in Na2O and TiO2. Chlorites are ripidolites.Albitized clastic plagioclase has the composition An0.7–1.6and albite in clinozoisite-calcite-albite-phengite-chloriteveins An2.1–2.3. Calcites carry minor Mn > Fe ? Mg.New-formed iron oxides are absent, whereas pyrrhotite and minorpyrite occur in one rock, buffering fs2 and indicating low fo2. Ratios Mg: Fe* (Fe* = total Fe) in coexisting chlorites andA1, Na-poor actinolites vary sympathetically both in the Lo?cheand southern New Zealand rocks here considered, giving KD =(Mg/Fe*) actlnolIte/(Mg/Fe*)chlorle = 1.72. Mg/Fe* ratios inpumpellyites tend to vary sympathetically with those of coexistingchlorites and actinolites but are more variable. Substitutionof (Fe, Mg)Si for A12 in phengitic micas and chlorites variessympathetically in the same suites between mafic volcanic andmore pelitic extremes. Various minor elements also behave ina consistent fashion, indicating an encouraging tendency towardsequilibrium. Variable (though small) A12O3 contents of actinolite,Fe: Al ratios in epidotes and pumpellyites, and Mg: Fe* ratiosin phengites, even within a single grain, are evidence of short-rangedisequilibrium; metamorphic equilibration is evidently easierbetween some crystal structures and structural sites than betweenothers. In phase rule analysis of assemblages in such rocks it is commonlynecessary to treat Fe2O3, FeO, and MgO as separate componentsand it may also be necessary to regard CO2 as an inert componentand/or to interpret observed assemblages as of low variance.The presence of the Ca-Al silicates and sphene indicates verylow Xco2 in the metamorphic fluids in all rocks examined exceptan albite-chlorite-calcite-quartz-anatase assemblage. But higherAn in albites than in isofacial and in greenschist facies rocksof southern New Zealand can be ascribed to significantly higherXco2 at Lo?che, especially in the veins, than in New Zealand. Pumpellyite and epidotes of the pumpellyite-actinolite faciestend to be lower in Fe and richer in Al than those of lowergrade facies. Important reactions include those of the formpumpellyite-(Fe3+)+chlorite+quartz+H2=pumpellyite-(Al)+actinolite,and pumpellyite+chlorite+quartz- ‘epidote’+actinolite+water.Careful selection of pumpellyite and chlorite compositions isrequired for experimental and chemographic analysis of pumpellyitestability. In the absence of critical data, temperatures ofabout 250–350? and pressures of several kilobars are provisionallysuggested for the Lo?che metamorphism. 相似文献