ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1993.tb00127.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractThe 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 fromc.450° C in the ankerite‐albite zone toc.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, χCO2decreases 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 χCO2of 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 (cm3fluid/cm2rock), 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 large

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1993.tb00128.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractAt the basement‐cover boundary of the north‐eastern Tauern Window (Eastern Alps), the following AlpineP‐T‐ddevelopment has been reconstructed on the basis of macro‐ and micro‐structures as well as preferred crystallographic orientations, mineral parageneses and compositions.During increasingP‐Tconditions in the greenschist facies a first period of deformation produced imbrication of the basement gneisses and cover sediments, and then monoclinal folds up to the kilometre scale. Tectonic transport was continuously top‐to‐the‐ENE. A second period of deformation began at about peakP‐Tconditions of 9 kbar andc.540–560°C in the south, and about 7–9 kbar and 490–500° C in the north; this continued locally to lower temperature. During the second period, transport was continuously top‐to‐the‐SE. Crystallographic orientations of white mica and plagioclase give particularly useful information on the kinematic framework. In addition, data on the ductile behaviour of dolomite and plagioclase can be inferred. Atc.7–9 kbar, dolomite recrystallization starts at 450–480° C, and the beginning of plagioclase recrystallization coincides with the oligoclase boundary.In general, the Alpine geodynamic history of the basement‐cover boundary may be related to continental collision processes between a northerly plate (European or Briançonnais) and a southerly (Adriatic) one. The first deformation period possibly reflects subduction of the gneiss‐sediment boundary toward the WSW, to a depth of 31–32 km. The second period may be a result of obduction toward the NW, followed by late‐stage uplift. Most of the basement domes of the eastern Tauern Window appear as a result of the final stage of the first deformation, formed prior to the peak of metamorphism, possibly partly influenced by the final collision

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1993.tb00129.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractFluids with compositions in the system CO2‐H2O‐NaCl were trapped in quartz veins enclosed in low‐grade metamorphic rocks (chlorite zone) on the southern flank of the Canigó Massif, eastern Pyrenees. The veins, which also contain arsenopyrite crystals, were formed contemporaneously with the main Hercynian foliation and metamorphism. Volumetric properties of the fluid and the results of arsenopyrite geothermometry suggestP‐Ttrapping conditions of 4.6–6 kbar and 450–530° C. This implies that an episode of metamorphism with an average geothermal gradient of 25° C km−1occurred during the main deformation event. This episode preceded the low‐P/high‐Tmetamorphism described around domes and to date considered as characteristic of the Hercynian oro

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1993.tb00130.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractIn the Adirondack Highlands of New York State, the effect of granulite facies metamorphism on the physical and isotopic characteristics of zircon from anorogenic plutonic rocks has a distinct geographical pattern. The location of zircon populations which appear to have been altered describes a roughly circular area where metamorphic palaeotemperatures have been determined to be in excess of 750° C. Zircons from anorogenic plutonic rocks outside this area were undisturbed during metamorphism and yield well constrained ages.Granitic, charnockitic and mangeritic anorogenic plutonic rocks peripheral to the Marcy anorthosite massif have large, euhedral, prismatic zircons that display fine, internal, magmatic growth zonations and abundant, randomly orientated, mineral inclusions. Co‐genetic zircon fractions yield linear discordant arrays and well constrained upper intercepts of 1125–1157 Ma. Metamorphic zircon is limited to sporadically developed and volumetrically insignificant, clear, low‐U overgrowths or protuberances.In marked contrast, zircons from petrographically and geochemically identical rocks adjacent to, or within, the Marcy anorthosite massif are typically large, limpid, anhedral to subhedral crystals or crystal fragments lacking internal features except for tubular cavities and CO‐2‐rich inclusions. Co‐genetic zircon fractions yield nearly concordant, non‐linear clusters with207Pb/206Pb minimum ages of 1073–1095 Ma. Metamorphic overgrowths cannot be readily identified by optical or cathodoluminscence techniques; however, many grains show complex and unusual external boundaries suggestive of post‐crystallization modification.These data indicate that temperatures as low as 750° C, in combination with other factors, may have been sufficient to facilitate recrystallization, and diffusion of radiogenic Pb from the zircon crystal structure, during the complex, protracted metamorphism of the A

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1993.tb00131.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractIn 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 recordP± 3 kbar andT± 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 isobaricT‐XCO2diagrams 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 × 104cm3/cm2and 4.8–12.9 × 104cm3/cm2, respectively. For an estimated duration of metamorphism of 105years, averagein situmetamorphic rock permeabilities, calculated from Darcy's Law, are 0.1–8 × 10−6D in the marbles and 10–27 × 10−6D 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

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1993.tb00132.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractThe Llano Uplift in central Texas is a Grenville aged (c.1.1 Ga) metamorphic terrane consisting predominantly of amphibolite facies mineral assemblages. The formation of these assemblages has been attributed to the emplacement of relatively late granite plutons throughout the area. Two types of granitic intrusion have previously been recognized: (1) Town Mountain Granites, which occur as relatively large, circular‐shaped bodies of coarse‐grained granite, and (2) Younger Granites which are present as smaller and more irregular bodies of finer‐grained granite. In the central part of the uplift, wollastonite‐bearing calc‐silicate rocks occur within the Valley Spring Gneiss. The development of these calc‐silicate rocks has been linked to infiltrating fluids presumably derived from spatially associated Younger Granites. The stability of coexisting quartz, calcite, wollastonite, grossular and anorthite and coexisting quartz, calcite, wollastonite, andradite and hedenbergite shows that the calc‐silicate rocks equilibrated under H2O‐rich conditions with χCO2<0.10. Fluid inclusions present within the calc‐silicate minerals are H2O‐rich with salinities of<17 wt% equivalent NaCl. The absence of any detectable CO2in the fluid inclusions may indicate entrapment of the inclusions at lower pressures and more H2O‐rich conditions compared to the stability of the peak metamorphic mineral assemblage. Homogenization temperatures, measured for texturally primary inclusions, range from 360 to 368° C corresponding to a density range from 0.53 to 0.82 g/cm3. Isochores for these fluid inclusions, when combined with the stability of the solid‐solid equilibria Grs + Qtz = Wo + An, yield formation conditions of 500–550° C at 1–2 kbar. This indicates that the granitic intrusions involved in the formation of the Blount Mountain calc‐silicates were emplaced at

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1993.tb00133.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractData are presented on a garnet population in a specimen of garnet‐biotite‐plagioclase‐quartz schist from the cordierite zone of an Archaean thermal dome in the Southern Slave Province of the Canadian Shield. Garnet crystals are bounded by planar dodecahedral faces and by trapezohedral faces which on the 10‐μm scale are corrugated. Crystal distribution, as revealed by dissection of a small cubic volume of rock, is random. The size distribution is normal, with a mean diameter of 0.81 mm and a standard deviation of 0.32 mm. In the largest crystal of the population (mean radius 0.83 mm), [Mn] = 100 Mn/(Fe + Mg + Mn + Ca) decreases from 14.5 at the centre to 7.5 and then increases in the outer margin to 8.5; [Fe]increases continuously from 67 at the centre to 77 at the surface; [Mg] increases from 12.5 to 13.5 and then falls sharply to 11; [Ca]remains unchanged at 4.0 and then drops to 3.3. Progressively smaller crystals have progressively lower [Mn] and higher [Fe]concentrations at their centres, while all crystals have the same margin composition. Growth vectors extending from given concentration contours to crystal surfaces are of equal length regardless of the size of the crystal in which the vector is located.A garnet‐forming model is presented in which reaction was initiated by a rise in temperature. Nucleation sites were randomly selected. The nucleation rate increased with time and then declined. Crystal faces advanced at a constant linear rate, which implies an increase in volume proportional to surface area. Initially, the composition of garnet deposited on crystal surfaces was determined by van Laar equations of equilibrium, which demanded the withdrawal of Mn and Fe from within chlorite crystals. This transfer reaction was then accompanied by an ion exchange reaction which moved Mn and Fe to garnet surfaces from biotite, in exchange for Mg. The exchange reaction provides an explanation for the high overall concentration of Mn and Fe in garnet and for the observed Mn and Mg reversals in the margins of crystals. The increase of garnet volume in the garnet population is found to be parabolic

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1993.tb00134.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractArchaean greenstone belts are often cut by major shear zones, for example the Cadillac tectonic zone (CTZ) of the southern Abitibi region in north‐western Quebec. At McWatters, the CTZ contains slices of metavolcanic units bounded by corridors of highly strained and altered rocks. Mineral assemblages of the metabasites record the metamorphic evolution of the CTZ.The McWatters metabasalts and metagabbros have similar chemistry but different mineral assemblages consisting of variable amounts of actinolite, hornblende, chlorite, albite, epidote, quartz, carbonates, titanite, biotite, rutile, magnetite, ilmenite and sulphides. The different mineral assemblages, which coexist in a single tectonic slice, can be divided into three types, characterized by (A) presence of hornblende and actinolite, (B) presence of actinolite and epidote, and (C) absence of amphibole and epidote. Partial replacements indicate that these mineral assemblages are not in equilibrium. The hornblende of the least altered and deformed samples of the type A assemblage is a relict of a prograde metamorphic event, contemporaneous with the development of the main schistosity. The prograde conditions are estimated atP= 5 kbar,T= 475° C with lowPf. The more altered and deformed samples of the type C assemblage record a later retrograde metamorphic event. Conditions of the later event are estimated atP= 4 kbar,T= 400° C with higherPf. Widespread calcite precipitation occurred during a later episode. The diversity of the mineral assemblages results from permeability variations along the high‐strain zones of th

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1993.tb00135.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractThe St Malo region in north‐west France contains migmatites and anatectic granites derived by partial melting of metasedimentary protoliths during Cadomian orogenesis atc.540 Ma. Previously reported Rb–Sr model ages for muscovite and biotite range fromc.550 toc.300 Ma, and suggest variable resetting of mineral isotopic systems. These rocks display microscopic evidence for variably intense Cadomian intracrystalline plastic strain but record no obvious evidence of penetrative Palaeozoic regional deformation.40Ar/39Ar mineral ages have been determined to evaluate better the extent, timing and significance of Palaeozoic overprinting.Eleven muscovite concentrates and one whole‐rock phyllite have been prepared from various units exposed in the St Malo and adjacent Mancellian regions. In the Mancellian region, muscovite from two facies of the Bonnemain Granite Complex record40Ar/39Ar plateau ages ofc.527 and 521 Ma. An internally discordant40Ar/39Ar release spectrum characterizes muscovite from protomylonitic granite within the Cadomian Alexain‐Deux Evailles‐Izé Granite Complex, and probably records the effects of Variscan displacement along the North Armorican Shear Zone. Muscovite concentrates from anatectic granite and from Cadomian mylonites along ductile shear zones within the north‐western sector of the St Malo region exhibit internally discordant40Ar/39Ar release spectra which suggest variable and partial late Palaeozoic rejuvenation. By contrast, muscovite concentrates from samples of variably mylonitic Brioverian metasedimentary rocks exposed within the south‐eastern sector of the St Malo region display internally concordant apparent age spectra which define plateaux of 326–320 Ma. A whole‐rock phyllite sample from Brioverian metasedimentary rocks exposed along the eastern boundary of the St Malo region displays an internally discordant argon release pattern which is interpreted to reflect the effects of a partial late Palaeozoic thermal overprint. Muscovite from the Plélan granite, part of the Variscan Plélan‐Bobital Granite Complex, yields a40Ar/39Ar plateau age ofc.307 Ma.The40Ar/39Ar results indicate that Cadomian rocks of the St Malo region have undergone a widespread and variable Palaeozoic (Carboniferous) rejuvenation of intracrystalline argon systems which apparently did not affect the Mancellian region. This rejuvenation was not accompanied by penetrative regional deformation, and was probably of a static thermal–hydrothermal origin. The heat source for rejuvenation was probably either the result of heating during Variscan extension or advection from Variscan granites which are argued to und

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1993.tb00136.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY