摘要:

AbstractThe Berzosa fault is a major ductile shear zone, the Berzosa Shear Zone (BSZ), which separates the ‘Ollo de Sapo’anticline from the inner higher‐grade crystalline axis of the Iberian Hercynian Belt. This shear zone is the site of abundant early kinematic quartz (± Al‐silicates) segregations, rich in fluid inclusions. Host rocks are medium‐grade staurolite schists and sillimanite gneisses.Fluid inclusions in selected quartz segregations across the Berzosa shear zone have been studied by microthermometric methods as well as, in some instances, by Raman analysis. The recorded fluid inclusion history begins at the end of an intense secondary recrystallization period during late‐peak metamorphic conditions and lasts until late in the uplift history of the zone.Three types of inclusions have been found, which in a time sequence are: CO2± H2O; H2O+salt (B‐type); and, N2+CH4. Three types of B inclusion may be distinguished in turn, depending on whether they were trapped during an earlier dynamic‐recovery phase (B1‐type), formed later as intergranular trails (B2‐type), or were trapped apparently along with N2+CH4in clusions from a heterogeneous fluid (B3‐type).Considerations from isochores confirm that CO2± H2O inclusions were trapped during late‐peak and high‐Tretrograde metamorphic conditions (in the range 650–500°C and 5–2 kbar), whilst N2+CH4inclusions, along with the B3‐type of inclusions, formed at low‐pressures (<1 kbar) and temperatures (± 300°C). B2‐type inclusions were trapped chronologically between these two in a period in which strong inverse lateral thermal gradients developed in the zone. InferredP‐

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1986.tb00342.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY

摘要:

AbstractOxygen and hydrogen isotope analyses have been made of coexisting quartz, ilmenite, muscovite, and biotite from Late Precambrian metapelitic rocks, staurolite‐kyanite to K‐feldspar‐muscovite‐sillimanite zones, from Mica Creek, British Columbia. The δ18O and †D values of these minerals are generally uniform and do not decrease significantly with increasing metamorphic grade. This implies that there has not been significant infiltration of deep crustal, possibly magmatic, fluids into the metapelites that has been suggested for other high‐grade metamorphic terranes. The uniformity of oxygen isotope compositions of the Mica Creek metapelite rocks may reflect isotopic uniformity in the sedimentary protolith rather than widespread exchange with an isotopically homogeneous metamorphic pore fluid.Temperature estimates based upon18O exchange thermometry for samples below the sillimanite zone are in reasonable agreement with the results of garnet‐biotite Fe–Mg exchange thermometry. In the higher grade rocks, the oxygen isotope and garnet‐biotite thermometry yield results which disagree by about 100°C. The highest temperatures recorded by oxygen isotope thermometry, 595°C, are at least 60°C below the minimum temperatures required by phase equilibria. These discrepancies appear to result from pervasive equilibrium retrograde exchange of oxygen isotopes between coexisting minerals. In addition, there are problems with calibration of garnet‐biotite thermometry at higher temperatures. Retrograde oxygen isotope exchange may be a general characteristic of high‐grade metamorphic rocks and oxygen isotope thermometry may not usually record peak metamorphic temperatures if they s

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1986.tb00343.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY

摘要:

AbstractThe orthopyroxene‐clinopyroxene, garnet‐orthopyroxene and garnet‐clinopyroxene geothermometers, and the garnet‐orthopyroxene‐plagioclase, garnet‐clinopyroxene‐plagioclase and anorthite‐ferrosilite‐grossular‐almandine‐quartz geobarometers are applied to metabasites and the garnetplagioclase‐sillimanite‐quartz geobarometer is applied to a metapelite from the Proterozoic Arendal granulite terrain, Bamble sector, Norway.P–Tconditions of metamorphism were 7.3 ± 0.5 kbar and 800 ± 60°C.This terrain shows a regional gradation from the amphibolite facies, into normal LILE content granulite facies rocks and finally strongly LILE deficient granulite facies gneisses. NeitherPnorTvary significantly across the entire transition zone. The change in ‘grade’parallels the increasing dominance of CO2over H2O in the fluid phase.LILE‐depletion is not a pre‐condition of granulite facies metamorphism: granulites may have either ‘depleted’or ‘normal’chemistries. The results presented herein show that LILE‐deficiency in granulite facies orthogneisses is not necessarily related to variations in eitherPorT. The important mechanisms in the Arendal terrain were (a) direct synmetamorphic crystallization from magma, with primary LILE‐poor mineralogies imposed by the prevailing fluid regime, and (b) metamorphic depletion, involving scavenging of LILEs during flushing by mantle‐derived CO2‐rich fluids. The latter process is constrained by U–Pb and Rb–Sr isotopic work to have occurred no later than 50 Ma after intrusion of the acid‐intermediate gneisses, and was probably associated with contemporary basic magmatism in a tectonic environ

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1986.tb00344.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY

摘要:

AbstractA study has been made of the high‐pressure early‐Alpine re‐equilibration in the eclogites and metasedimentary cover of the Val d'Ala di Lanzo ophiolite. All of the main high‐pressure minerals have been analysed and their compositions used to determine re‐equilibration temperatures. The minimum conditions proposed (P= 1.3 GPa,T= 450–460°C) are also indicated by the presence of a jadeite+quartz‐bearing metagranite.The temperatures are compared with those reported for similar eclogites from the Voltri Group, the Aosta Valley and the Valais. Comparison of recalculated temperatures shows that the temperature (and probably the pressure) of the eclogitic re‐equilibration increased in the Aosta Valley and the Valais, in keeping with what has been observed in the internal Penninic basement of the Gran Paradiso and Monte Rosa cry

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1986.tb00345.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY

摘要:

AbstractPeridotite and infolded marble of the Seiad ultramafic complex were recrystallized in the upper amphibolite facies as part of the regional progressive metamorphism of the Rattlesnake Creek terrane. Field relations, including the occurrence of metarodingites, and metasomatic zones between dissimilar rock types, demonstrate that the metasediments and serpentinized ultramafic rocks were juxtaposed prior to regional, barrovian metamorphism. Temperatures are estimated to have reached 760–800°C at pressures of 7–8 kbar during the peak of metamorphism. Four low‐variance parageneses have been identified within a small (3 km2) area of the complex, which may reasonably be assumed to have formed under the samePandTconditions. IsobaricT‐Xco2diagrams of appropriate equilibria are presented for three different internally consistent sets of thermodynamic data. Despite the seemingly small numerical differences between the standard state thermodynamic properties of the data sets, only one diagram allows the four observed assemblages to coexist within a reasonable temperature range. All three phase diagrams require differences in fluid composition on the scale of a thin section; strong evidence for effective control of pore fluid composition by local mineral reactions during meta

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1986.tb00346.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY

摘要:

AbstractThis work uses a simplified model of equilibrium to predict the assemblage sequence and compositional zoning in garnet that should result from prograde metamorphism of common bulk compositions of pelitic rocks. An internally‐consistent set of model thermodynamic data are derived for natural mineral compositions from natural assemblages. Equilibrium assemblages can be calculated for pelitic compositions with excess quartz and either muscovite or K‐feldspar at any pressure and water pressure. The compositions and abundances of phases in equilibrium assemblages can be calculated where the elements Mg, Fe and Mn are exchanged among phases. The prograde metamorphic assemblage sequences and the effects of pressure on assemblages, predicted by the simulation method presented here, are similar enough to natural observations to suggest that the simulations can be used to analyse natural equilibrium and growth processes.The calculated phase diagrams at moderate and high crustal pressures explain the mineral assemblage sequence produced by prograde metamorphism in common pelitic compositions. Garnet appears by continuous reaction of biotite and chlorite as the garnet‐biotite‐chlorite divariant field migrates toward higher Mg/Fe ratios over the bulk composition. Staurolite appears in common bulk compositions when garnet and chlorite become incompatible. An aluminum silicate phase can appear when staurolite and chlorite react. Staurolite breaks down at an extremum point to produce garnet. Continuous reaction of biotite and sillimanite causes growth of abundant garnet.The reaction sequence involving garnet, staurolite and aluminum silicates is probably different at low pressure, but the main reason that staurolite and garnet are rare is the restricted compositional range over which their assemblages exist. Andalusite appears by the divariant reaction of chlorite and cordierite appears at low temperature in low pressure assemblages for common bulk compositions by the extremumpoint breakdown reaction of chlorite.Compositional zoning of garnet and the systematic variation of biotite composition in metamorphic sequences indicate that garnet is probably fractionated during growth. Fractionation of garnet causes garnet‐consuming, univariant reactions to become multivariant. The metastable persistence of garnet should reduce the abundance and stability range of staurolite. Fractionation of even small quantities of garnet should deplete the equilibrating bulk composition of Mn, but have little effect otherwise. The simulations show that the prograde assemblage sequence in pelitic rocks can be complex in detail, with some assemblages lasting over temperature intervals of only a few degrees.The major prograde reactions that release water are the breakdown of chlorite to form garnet at low grade and the breakdown of muscovite at high grade. The volume of water released by formation of garnet at high grade is also important. These reactions have the capacity to buffer water pressure.The density of anhydrous pelitic rock increases markedly when chlorite breaks down and by the continuous reaction forming garnet at high grade. The heat content is controlled principally by heat capacity and continuous reactions. Discontinuous reactions have little thermal buffering capacity.Simulations of garnet fractionation show that commonly‐observed garnet zoning profiles can be formed by garnet growth in the assemblage garnet‐biotite‐chlorite in common bulk compositions. A reversal of Fe‐zoning in garnet can occur when garnet resumes growth above staurolite grade in the assemblage garnetbiotite‐sillimanite. Discontinuities in zoning profiles can be caused only by disequilibrium. The disequilibrium can be due to either metastable persistence during a hiatus in growth or to growth by irreversible reaction.Because the appearance of garnet is controlled by a continuous rather than a discontinuous reaction, the appearance of garnet is very sensitive to bulk composition. The early development of garnet is also sensitive to the pressure and water pressure of metamorphism. As a consequence the first garnet isograd is of limited thermometric value.Metastable persistence of kyanite and manite at high grades could reduce the abundance of garnet and allow biotite to persist. Metastable persistence would also limit the of c

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1986.tb00347.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY

摘要:

MIGMATITES

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1986.tb00348.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1986.tb00349.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY