摘要:

AbstractThis, the first two papers, sets out the philosophy and methods of determining an internally consistent thermodynamic dataset for minerals using the least squares method. The applicability of the least squares method is discussed, and it is applied to a small set of experimental equilibria in the system Na2O–Al2O3–SiO2–H2O. The importance is stressed of defining not only the enthalpies of formation of minerals, but also the uncertainties and the correlations among them. The system which has been used as an illustration for this paper serves as a visual guide to the method, as it is small enough to represent graphically in two dimensions. In the paper which follows, we extend the method to a system of 60 equations (experimentally determined equilibria) involving 34 unknowns (enthalpies of formation of mineral end‐m

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1985.tb00324.x

出版商:Blackwell Publishing Ltd    

年代:1985

数据来源: WILEY

摘要:

AbstractThis, the second of two papers, represents the application of a least squares approach, discussed in the previous paper, to the generation of an internally consistent thermodynamic dataset involving 60 reactions among 43 phases, in the system K2O–Na2O–CaO–MgO–Al2O3–SiO2–H2O–CO2. We make the assumption that all the thermodynamic data, with the exception of enthalpies of formation of the phases, are well known, and solve for an internally consistent set of enthalpies which reproduces the 60, experimentally determined, phase equilibrium reactions. An important difference between our dataset and that of previous alternatives in the literature is that we are able to determine the uncertainties on, and correlations between, the enthalpies of formation for all phases in the set, and hence are able to apply simple error propagation techniques to determine the uncertainties in any phase equilibrium calculations performed using this dataset. Selection of reactions, for geothermometry and geobarometry, may be more readily made by choosing equilibria with small uncertainties in their thermodynamics. Our data are in reasonably close agreement with the high temperature molten oxide calorimetry results on silicate minerals where available, a fact which lends a degree of confidence t

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1985.tb00325.x

出版商:Blackwell Publishing Ltd    

年代:1985

数据来源: WILEY

摘要:

AbstractThe Pennine rocks exposed in the south‐east Tauern Window, Austria, contain mineral assemblages which crystallized in the mid‐Tertiary ‘late Alpine’regional metamorphism. The pressure and temperature conditions at the thermal peak of this event have been estimated for rocks at four different structural levels using a variety of published and thermochemically derived geobarometers and geothermometers. The results are:(a) In the garnet+chlorite zone, 2–5 km structurally above the staurolite+biotite isograd:T= 490.50°C,P= 7° 1 kbar;(b) Within 0.5 km of the staurolite+biotite isograd:T= 560±300C,P=7.1 kbar;(c) In the staurolite+biotite zone,c.2.5 km structurally below the staurolite+biotite isograd:T= 610±30°C,P=7.6±1.2 kbar;(d) In the staurolite+biotite zone, 3–4 km structurally below the staurolite+biotite isograd:T= 630±40°C,P= 6.6±1.2 kbar.The pressure estimates imply that the total thickness of overburden above the basement‐cover interface in the mid‐Tertiary wasc.26.4 km. This overburden can only be accounted for by the Austro‐Alpine units currently exposed in the vicinity of the Tauern Window, if the Altkristallin (the ‘Middle Austro‐Alpine’nappe) was itself buried beneath an ‘Upper Austro‐Alpine’nappe or nappe‐pile which was 7.4 km thick at that time.The occurrence of epidote + margarite + quartz pseudomorphs after lawsonite in garnet, indicates that part of the Mesozoic Pennine cover sequence in the south‐east Tauern experienced blueschist‐facies conditions (T<450°C,P<12 kbar) in early Alpine times. Evidence from the central Tauern is used to argue that the blueschist‐facies imprint post‐dated the main phase of tectonic thickening (D1A) and was thus a direct consequence of continental collision.Combined oxygen‐isotope and fluid‐inclusion studies on late‐stage veins, thought to have been at lithostatic pressure and in thermal equilibrium with their host rocks during formation, suggest that they crystallized from aqueous fluids at 1.1±0.4 kbar and 420.20°C.Early Alpine, late Alpine and vein‐formationP–Tconstraints have been used to construct aP–Tpath for the base of the Mesozoic cover sequence in the south‐east Tauern Window. The prograde part of theP–Tpath, between early and late Alpine metamorphic imprints, is unlikely to have been a smooth curve and may well have had a low dP/dToverall; the decompression (presumably due to erosion) which occurred immediately before the thermal peak and possibly also earlier in the Tertiary, was probably partly or completely cancelled by the effects of early‐ to mid‐Tertiary (D2A) tectonic thickening. The thermal peak of metamorphism was followed by a phase of almost isothermal decompression, which implies a period of rapid uplift in the middle Tertiary.The peak metamorphicP–Testimates are compared with the solutions of England's (1978) one‐dimensional conductive thermal model of the Eastern Alps, and are shown to be consistent with the idea that the late Alpine metamorphism was caused by tectonic burial of the Pennine Zone beneath the Austro‐Alpine nappes

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1985.tb00326.x

出版商:Blackwell Publishing Ltd    

年代:1985

数据来源: WILEY

摘要:

AbstractExisting geochronological data are reviewed and new Rb‐Sr, K‐Ar and39Ar–40Ar ages are presented, including a suite of 33 mica ages from a 20 km north–south tunnel section. These data are discussed in relation to the thermal history from the overthrusting of the Autroalpine nappesc.65 Myr ago to the present. The earliest phase of metamorphism, involving lawsonite crystallization, is associated with emplacement of these nappes. Subsequently, temperatures in the rocks beneath rose, at a mean rate of 3–6°C/Myr, until the climax of metamorphism.At high structural levels, published data indicate an age>35 Myr for the metamorphic climax. In contrast, a new39Ar–40Ar step‐heating age of 23.8 ± 0.8 Myr on amphibole, from near the base of Peripheral Schieferhülle, closely approximates the age of metamorphism and provides the first clear indication that the climax of metamorphism occurred later at deeper structure levels. Following the climax, near‐isothermal uplift and erosion reduced pressure toc.1 kbar before white mica closure at 19 Myr; this implies uplift at>3 mm/yr.Along the tunnel section, white mica K‐Ar ages vary systematically from 24 Myr to 16.5 Myr with position relative to a late 4 km amplitude dome whereas biotite Rb‐Sr ages are uniform at 16.5 Myr across the whole profile; doming is thus dated at 16.5 Myr with transient uplift rates>5 mm/yr. At other times upl

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1985.tb00327.x

出版商:Blackwell Publishing Ltd    

年代:1985

数据来源: WILEY

摘要:

AbstractNew occurrences of crossite and jadeitic pyroxene are described from a thick metabasite unit within the upper levels of the Peripheral Schieferhülle in the Tauern Window, Austria. Unusual textures are preserved which provide evidence for the reactions and mechanisms involved in the breakdown of crossite and jadeitic pyroxene. Zones of albite and chlorite, produced by reaction between crossite and paragonite, have been preserved due to sluggish reaction kinetics during decompression from the blueschist to the greenschist facies. The zonal sequence is interpreted in terms of chemical potential gradients in Na, Mg and Al, which have been established by overstepping the equilibrium boundary. Breakdown textures of jadeite‐acmite pyroxene to a symplectite of albite + hematite + actinolite, and of crossite to talc and actinolite are also described.The occurrence of crossite and jadeitic pyroxene at high levels within the Peripheral Schieferhülle implies that even upper levels of the structural sequence have undergone blueschist facies metamorphism with pressures in excess of 8 kbar during the Alpine collisional ev

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1985.tb00328.x

出版商:Blackwell Publishing Ltd    

年代:1985

数据来源: WILEY

摘要:

AbstractPetrological data from intercalated pelitic schists and greenstones are used to construct a pressure–temperature path followed by the Upper Schieferhülle (USH) series during progressive metamorphism and uplift in the south‐west Tauern Window, Italy. Pseudomorphs of Al–epidote + Fe‐epidote + albite + oligoclase + chlorite after lawsonite and data on amphibole crystal chemistry indicate early metamorphism in the lawsonite‐albite‐chlorite subfacies of the blueschist facies atP± 7–8 kbar. Geothermometry and geobarometry yield conditions of final equilibration of the matrix assemblage of 475±25°C, 5–6 kbar; calculations with plagioclase and phengite inclusions in garnet indicate early garnet growth at pressures of ∼ 7.5 kbar. Garnet zoning patterns are complex and reversals in zoning can be correlated between samples. Thermodynamic modelling of these zoning profiles implies garnet growth in response to four distinct phases of tectonic activity. Fluid inclusion data from coexisting immiscible H2O–CO2–NaCl fluids constrain the uplift path to have passed through temperatures of 380 + 30°C at 1.3 + 0.2 kbar.There is no evidence for metamorphism of USH at pressures greater than ∼ 7.5 kbar in this area of the Tauern Window. This is in contrast to pressures of ± 10 kbar recorded in the Lower Schieferhülle only 2–3 km across strike. A history of differential uplift and thinning of the intervening section during metamorphism is necessary to reconcile theP–Tdata obtained

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1985.tb00329.x

出版商:Blackwell Publishing Ltd    

年代:1985

数据来源: WILEY

摘要:

AbstractThe chemical evolution of garnets from pelitic rocks of probable Palaeozoic age corresponds to a complex metamorphic evolution of the host rocks.Among the almandine‐rich garnets (Alm60–80), two main types of evolution can be distinguished. Early Mn‐rich garnets coexisting with kyanite may be replaced by plagioclase and then, during a late stage, by biotite and/or sillimanite. The second type of evolution corresponds to an overgrowth of Mn‐poor late‐stage garnet on older Mn‐rich garnets which corresponds to a thermal peak with sillimanite‐type of metamorphism. This new garnet may appear either as an overgrowth with a strong discontinuity, or as small, new euhedral garnet or as skeletal garnet.This chemical evolution of garnet corresponds to an early collisional stage of metamorphism (of high pressure type with high Mn values) of probable Ordovician age followed by uplift and a thermal peak (low Mn values) in D

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1985.tb00330.x

出版商:Blackwell Publishing Ltd    

年代:1985

数据来源: WILEY

摘要:

AbstractSmall unexploited copper‐lead‐zinc deposits, characterized by a distinctive wall‐rock association of cordierite quartzite, silica‐undersaturated rocks, calc‐silicate rocks and impure marbles, occur in quartzofeldspathic gneisses and mafic granulites of the Strangways Metamorphic Complex, central Arunta Block, central Australia. Available data support the hypothesis that these are metamorphosed volcanogenic ore bodies. The chemical compositions of the quartzofeldspathic gneisses are comparable with those of less metamorphosed felsic igneous rocks, particularly the felsic igneous rocks emplaced in the North Australian Orogenic Province in the interval 1880–1800 Ma; and the mafic granulites are chemically similar to basalts (olivine‐normative tholeiites). The wall‐rock suite can be correlated from chemistry and lithological association with the suites of wall rocks found in unmetamorphosed volcanogenic ore deposits. That the protolith of the cordierite quartzites may well have been leached tuff, similar to the illite‐chlorite‐quartz tuff found in volcanogenic ore deposits, is also shown by retrogression of the granulitefacies assemblage: cordierite‐garnet‐ortho‐pyroxene‐biotite‐quartz in the cordierite quartzites to cordierite‐anthophyllite‐bearing assemblages and thence to chlorite‐muscovite‐quartz assemblages. Lenses of silica‐undersaturated rocks with spinel and, less commonly, sapphirine are interpreted as the metamorphosed equivalents of chlorite‐rich pods found within leached tuffs in volcanogenic ore deposits. The wall rocks form sheet‐like bodies; this suggests that they were deposited in relatively shallow water, thus pr

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1985.tb00331.x

出版商:Blackwell Publishing Ltd    

年代:1985

数据来源: WILEY

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1985.tb00332.x

出版商:Blackwell Publishing Ltd    

年代:1985

数据来源: WILEY

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1985.tb00333.x

出版商:Blackwell Publishing Ltd    

年代:1985

数据来源: WILEY