摘要:

AbstractEdenite/tremolite and edenite/magnesio‐hornblende in equilibrium with plagioclase, chlorite, epidote, quartz and vapour involve several types of reactions for whichKDcan be related toTandP.Thermodynamic calculation of these equilibria leads to isopleth systems. Given knowledge of the progressive changes of end‐member activities in zoned Ca–Mg amphiboles (based on microprobe analyses), it is possible to construct precise pressure–temperature–time paths (P–T–tpaths) which have been followed by metabasites during polyphase metamorphism. When applied to basic rocks from the River Vilaine area, this method allows us to construct aP–T–tpath that can be compared directly to theP–T–tpath constructed from interbedded acid rocks (aluminous micaschists) in the same structural unit. Through time, both basic and acid rocks underwent the same complex deformation history that can be described conveniently in the L–S fabric system of Flinn. This allows us to construct aP–T–tdeformation path for this structural unit.These paths are interpreted in terms of an under/overthrusting continental collision belt (the Hercynian belt), and represent an illustration of the time delay caused by stacking of

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1988.tb00412.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractA detailed study of garnet–chloritoid micaschists fom the Sesia zone (Western Alps) is used to constrain phase relations in high pressure (HP) metapelitic rocks. In addition to quartz, phengite, paragonite and rutile, the micaschists display two distinct parageneses, namely garnet + chloritoid + chlorite and garnet + chloritoid + kyanite. Talc has never been observed. Garnet and chloritoid are more magnesian when chlorite is present instead of kyanite. The distinction of the two equilibria results from different bulk rock chemistries, not fromP–Tconditions or redox state. EstimatedP–Tconditions for the eclogitic metamorphism are 550–600°C, 15–18 kbar.The presence of primary chlorite in association with garnet and chloritoid leads us to construct two possibleAFMtopologies for the Sesia metapelites. The paper describes a KFMASH multisystem for HP pelitic rocks, which extends the grid of Harte&Hudson (1979) towards higher pressures and adds the phase talc. Observed parageneses in HP metapelites are consistent with predicted phase relations. Critical associations are Gt–Ctd–Chl and Gt–Ctd–Ky at relatively low temperatures and Gl–Chl–Ky and Gt–Tc–Ky at re

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1988.tb00413.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractTextural relationships between porphyroblasts of biotite and garnet in metasediments in the Nordkinn Peninsula area of the Finnmarkian Caledonides of North Norway are apparently complex. There is evidence for two textural zones in both mineral phases and superficially the development of these appears to have overlapped, at least in part, in time and space. This apparently complex porphyroblast growth history can be considerably simplified if only one period of garnet growth occurred and if different inclusion fabrics developed where garnet replaced biotite porphyroblasts and where it overgrew the matrix foliation. The possibility that porphyroblasts with textural evidence for multiphase growth histories actually grew during a single crystallization event is of importance in the interpretation and elucidation of tectonometamorphic relationships.

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1988.tb00414.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractThis paper provides methods and a description of a Pascal computer program,thermocalc, for various thermodynamic calculations using the thermodynamic dataset presented in earlier papers in this series (Holland&Powell, 1985; Powell&Holland, 1985). The dataset involves uncertainties on the thermodynamic parameters and therefore allows uncertainties to be calculated on results, for example in geothermometry and geobarometry. Recommendations are made for the uncertainties on activities to be used in calculations on rocks, particular emphasis being placed on preventing underestimates of these uncertainties at small mole fractions. Apposite examples of phase diagram and rock calculations are presented with ouput fromthermocalc, demonstrating the utility of the program. Of the rock calculations, the most valuable are considered to be those involving simultaneous combination ‘least squares’of calculated conditions for a set of reactions applicable to a rock. This set of reactions involves the independent reactions which can be written between the end‐members in the minerals in a rock and in the thermodynamic dataset. In contrast to an approach based on specific geothermometers and geobarometers, this approach maximizes the benefit of having an internally consistent thermodynamic dataset.thermocalcis available in IBM PC and Mac versions, from Roger Powell for A$25 or Tim Holland for £10 per v

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1988.tb00415.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractThe Western Baja terrane (WBt) of west‐central Baja California is an uplifted subduction complex that is divided into smaller ‘subterranes’on the basis of bounding faults and petrological differences. Each subterrane contains coherent Early Jurassic to Early Cretaceous sedimentary and mafic volcanic rocks (not melange) that have been metamorphosed under blueschist facies conditions. Key phases in metabasites and metaturbidites include jadeitic to acmitic clinopyroxene, sodic amphibole, lawsonite, aragonite, chlorite, titanite and white mica. Pressure indicators include the jadeite content of clinopyroxene and the presence of aragonite. Temperature indicators include the presence of lawsonite, the absence of greenschist facies minerals and results from vitrinite reflectance studies. Conditions at the peak of metamorphism were>8 kbar, 225–325°C for subterrane 1, 7–8 kbar, 170–220°C for subterrane 2, and 5–6 kbar, 175–200°C for subterrane 3; these correspond to cold geothermal gradients (6–9/km). Vein assemblages that include aegerine–jadeite and aegerine, albite, aragonite, lawsonite and sodic amphibole indicate uplift during continued cold conditions, probably during s

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1988.tb00416.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractRetrograde and prograde mineral assemblages from metapelitic and metabasic rocks of the Iforas Granulitic Unit (Mali) were generated by the superimposition of two granulite facies metamorphic events. They clearly result from a polycyclic evolution and can be related to a late Eburnean unroofing followed by a Pan‐African burial.Thermobarometry on Pan‐African garnet‐bearing assemblages yields (P, T) estimates of 620±50°C and 5± Ikbar. The nearly anhydrous conditions produced in the Eburnean appear to be the direct cause of the unusually lowtemperature granulite‐facies metamorphism in the Pan‐African. TheseP, Testimates are compared with those obtained on the underlying unit (Kidal Assemblage) upon which the Iforas Granulitic Unit was thrust.A P‐T‐tpath, during the Pan‐African orogeny, is proposed and discussed for both the Iforas Granulites an

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1988.tb00417.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractLow‐pressure prograde metamorphism of pelitic rocks in the Cooma Complex, south‐east Australia, has produced cordierite‐andalusite schists at intermediate grades. The first foliation (S1) is preserved largely as inclusion trails in cordierite porphyroblasts. Microstructural evidence indicates that the cordierite porphyroblasts grew during the early stages of development of a crenulation‐foliation (S2) and that andalusite porphyroblasts grew during the development of a later crenulation‐foliation (S3). Microstructural evidence also indicates that the andalusite was a product of the prograde reaction: cordierite + muscovite ± andalusite + biotite + quartz. The occurrence of the products of this reaction in ‘beard’structures between cordierite microboudins formed by extension in S3confirms that the andalusite grew during the development of S3. The investigation shows that porphyroblast‐matrix relationships can preserve the orientation of an early S‐surface that has been largely obliterated from the matrix, as well as providing relatively direct evidence of sequential mineral growth and met

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1988.tb00418.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

LOW TEMPERATURE METAMORPHISM. Edited by M. Frey. Blackie&Son Limited, Glasgow and London. 1987. pp. 364.

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1988.tb00419.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1988.tb00420.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY