摘要:

AbstractLayer‐parallel (i.e. parallel to foliation or bedding) vein formation in the graywackes and pelites of the Quetico Metasedimentary Belt occurred during synchronous prograde metamorphism and regional (D2) compression. In a traverse across metasediments which change in metamorphic grade from greenschist to upper amphibolite (migmatite) facies, layer‐parallel veins show the following trends: (1) an increase in thickness and internal complexity, the latter due to successive boudinage; (2) low‐grade veins are parallel to planes of anisotropy due to the original sedimentary fabric of the host rocks, but at higher grades other sites are also used and (3) a systematic increase in plagioclase/quartz ratio in the veins towards higher grade, adjacent mafic selvedges first exhibit quartz depletion then, in the amphibolite facies, plagioclase depletion. Mineralogical zoning is often preserved in a single vein, older parts are more quartz‐rich than younger.Mass balance calculations and whole‐rock geochemistry based on veins, mafic selvedges and country rock are consistent with a closedsystem subsolidus segregation origin. The layerparallel veins are syntectonic, and migration of the mobile components required to form their mineralogy is a stress‐induced mass transfer. The source of these components appears to be dominantly pressure solution of the same minerals in the host rocks, although metamorphic reactions may also have contributed. Veins nucleated first at those sites where initial sedimentary heterogeneites, such as fine‐scale graded bedding, provided gradients of normal stress across grain boundaries, and hence of chemical potential, necessary to drive the subsolidus segregation process. The earliest veins are thus parallel to bedding. Later, nucleation of the veins could also occur along more randomly distributed sites within the metasediments, and these veins grew parallel to the schistosity rather than bedding, if the two were distinct. Once formed, the veins themselves, which are more competent than the surrounding rock, provide the stress heterogeneity required for their further growth. The increasing plagioclase/quartz ratio in the veins may be due to a temperature dependent increase in plagioclase component mobility relative to quartz. Alternatively, the increasing transfer distances for silica, resulting from prior quartz depletion in the inner parts of the mafic selvedge, may increase the relative mobility of plagiocl

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1986.tb00350.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY

摘要:

AbstractCompositions of actinolite, hornblende and cummingtonite, together with pyroxene and plagioclase, are studied in basic intrusions in the Dalradian of north‐east Scotland, and the Glen Scaddle complex in the West Moine. Amphibolitization is due to influx of water from the country rocks. Pyroxene compositions are found to have adjusted to the regional metamorphic environment. Owing to the difficulty of diffusion of Al and Si, calcic amphiboles are zoned and commonly contain quartz blebs. Discontinuities in zoning give rise to actinolite‐hornblende pairs. Compared with north‐east Scotland, disequilibrium is less strong in the Glen Scaddle area: in the latter, plagioclase compositions have been greatly changed, Na partition between hornblende and plagioclase is close to equilibrium, the maximum Al content of hornblende is lower and zoning patterns are more consistent. The Fe/Mg ratio in calcic amphiboles varies with Al content, while approaching equilibrium partition with other minerals. Both zoning patterns and Fe/Mg partition with cummingtonite suggest that Fe/Mg of the calcic amphiboles increases more strongly with increasing (Alvi+Fe3+) than can be explained simply by substitution of Al,Fe3+for Mg on M2. Model reactions for amphibole formation are constructed. Cummingtonite formed at lower chemical potential of CaO than actinolite: Ca was exchanged for Mg,Fe between orthopyroxene‐derived and clinopyroxene‐derived local systems. Both cummingtonite and actinolite were formed because of kinetic constraints, as intermediate reaction products: actinolite‐hornblende pairs represent disequilibrium. This work suggests that many occurrences of actinolite with hornblende, where the minerals are zoned, may also be due to diffusi

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1986.tb00351.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY

摘要:

AbstractContact aureoles of plutons emplaced into regionally metamorphosed terranes can provide indicators of physical conditions along a portion of regional metamorphicP‐Ttrajectories, thereby allowing reconstruction of more completeP‐Tloops than would be otherwise possible. In the Northern Alabama Piedmont of the southern Appalachians, Wedowee Group metapelites preserve evidence for two regional metamorphic phases overprinted by contact metamorphism adjacent to the Blakes Ferry Trondhjemite. Textural evidence indicates that an early bt+st+grt assemblage was replaced by bt+chl+grt during the latter stages of regional metamorphism. Changes in AKFM topology, complex Fe‐Mg‐Ca garnet zoning, and the latestage appearance of epidote indicate that a sequence of continuous reactions (bt+st = grt+ ms followed by chl+ms+Ca‐grt+Ca‐pl=bt+ (Fe+Mg)‐grt+ep) occurred in response to increasing pressure and resulted in the observed changes in mineral assemblage. Pl‐ms‐bt‐grt thermobarometry indicates conditions of 580° 65°C, 8.5±0.8 kbar for equilibration of grt+ bt+chl. Pluton emplacement, subsequent to penetrative deformation, caused textural annealing and mineral re‐equilibration by the continuous reaction bt+(Fe+Mg)‐grt+ep = chl+ms+Ca‐grt+Ca‐pl within 50 m of the pluton. Conditions of 510±65°C, 5‐7 kbar are inferred. A reconstructedP‐Ttrajectory for this area is characterized by (1) early moderate‐T, moderate‐Pmetamorphism; (2) an increase inPto approximately 8.5 kbar; and (3) decompression and slight cooling prior to pluton emplacement. The compressional phase of this path is interpreted to result from underthrusting of the Wedowee metasediments to mid‐crustal levels during Palaeozoic crustal thickening. Late‐stage decompression prior to intrusion records uplift of these rocks in respon

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1986.tb00352.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY

摘要:

AbstractMineralogical and petrological studies of Triassic Verrucano metasediments of the Northern Apennines are reported. The widespread occurrence of Al‐silicates allows the delineation of four metamorphic zones with increasing metamorphic grade: (1) kaolinite zone (well Perugia 2, Umbria); (2) kaolinite‐pyro‐phyllite zone (Monte Argentario and part of the Verrucano of the Monticiano‐Roccastrada area and Monti Leoni); (3) pyrophyllite zone (Monti Pisani, Iano, Monti Leoni, the Monticiano‐Roccastrada area and some wells in the Larderello region); (4) kyanite zone (Massa area and some wells in the Larderello area).The four metamorphic zones correspond to temperatures ranging from 300°C to about 450°C. On the basis of the Si content of muscovite and geological arguments, pressures of between 3 and 5 kbar are estimated. The metamorphic zones are located more or less parallel to the bent north‐west‐south‐east trending structural zonation of the Northern Apennines, with the concave side towards the Tyhrrenian Sea.During the Alpine orogeny, the Verrucano metasediments underwent three folding phases each of which has produced an axial plane schistosity (S1, S2, S3). During the first folding phase the Verrucano sediments were buried increasingly deeply within the crust from east to west. The climax of Alpine metamorphism was attained prior to the second folding phase with crystallization of porphyroblasts of kyanite and chloritoid in a central area located between Massa and Larderello. The inferred paleo‐temperature distribution pattern resembles an asymmetric thermal high defined by the kyanite zone, and surrounded by the pyrophyllite zone. A similar pattern is still present in the Tuscan crust, as indicated by a series of geothermal anomalies passing through the

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1986.tb00353.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY

摘要:

AbstractGneisses from the southern part of the Bunte Series have been studied in order to determine their metamorphic evolution. They all belong to the paragenetic facies type Bt+Sil+ Kfs+Qtz+Grt. The garnets of these rocks generally exhibit typical features of garnets from this high‐temperature facies type: diffusional homogenization throughout the bulk of the garnet and retrograde zoning patterns at the outer rim. Ca‐ and Mn‐richer relict cores in some garnets are evidence of the early metamorphism. These cores are enveloped by the high‐temperature homogenized garnet. The element distribution curves of the relict cores show evidence of partial element‐specific modification by volume diffusion.Paragenetic and textural relations as well as temperature and pressure calculations suggest the following conditions of the main metamorphism: 700–770°C at 7‐9 kbar andPH2OPtotDespite the uncertainty due to the incomplete equilibration among garnet rims and other matrix minerals, an attempt is made to model the cooling and uplift history of the rocks using textural, paragenetic, chemical and literature data.ZusammenfassungGneise aus dem südlichen Teil der Bunten Serie wurden im Hinblick auf ihre Metamorphoseentwicklung untersucht. Sie gehören allgemein dem Faziestyp Bt+Sil+Kfs+Qtz+Grt an. Die Granate weisen alle für diesen Faziestyp charakteristischen Merkmale auf: weitgeheride Homogenisierung durch Diffusion und Abkühlungserscheinungen am äuβeren Rand. Ca‐ und Mn‐ reiche reliktische Kerne mancher Granate weisen auf ein älteres Ereignis hin. Diese Relikte werden im homogenisierten Granat höherer Bildungstem‐peratur eingeschlossen. Ihre Element‐ Verteilungskurven sind teilweise durch die intrakristalline Diffusion dem Kationenradius entsprechend abgeflacht.Paragenetische Beziehungen undT‐ undP‐Berechnungen ergeben folgende Bedingungen für die Hauptmetamorphose: 700–770°C bei 7‐9 kbar undPH2OPtot.Trotz der Unsicherheit, die auf eine unvoll‐ständige Einstellung des Gleichgewichtes zwischen Granaträndern und den übrigen Matrixmineralen zurückzuführen ist, wurde ver‐sucht, ein Modell über die Abkühlungs‐ und Heb

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1986.tb00354.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY

摘要:

AbstractLewisian grey gneisses from Gruinard Bay, North‐west Scotland retain mineralogical and geochemical evidence for Scourian horn‐blende‐granulite facies metamorphism, and they may be used to assess current models of elemental depletion at granulite grade. Their ‘immobile’major and trace element geochemistry is indistinguishable from that of Lewisian amphibolite and pyroxene‐granulite facies counterparts. The K, Rb, Th and U contents of the Gruinard Bay gneisses are depleted relative to amphibolite facies gneisses, but generally the abundances of these elements are above those of comparable pyroxene granulites. U and Th have reached an advanced stage of depletion, but allanite appears to be crucial in maintaining significantly higher U and Th abundances at Gruinard Bay than in pyroxene granulites. K and Rb loss is less extreme, and depends on the stability of the rock‐forming minerals: K‐feldspar; biotite; and, amphibole. Early removal of K and Rb has resulted in a small rise in K/Rb, but further preferential Rb loss would have been required to generate the characteristically high K/Rb ratios of Lewisian pyroxene granulites.The residence of U and Th in the accessory minerals of granulite facies gneisses, which are often correlated with the residua of intracrustal partial melting, renders unlikely their extreme incompatibility required by such models. Even if such phases are ignored, high mineral‐melt partition coefficients for silicic melts argue against partial fusion as an efficient depletion mechanism. On the other hand, the advanced stage of U and Th depletion reached in Gruinard Bay gneisses, which were still partly hydrous, severely restricts the role played by CO2‐dominated fluids and a hydrous

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1986.tb00355.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.tb00357.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY