ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1986.tb00334.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY

摘要:

AbstractMctamorphic rocks of the St Anthony Complex of north‐western Newfoundland are best interpreted in terms of a high‐temperature shear zone formed between down‐going continental margin rocks and overriding oceanic lithosphere in a subduction zone. High‐grade rocks, immediately beneath the oceanic lithosphere peridotite, display retrograde meta‐morphism in high‐strain zones, whereas lower grade rocks, near the base of the metamorphic complex, display prograde metamorphism in high‐strain zones. Mylonite zones in meta‐basitcs at all levels in the complex contain the assemblage epidote‐hornblende‐albite‐sodic oligoclase. These observations suggest that the ‘inverted metamorphic gradient’within the St Anthony Complex results from the fortuitous preservation of residual metamorphic assemblages from different crustal levels within an epidote amphibolite facies shear zone. The degree of re‐equilibration is strongly dependent on the degree of strain, and is best achieved in synmetamorphic mylonite zones. This interpretation of the St Anthony Complex can be extended to other sub‐ophiolite metamorphic sheets, which show very similar relationships. It is proposed that most metamorphic sheets beneath ophiolites are high temperature shear zones, theP‐Tpaths of which preserve records of burial and

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1986.tb00335.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY

摘要:

AbstractThe paragenetic relations of epidote‐glaucophane schists are described in terms of the system Al2O3‐Fe2O3‐Fe2O3‐MgO‐CaO with excess of quartz, albite and epidote. If alkali‐amphibole is free from Ca and AlIV, its composition when associated with epidote is invariant, univariant or divariant at a given pressure and temperature on Miyashiro's (1957) diagram of alkali‐amphibole solid solution if it is also associated, respectively, with three, two or one additional minerals in the system.Using a group of epidote‐glaucophane schists from the Kotu area of the Sanbagawa metamor‐phic belt in Shlkoku, Japan (isophysical compositional),univariant boundary lines were determined for the assemblages that, in addition to the ubiquitous quartz + albite + phengitic mica, contain hematite + chlorite, garnet + chlorite and actinolite + chlorite, respectively. The slopes of the univariant boundary lines obtained from petrographical data are in good agreement with those calculated in a model system.The positions of isophysical univariant boundary lines on the amphibole compositional diagram serve to distinguish the grade of metamorphism among the rocks of the same mineral facies. The hematite‐chlorite univariant boundary line can be used to divide the zone of epidote‐glaucophane schists of the Sanbagawa metamorphic belt into three, and the garnet‐chlorite‐paragonite invariant equilibrium can be used to divide the epidote zone of

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1986.tb00336.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY

摘要:

AbstractIn regional metamorphic rocks, the partitioning of deformation into progressive shearing and progressive shortening components results in strain and strain‐rate gradients across the boundaries between the partitioned zones. These generate dislocation density gradients and hence chemical potential gradients that drive dissolution and solution transfer. Phyllosilicates and graphite are well adapted to accommodating progressive shearing without necessarily building up large dislocation density gradients within a grain, because of their uniquely layered crystal structure. However, most silicates and oxides cannot accommodate strain transitions within grains without associated dislocation density gradients, and hence are susceptible to dissolution and solution transfer.As a consequence, zones of progressive shearing become zones of dissolution of most minerals, and of concentration of phyllosilicates and graphite. Exceptions are mylonites, where strain‐rates are commonly high enough for plastic deformation to dominate over diffusion rates and therefore over dissolution and solution transfer. Porphyroblastic minerals cannot nucleate and grow in zones of active progressive shearing, as they would be dissolved by the effects of shearing strain on their boundaries. However, they can nucleate and grow in zones of progressive shortening and this is aided by the propensity for microfracturing in these zones, which allows rapid access of fluids carrying the material presumed to be necessary for nucleation and growth. Zones of progessive shortening also have a number of characteristics that help to lower the activation energy barrier for nucleation, this includes a build up of stored strain‐energy relative to zones of progressive shearing, in which dissolution is occuring.Porphyroblast growth is generally syndeformational, and previously accepted criteria for static growth are not valid when the role of deformation partitioning is taken into account.Porphyroblasts in a contact aureole do not grow statically either, as microfracturing, associated with emplacement, allows access of fluids in a fashion that is similar to microfracturing in zones of progressive shortening.The criteria used for porphyroblast timing can be readily accommodated in terms of deformation partitioning, reactivation of deforming foliations, and a general lack of rotation of porphyroblasts, with the spectacular exception of genuinely spiralling garnet porphyrob

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1986.tb00337.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY

摘要:

AbstractChromian omphacite which contains up to 4 wt % Cr2O3has been identified from low‐grade metamorphic rocks in Nishisonogi, Kyushu, Japan. It occurs as aggregates, forming a thin horizon ([20 mm thick) in alayered metagabbro within a serpentinite melange zone, together with Cr‐free omphacite, actino‐lite, epidote and sphene. It may have been formed by the metasomatic introduction of Cr into the metagabbro from the serpentinite rather than by reaction with chromite. The structural formula, based on EPMA analyses, and the optical absorption spectrum of the chromian omphacite show that the Cr is positioned in the octahedral

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1986.tb00338.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY

摘要:

AbstractRare layers of an aluminous, muscovite‐rich rock from the Lewisian Complex at Stoer, North‐West Scotland, display evidence which suggests that the rock has undergone local partial melting to form quartz‐bearing veins and a corundum‐bearing restite. The assemblages observed in these rocks match those predicted by modelling in the system KAlO2‐NaAlO2‐Al2O3‐SiO2‐H2O (KNASH) where certain bulk compositions melt peritectically to give corundum‐bearing restites and quartz‐normative melts. Study of the model system shows that the observed parageneses could have formed from a range of bulk compositions with a variety of possible values ofaH2O which could have been internally or externally buffered. The KNASH petrogenetic grid, together with another in the system CaO‐Na2O‐FeO‐Al2O3‐SiO2‐H2O (CNFASH), allows theP–Tpath of the rocks to be delineated and an estimate to be made of the conditions at the peak of metamorphism as>11 kbar and 900‐925°C. This estimate is in agreement withP–Testimates using thermobarometric methods on adjacent lithologies: The activity of H2O in the system throughout metamor

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1986.tb00339.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY

摘要:

AbstractA diagenctic through anchizone to epizone transition is demonstrated in pelitic rocks of the Lower Palaeozoic marginal basin of Wales by examination of variations in phyllo‐silicate mineralogy, illite crystallinity and boparameter of white micas. This transition represents a temperature range from ∼ 150°C to ∼ 400°C and the metamorphism is of a low‐pressure facies series type, with a geothermal gradient of ∼ 40°Ckm‐1. Variations in grade can be correlated largely with the original basin and shelf form, suggesting a depth‐related metamorphism. However, in areas closer to the site of Caledonian plate collision an increasingly syn‐tectonic metamorphic event is apparent.Correlation of pelite data with metabasite assemblages is variable, the most consistent relationship being between epizone crystallinity values andepidote‐actinolite (greenschist facies) assemblages. Diagenetic clay mineral assemblages are found associated with prehnite‐pumpellyite assemblages in metabasites and it is suggested that the latter represent non‐buffered, and therefore non

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1986.tb00340.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.tb00341.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY