摘要:

AbstractMost porphyroblasts never rotate during ductile deformation, provided they do not internally deform during subsequent events, with the exception of relatively uncommon but spectacular examples of spiralling garnets. Instead, the surrounding foliation rotates and reactivates due to partitioning of the deformation around the porphyroblast. Consequently, porphyroblasts commonly preserve the orientation of early foliations and stretching lineations within strain shadows or inclusion trails, even where these structures have been rotated or obliterated in the matrix due to subsequent deformation. These relationships can be readily used to help develop an understanding of the processes of foliation development and they demonstrate the prominent role of reactivation of old foliations during subsequent deformation. They can also be used to determine the deformation history, as porphyroblasts only rotate when the deformation cannot partition and involves progressive shearing with no combined bulk shortening component.

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1985.tb00309.x

出版商:Blackwell Publishing Ltd    

年代:1985

数据来源: WILEY

摘要:

AbstractThe high‐grade meta‐plutonic rocks of this study lie entirely within the Jotun Nappe of the southern Norwegian Caledonides. They are divisible on the basis of metamorphic grade and petrographic character into three units, the Storadalen Complex (SCX), the Svartdalen Gneiss (SG), and the Mjølkedøla Purple Gabbro (MPG). The SCX is a differentiated series of ultrabasic to intermediate rocks now showing only tectonite fabrics. It has been metamorphosed to spinel‐Iherzolite granulite facies grade. The broadly monzonitic SG is weakly tectonized and internally differentiated. Its metamorphic grade does not exceed plagioclase‐lherzolite granulite facies grade. The mis‐named MPG is also broadly of monzonitic composition but it retains a coarse ophitic texture, and is of amphibolite facies grade. A gradational boundary exists between the MPG and SG, but the contact between these two units and the SCX is the steeply dipping Tyin‐Gjende Fault. The three units represent a comagmatic body of mid‐Proterozoic age, metamorphosed during a Sveconorwegian event and finally dismembered and upthrust during the Caledonian Orogeny.The new trace element analyses reported here show that the three rock units have remarkably similar trace element abundances and trends. K‐Rb covariation shows increasing K/Rb ratios with increasing K. These patterns were produced by magmatic fractionation processes acting at deep crustal levels, possibly in the presence of a non‐aqueous fluid phase. With the exception of K and Sr, close similarities exist between the rocks of this study and present‐day calc‐alkaline basalts and andesites from island arcs. The high K content is regarded as a primary magmatic feature, but the available data are insufficient to indicate its origin. The Sr contents are abnormally high and are ascribed to metasomatism which occurred during either high‐grade metamorphism or post‐climactic cooling. There are no systematic geochemical variations with metamorphic gra

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1985.tb00310.x

出版商:Blackwell Publishing Ltd    

年代:1985

数据来源: WILEY

摘要:

AbstractChemical analysis (including H2, F2, FeO, Fe2O3) of a Mg‐vesuvianite from Georgetown, Calif., USA, yields a formula, Ca18.92Mg1.88Fe3+0.40Al10.97Si17.81‐O69.0.1(OH)8.84F0.14, in good agreement on a cation basis with the analysis reported by Pabst (1936). X‐ray and electron diffraction reveal sharp reflections violating the space group P4/nnc as consistent with domains having space groups P4/n and P4nc. Refinement of the average crystal structure in space group P4/nnc is consistent with occupancy of the A site with Al, of the half‐occupied B site by 0.8 Mg and 0.2 Fe, of the half‐occupied C site by Ca, of the Ca (1,2,3) sites by Ca, and the OH and O(10) sites by OH and O. We infer an idealized formula for Mg‐vesuvianite to be Ca19Mg(MgAl7)Al4Si18O69(OH)9, which is related to Fe3+‐vesuvianite by the substitutions Mg + OH = Fe3++ O in the B and O(10) sites and Fe3+= Al in the AlFe site.Thermodynamic calculations using this formula for Mg‐vesuvianite are consistent with the phase equilibria of Hochella, Liou, Keskinen&Kim (1982) but inconsistent with those of Olesch (1978). Further work is needed in determining the composition and entropy of syntheticvsnatural vesuvianite before quantitative phase equilibria can be dependably generated. A qualitative analysis of reactions in the system CaO‐MgO‐Al2O3‐SiO2‐H2O‐CO2shows that assemblages with Mg‐vesuvianite are stable to high T in the absence of quartz and require water‐rich conditions (XH2O>0.8). In the presence of wollastonite, Mg‐vesuvianite requires very wa

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1985.tb00311.x

出版商:Blackwell Publishing Ltd    

年代:1985

数据来源: WILEY

摘要:

AbstractGranulites at Fyfe Hills in Enderby Land, Antarctica, crystallized at temperatures in excess of 850°C, and possibly as high as 1000°C, and at pressures of 8‐10kbar during the mid to late Archaean. A number of features, including repeated retrograde metamorphism at 5.5‐8kbar, retrograde reaction textures, and rimward zoning in pressure sensitive systems, suggest that following peak metamorphism the granulites stabilized at a depth of 18‐26 km. After stabilization, the granulites cooled near‐isobarically to temperatures of 600‐700°C. Assuming a total crustal thickness of 35‐40 km during this late Archaean interval of isobaric cooling, the peak metamorphic crustal thickness is estimated at 35‐56 km. This estimate is significantly less than the 60‐70 km obtained by summing the depths of the present levels of exposure (26‐34 km) and the thickness of the crust presently beneath Fyfe Hills (approxi‐mately 35km) and is, therefore, consistent with independent evidence for extensive post‐Archaean thickening o

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1985.tb00312.x

出版商:Blackwell Publishing Ltd    

年代:1985

数据来源: WILEY

摘要:

AbstractPolymetamorphic orthoamphibole‐bearing gneisses from the vicinity of shear zones in Casey Bay, Enderby Land, Antarctica, record both the overprinting of Archaean granulite lithologies by Proterozoic metamorphism and the subsequent evolution of the latter episode during localized deformation.Mineral chemistry and zoning relationships in orthoamphibole‐garnet‐kyanite‐quartz and later orthoamphibole‐garnet‐cordierite‐quartz assemblages are used together with interpretation of reaction and corona textures to constrain the Proterozoic pressure‐temperature path experienced by the rocks. Consideration of reaction topologies, P‐T‐X(Fe‐Mg‐A1) relationships in orthoamphibole‐bearing assemblages, and standard geothermobarometry indicate that the gneisses underwent a near‐isothermal decompression P‐T history (steep positive dP/dT) from ± 8 kbar and 700°C to<5.5kbar and 650°C. This uplift path is correlated with the general effects of Rayner Complex metamorphism and deformation which occurred after 1100 Ma in a major

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1985.tb00313.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.tb00314.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.tb00315.x

出版商:Blackwell Publishing Ltd    

年代:1985

数据来源: WILEY