摘要:

ABSTRACTDetailed microstructural analysis of inclusion trails in hundreds of garnet porphyroblasts from rocks where spiral‐shaped inclusion trails are common indicates that spiral‐shaped trails did not form by rotation of the growing porphyroblasts relative to geographic coordinates. They formed instead by progressive growth by porphyroblasts over several sets of near‐orthogonal foliations that successively overprint one another. The orientations of these near‐orthogonal foliations are alternately near‐vertical and near‐horizontal in all porphyroblasts examined. This provides very strong evidence for lack of porphyroblast rotation.The deformation path recorded by these porphyroblasts indicates that the process of orogenesis involves a multiply repeated two‐stage cycle of: (1) crustal shortening and thickening, with the development of a near‐vertical foliation with a steep stretching lineation; followed by (2) gravitational instability and collapse of this uplifted pile with the development of a near‐horizontal foliation, gravitational spreading, near‐coaxial vertical shortening and consequent thrusting on the orogen margins. Correlation of inclusion trail overprinting relationships and asymmetry in porphyroblasts with foliation overprinting relationships observed in the field allows determination of where the rocks studied lie and have moved within an orogen. This information, combined with information about chemical zoning in porphyroblasts, provides details about the structural/metamorphic (P‐T‐t) paths the rocks have followed.The ductile deformation environment in which a porphyroblast can rotate relative to geographic coordinates during orogenesis is spatially restricted in continental crust to vertical, ductile tear/transcurrent faults across which there is no component of bulk shor

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1989.tb00598.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

ABSTRACTMicrostructures in slate belt rocks at the Elura Mine, near Cobar, south‐eastern Australia, indicate that volume loss by syntectonic dissolution is coupled with mass accretion by reprecipitation of the dissolved material in dilational sites. The mass accretion is sustained primarily by repetitive tensile microfracturing at high pore‐fluid pressures. Oriented growth in the inter‐ and intragranular microcracks is locally host‐controlled, creating lattice‐ and shape‐preferred orientations. The grain‐scale crack‐seal features throughout the rock reflect rhythmic fluid pressure fluctuations; a balance is achieved between the fracture‐induced permeability (and consequent flushing rates), and the rate of fluid build‐up in a relatively sealed environment.Instability in the balancing factors can lead to localization and intensification of tensile failure (and hence, tension vein formation) in the grain aggregate. Growth of veins by crack‐seal also reflects a steady state, but with more localized fluctuations of fluid flow on the aggregate scale. Still larger imbalances between flushing and fluid accumulation (i.e. pressure variations) induce breccia veining. The larger pressure gradients over greater distances, associated with dilation localization (from pervasive microfracturing to spaced breccia domains), allow fluid channelling with an increased potential for chemical fluid/rock disequilibrium. Therefore, large breccia vein systems tend to be sites of extensive fluid/rock interaction and replacement, as spectacularly illustrated by the syntectonic sulphide orebodies at Elura. The huge amounts of silicate, carbonate and sulphide accumulated during folding at Elura illustrate the large scale of source and sink couples possible in

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1989.tb00599.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

ABSTRACTLow‐pressure granulite facies metasedimentary gneisses exposed in MacRobertson Land, east Antarctica, include hercynitic spinel‐bearing metapelitic gneisses. Peak metamorphic mineral assemblages include spinel + rutile + ilmenite + sillimanite + garnet, spinel + ilmenite + sillimanite + garnet + cordierite, ortho‐pyroxene + magnetite + ilmenite + garnet, spinel + cordierite + biotite + ilmenite and orthopyroxene + cordierite + biotite, each with quartz, K‐feldspar and melt. The presence of garnet + biotite‐ and cordierite + orthopyroxene‐bearing assemblages implies crossing tie‐lines in AFM projection for the K2O‐FeO‐MgO‐Al2O3‐SiO2‐H2O (KFMASH) system. This apparent contradiction, and the presence of spinel, rutile and ilmenite in the assemblages, is acounted for by using the KFMASH‐TiO2‐O2system, i.e. AFM + TiO2+ Fe2O3. We derive a petrogenetic grid for this system, applicable to low‐pressure granulite facies metamorphic conditions. Retrograde assemblages are interpreted from corona textures on hercynitic spinel and Fe‐Ti oxides. The relative positions of the peak and retrograde metamorphic assemblages on the petrogenetic gridsuggestthat corona development occurred duri

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1989.tb00600.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

ABSTRACTThere is no significant difference in the diffusion profiles across albite‐adularia bicrystals that were simultaneously deformed at a strain rate of 10‐6S‐1and those from hydrostatic experiments at the same conditions (1500 MPa and 1000°C for 156 h). This indicates that the bulk alkali diffusion rate, which is the sum of lattice diffusion (D,1) and dislocation pipe diffusion (Dp), is not significantly enhanced by dislocations at these conditions, and that the maximum value for the ratio ofDp/D1is about 105. This is equal to the value previously reported for‘oxygen’diffusion in albite. If this ratio is independent of temperature, the contribution of either static (pre‐deformed) or moving (syn‐deformed) dislocations to the bulk diffusion rate of alkalis is probably minor at all metamorphic conditions. For Al and Si diffusion the ratio ofDp/D1may be larger ifD1is lower. Thus a significant contribution from dislocations to bulk diffusion cannot be ruled out, especially during simultaneo

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1989.tb00601.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

ABSTRACTThe western metamorphic belt of the Coast Plutonic Complex, south‐east Alaska and adjacent British Columbia, contains strongly deformed rocks and a prominent topographic low: the Coast Range megalineament. Near Holkham Bay, south‐east Alaska, the lineament separates the western metamorphic belt into: a western low‐grade (greenschist facies) terrane, and an eastern medium‐grade (amphibolite facies) terrane.Sphalerite compositions of grains in direct contact with pyrite and pyrrhotite in chlorite‐muscovite zone rocks in the low‐grade terrane give pressures of about 8 kbar; compatible with pressures of 8‐10 kbar at 500°C calculated from plagioclase‐biotite‐garnet‐muscovite assemblages adjacent to the Windham Bay pluton about 15 km away. A pressure of 4.8 ± 0.7 kbar was calculated from sphalerite compositions in staurolite zone rocks east of the Coast Range megalineament. This is indistinguishable from pressures of 4.8 ± 1 kbar at 585°C and 5.1 ± 1 kbar at 680°C (plagioclase‐garnet‐aluminum silicate‐quartz equilibria), and 4.1 ± 1 kbar at 585°C (plagioclase‐biotite‐garnet‐muscovite equilibrium) determined for the medium‐grade terrane. An identical pressure of 4.8 ± 0.7 kbar was calculated from sphalerite compositions in biotite zone rocks adjacent to the lineament; this is considerably higher than a pressure of 3.1 ± 1 kbar at 525°C obtained using plagioclase‐biotite‐garnet‐muscovite geobarometry from shear zones within the lineament. The discrepancy may be explained by later equilibration of mineral phases within the shear zones.The geothermobarometry suggests relatively low temperatures and high pressures for the low‐grade terrane (6‐10 kbar), and intermediate temperatures and pressures for the medium‐grade terrane to the east (4‐6 kbar). Comparison of the barometers indicate that sphalerite can be used to estimate metamorphic pressures, similar to those estimated from silicate mineral chemis

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1989.tb00602.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

ABSTRACTThe Internal Zone of the Betic Cordilleras consists of several superimposed major thrust sheets with differentP‐T‐tevolutions. On the basis of an integrated field, microscopic and laboratory study, the tectono‐metamorphic history of the Mulhacen Complex and Almanzora Unit has been reconstructed in detail. The Mulhacen Complex has been affected by at least five phases of penetrative deformation, which have been labelled Dx‐1, Dx, Dx+1, Dx+2and Dx+3. Dx‐1, and Dxare related to continent‐continent collision, which is indicated by high pressure‐low temperature (HP/LT) and subsequent intermediateP/Tmetamorphic conditions. Dx+1is related to crustal thinning and heterogeneous extension. During this event the Almanzora Unit was juxtaposed against the Mulhacen Complex. This phase was succeeded by the establishment of low pressure‐high temperature (LP/HT) conditions and at least two phases of folding and overthrusting. The Almanzora Unit shows a comparable tectono‐metamorphic evolution post Dx+1. However, theP/Tconditions prior to Dx+1indicate a higher crustal position with respect to the Mulhacen Complex during the

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1989.tb00603.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

ABSTRACTSequential reaction textures in Archaean garnet‐corundum‐sapphirine granulites from the Central Zone of the Limpopo Belt document a progression from early, coarse‐grained, high‐pressure (P>9.5 kbar) granulite‐facies assemblages (M1) to late, low‐pressure (P5 kbar, indicative of rapid uplift. TheP‐Tpath is interpreted as the product of a single metamorphic cycle which probably took place in response to tectonic thickening of the crust. Such a process contrasts with the extensional origin recently prop

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1989.tb00604.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1989.tb00605.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY