摘要:

AbstractOptical and X‐ray studies of carbonaceous material in the Tono contact metamorphic aureole, Kitakami Mountains, northeast Japan, have revealed that metamorphic graphitization proceeded through two discontinuous changes: first, optically anisotropic domains develop within the coaly phytoclast, forming transitional material, and then, ordered graphite crystallizes by the decomposition of pre‐existing carbonaceous materials. Coaly material disappears in the uppermost chlorite zone. Transitional material appears in the middle of the lower chlorite zone. Graphite appears in the upper chlorite zone and its modal abundance increases across the andalusite iso‐grad to the cordierite isograd where all the carbonaceous materials have converted to graphite. The apparently continuous variation in the crystallographic parameters of thebulkcarbonaceous material during graphitization is largely due to variation in the modal proportions of three types of carbonaceous materials. The temperature of graphitization in the present area is at least 100°C higher than the temperature in the Sanbagawa and New Caledonia high‐pressure metamorphic terrains, probably due to the slow reaction rate of metamorphic graphitization and to the short duration of contact meta

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1987.tb00375.x

出版商:Blackwell Publishing Ltd    

年代:1987

数据来源: WILEY

摘要:

AbstractThe effect of ductile deformation (dislocation creep) on the kinetics of the aragonite‐calcite transformation has been studied at 1 atm (330° C and 360° C) and 900‐1500 MPa (500° C) using undeformed and either previously or simultaneously deformed samples (500° C and a strain rate of 10‐6s). Deformation enhances the rate of the transformation of calcite to aragonite, but decreases the rate of transformation of aragonite to calcite. The difference results from a dependence of transformation rate on grain size, coupled with a difference in the accommodation mechanisms, climb versus recry‐stallization, of these minerals during dislocation creep. Dislocation climb is relatively easy in calcite and thus plastic strain results in high dislocation densities without significant grain size reduction. The rate of transformation to aragonite is enhanced primarily because of the increase in nucleation sites at dislocations and subgrain boundaries. In aragonite, on the other hand, dislocation climb is difficult and thus plastic strain produces extensive dynamic recry‐stallization resulting in a substantial grain size reduction. The transformation of aragonite is inhibited because the increase in calcite nucleation sites at dislocations and/or new grain boundaries is more than offset by the inability of calcite to grow across high angle grain boundaries. Thus the net effect of ductile deformation by dislocation creep on the kinetics of polymorphic phase transformations depends on the details of the accommodat

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1987.tb00376.x

出版商:Blackwell Publishing Ltd    

年代:1987

数据来源: WILEY

摘要:

AbstractSeveral small bodies of metabasite (maximum dimensions of 1000 m x 500 m) are included in the metamorphic rocks of the Nevado‐Filabride Complex in the Betic Cordilleras (Almeria Region). The body of 400 m x 100 m, located 200 m due west of the Lubrin village, contains troctolitic gabbro with well‐preserved igneous textures and mineral compositions, wholly amphibolitized gabbro, garnet‐bearing metagabbro eclogite. Along with the textural and mineral changes, sensible and regular geochemical variations can be observed, where the content of MgO decreases from 24% to 11%, while that of CaO and Na2O increases from 7% to 11% and from 2% to 3%, respectively. In addition, the content of some minor elements such as Sr, Y, Nb, Zr and Sc increases while that of Ni and Cr decreases from troctolitic gabbro to the eclogite. The amphibolitized gabbro shows values scattered around those of the troctolitic gabbro. These geochemical variations are ascribed to inherited differences in the pre‐metamorphic protolith, i.e. a fractionated gabbro which varies from olivine‐rich to clinopyroxene‐rich gabbro. Nevertheless, some metasomatism affected the Lubrin body without changing the main chemical trends, as documented by the significantly different87Sr/86Sr ratios of each rock‐type. This points to a metasomatism which involved the introduction of crustal radiogenic strontium. The petrographical and mineral chemical features are interpreted to be the result of syn‐metamorphic fluid circulation possibly combined with deformation by shearing. The igneous texture and mineral chemistry have been retained wherever both fluid circulation and shearing were ineffective. On the contrary, where both events were effective, the formation of eclogite occurred. Later, the entire body underwent a retrogressive amphi‐bolitic stage under greenschist facies conditions, which was probably responsible for the formation of the amphibolitized gabbro portion and for the retrogressio

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1987.tb00377.x

出版商:Blackwell Publishing Ltd    

年代:1987

数据来源: WILEY

摘要:

AbstractThe effects of Tertiary Alpine metamorphism on pelitic Mesozoic cover rocks have been studied along a cross‐section in the central Lepontine Alps in the Nufenen Pass area, Switzerland.Greenschist facies to amphibolite facies conditions are indicated by the formation of the index minerals chloritoid, garnet, staurolite and kyanite in pelitic rocks. Regional metamorphism reached maximum conditions during the interkinematic period between a main Alpine penetrative (D2) and a late Alpine (D3) crenulation type deformation phase or synchronous with the late Alpine deformation. Based on AFM phase relationships four different metamorphic zones can be distinguished: (1) chloritoid zone; (2) staurolite + chlorite zone; (3) staurolite + biotite zone; and, (4) kyanite zone.The isograds that separate these zones can be modelled by univariant reactions in the KFMASH system. The conditions of metamorphism calculated from geological ther‐mobarometers for the maximum post‐D2 por‐phyroblast stage are from North to South: 500° C at 5‐6 kbar and 600° C at 7‐8 kbar.Detailed thermobarometry of garnet por‐phyroblasts with complex textures suggests that maximum temperature was reached later than maximum pressure. Early garnet growth occurred along a progradeP‐T‐path, post‐D2 rims grew with increasing temperature but decreasing pressure, and finally post‐D3 garnet formed along a retrogradeP‐T‐path.It may be concluded from the calculated pressure and temperature difference over a short distance (3 km) across the mapped area that the isogradic surfaces of the post‐D2 metamorphism are steeply oriented. The data also suggest that isobaric and isothermal surfaces are parallel.Much of the observed metamorphic pattern can be explained as the result of a significant post‐D2 differential uplift of the hot Pennine area relative to the Helvetic area along a tectonic contact zone. The closely spaced isograds (isotherms) in the North may then be interpreted as a thermal effect owing to the emplacement of the hot Pennine rocks against the Got‐thard massif with its cover. Whereas, in the Pennine metasediments, post‐D2 porphyroblast formation can be related to the decompression path which was steep enough for dehydration reactions to proceed. It is also remarkable that late kyanite porphyroblasts probably formed with decreasing pressure.The interpretation given here for the Nufenen Pass area may also apply to the Luk‐manier Pass area where similar metamorphic patterns have been reported by Fox (1975). The formation of the ‘Northern Steep Belt’;, as denned by Milnes (1974b), and the associated late Alpine fold zones may, therefore, have significantly modified the metamorphic patt

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1987.tb00378.x

出版商:Blackwell Publishing Ltd    

年代:1987

数据来源: WILEY

摘要:

AbstractDissolution of quartz and the recryst‐allization and re‐equilibration of phyllosilicates produce the dark anastomosing seams that dominate microstructures of tectonic melange which occur in a low‐grade, imbricated and multiply deformed, mid‐Palaeozoic, intracra‐tonic fiysch sequence in northeastern Australia. Seams are composed of very closely spaced or coalesced cleavage lamellae, which are very thin layers of extremely fine‐grained phyllosilicates. Cleavage seams enclose lenses of silt‐stone or greywacke, which formerly occurred in continuous sedimentary layers, indicating extremely heterogeneous and disruptive deformation. Microphacoids enclosed by cleavage seams have subtle shape asymmetries analogous to those of porphyroclasts. Phyllosilicate‐preferred orientations within microphacoids commonly lie at a low angle to enclosing seams, and asymmetric relationships occur within seams. The shape and fabric asymmetries appear to be constant, and are regarded as analgous to S and C planes. The number and extent of seams, the amount of dissolution they indicate, and the efficiency of deformation partitioning imply some enhancement of chemical activity and substantial silica loss from the system. This, in turn, suggests the passage of large amount of silica‐undersaturated fluid, and melanges may be zones of high fluid flow. However, the microstructures and the disruptive nature of the fabrics may also reflect the influence of high bulk shear strains and suggest some relationship between the shearing component of deformation and the development of cleavag

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1987.tb00379.x

出版商:Blackwell Publishing Ltd    

年代:1987

数据来源: WILEY

摘要:

AbstractScapolite, wollastonite, calcite, diopside, grossular‐andradite garnet and sphene occur in calc‐silicate rocks in the granulite terrain of the Arunta Block, central Australia. This assemblage buffers the CO2activity at a low value, so that any coexisting fluid phase must be H2O rich and CO2poor (Xco2= 0.2‐0.3). In contrast, the H2O activity in the surrounding felsic and mafic granulites was low. Thus fluid activities during granulite facies metamorphism were locally buffered in various rock units and fluid flow appears to have been restricted or fluid may have been absent. Late retrograde rims of garnet and garnet‐quartz separate phases formed in the high‐grade stage. Formation of these rims would have required either an influx of water‐rich fluid or a decrease in pressure. Evidence from the surrounding granulites shows that in one locality, the calc‐silicate rocks had undergone late isobaric hydration; in another locality, minor uplift had occurred soon after peakP‐Tconditions. In both, scapolite had partly broken down to plagioclase‐calite. A calc silicate rock from the granulite terrain of Enderby Land, Antarctica, contains scapolite, wollastonite, calcite, diopside, quartz and sphene; this assemblage also indicates low CO2activities. In this rock, wollastonite has broken down to calcite‐quartz, to indicate isobaric cooling without infl

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1987.tb00380.x

出版商:Blackwell Publishing Ltd    

年代:1987

数据来源: WILEY

摘要:

AbstractIn the Bois de Cené area, blueschist facies rocks, characterized by glaucophane and/ or chloritoid, provide evidence for a suture zone in the Variscan. This terrain is considered to be the eastern equivalent of the Ile de Groix high‐pressure metamorphic terrain. Petrological study of the two characteristic types of rocks found in the area shows that the primary high‐pressure paragenesis was modified during a retrogression which followed substantial decompression, probably at constant or decreasing temperature. The simplest interpretation is that this retrogression followed tectonic emplacement within a nappe

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1987.tb00381.x

出版商:Blackwell Publishing Ltd    

年代:1987

数据来源: WILEY

摘要:

AbstractChloritoid‐bearing metasedimentary rocks occur in close proximity to blueschists and eclogites in the Tertiary high‐pressure metamorphic belt of northern New Caledonia. The typical assemblage of chloritoid‐bearing rocks in the epidote zone is quartzchlorite‐muscovite‐garnet‐chloritoid. In the omphacite zone, epidote is an additional member of the chloritoid‐bearing assemblage. Paragonite is rare, plagioclase was not detected, and rutile and ilmenite are the Fe‐Ti oxide phases. Chloritoid‐glaucophane is not a common assemblage. Chloritoid‐bearing rocks have relatively low (Ca+K+Na)/Al ratios and the chloritoids are relatively Mg‐rich with Mg/ (Mg+Fe) up to about 0.4. A comparison of the mineral assemblages and mineral chemistry with experimental and computed phase equilibria suggest an upper temperature limit near 560° C in the omphacite zone and a minimum temperature limit near 450° C at 10 kbar. An empirical garnet‐chlorite Fe‐Mg exchange thermometer does not yield consistent results for the higher‐grade rocks, suggestingTs ranging from 390 to 535° C in the omphacite zone and 420–465° C in the epidote zone. The distribution coefficientKD= (Fe/Mg)ctd/(Fe/Mg)chlfor chloritoid and chlorite ranges from 3.9 to 6.4, values which are lower than those (=10) from lower greenschist facies rocks, but are near those of upper greenschist facies and

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1987.tb00382.x

出版商:Blackwell Publishing Ltd    

年代:1987

数据来源: WILEY

摘要:

AbstractConsidering the minerals cordierite (Cd), sapphirine (Sa), hypersthene (Hy), garnet (Ga), spinel (Sp), sillimanite (Si) and corundum (Co) in the system FeO‐MgO‐Al2O3‐SiO2(FMAS), the stable invariant points are [Co], [Ga], [Cd] and [Sa]. Constraints imposed by experimental data for the system MAS indicate that under lowPH2o conditions the invariant points occur at high temperature (>900° C) and intermediate pressure (7‐10 kbar). This temperature is higher than that commonly advocated for granulite facies metamorphism. In granulites Fe‐Mg exchange geothermometers may yield temperatures of 100–150° C below peak metamorphic conditions and evidence for peak temperatures is best preserved by relict high‐temperature assemblages and by Al‐rich cores in orthopyroxene. Application of the FMAS grid to some well‐documented granulite occurrences introduces important constraints on theirP‐Thistories. Rocks of different bulk compositions, occurring in close proximity in the field, may record distinct segments of theirP‐Tpaths. This applies particularly to rocks with evidence for reaction in the form of coronas, symplectites and zoned minerals. Consideration of curved reaction boundaries andXMsisopleths may explain apparently contradictory results for the stability of cordierite obtained from low‐temperature experiments and thermochemical calculations on the one hand and hightemperature exper

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1987.tb00383.x

出版商:Blackwell Publishing Ltd    

年代:1987

数据来源: WILEY

摘要:

AbstractIn the Fleur de Lys Supergroup, western Newfoundland, inclusion trails in garnet and albite porphyroblasts indicate that porphyroblasts overgrew a crenulation foliation, without rotation, probably during the deformation event that produced the crenulations. Further deformation of the matrix resulted in strong re‐orientation and retrograde metamorphism of the matrix foliation, which is consequently highly oblique to the crenulation foliation preserved in the porphyroblasts. The resulting matrix foliation locally preserves relics of the early crenulations, and also has itself been crenulated later in places. Thus the porphyroblasts grew before the later stages of deformation, rather than during the final stage, as had been suggested previously. The new interpretation is consistent with available40Ar/39Ar cooling ages which indicate a late Ordovician‐early Silurian metamorphic peak, rather than the Devonian peak suggested by previous workers. The inclusion patterns and microprobe data indicate normal outward growth of garnet porphyroblasts from a central nucleus, rather than as a series of veins as proposed by de Wit (1976a, b). However, the observations presented here support growth of porphyroblasts without rotation, which is implied by the de Wit mo

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1987.tb00384.x

出版商:Blackwell Publishing Ltd    

年代:1987

数据来源: WILEY