ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1996.t01-1-00001.x

出版商:Blackwell Science Inc    

年代:1996

数据来源: WILEY

摘要:

ABSTRACTSeventy‐five spatially orientated, serial thin sections cut from a single rock containing ‘millipede’ porphyroblast microstructure from the Robertson River Metamorphics, Australia, reveal the three‐dimensional (3‐D) geometry of oppositely concave microfolds (OCMs) that define the microstructure. Electronic animations showing progressive serial sections of the 3‐D microstructure are made available via the World Wide Web. The OCM amplitudes decrease regularly from a maximum in near‐central sections to a minimum in near‐marginal sections, whereas the OCM interlimb angles increase regularly from a minimum in near‐central sections to a maximum in near‐marginal sections. These observations illustrate that the OCMs are noncylindrical surfaces with culminations located in near‐central sections. Because the porphyroblast cores appear to have been present before significant development of the syn‐OCM foliation, all of the OCMs were formed by heterogeneous extension around these cores. The overall geometry of the OCMs described here reflects the strain state, and cannot be used to cons

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1996.00003.x

出版商:Blackwell Science Inc    

年代:1996

数据来源: WILEY

摘要:

ABSTRACTOppositely concave microfolds (OCMs) in and adjacent to porphyroblasts can be classified into five nongenetic types. Type 1 OCMs are found in sections through porphyroblasts with spiral‐shaped inclusion trails cut parallel to the spiral axes, and commonly show closed foliation loops. Type 2 OCMs, commonly referred to as ‘millipede’ microstructure, are highly symmetrical, the foliation folded into OCMs being approximately perpendicular to the overprinting foliation. Type 3 OCMs are similar to Type 2, but are asymmetrical, the foliation folded into OCMs being variably oblique to the overprinting foliation. Type 4 OCMs are highly asymmetrical, only one foliation is present, and this foliation is parallel to the local shear plane. Type 5 OCMs result from porphyroblast growth over a microfold interference pattern.Types 1 and 2 are commonly interpreted as indicating highly noncoaxial and highly coaxial bulk deformation paths, respectively, during porphyroblast growth. However, theoretically they can form by any deformation path intermediate between bulk coaxial shortening and bulk simple shearing. Given particular initial foliation orientation and timing of porphyroblast growth, Type 3 OCMs can also form during these intermediate deformation paths, and are commonly found in the same rocks as Type 2 OCMs. Type 4 OCMs may indicate highly noncoaxial deformation during porphyroblast growth, but may be difficult to distinguish from Type 3 OCMs. Thus, Types 1–3 (and possibly 4) reflect the finite strain state, giving no information about the rotational component of the deformation(s) responsible for their formation. Furthermore, there is a lack of unequivocal independent evidence for the degree of noncoaxiality of deformation (s) during the growth of porphyroblasts containing OCMs. Type 2 OCMs that occur independently of porphyroblasts or other rigid objects might indicate highly coaxial bulk shortening, but there is a lack of supporting physical or computer modelling.It is possible that microstructures in the matrix around OCMs formed during highly noncoaxial and highly coaxial deformation histories might have specific characteristics that allow them to be distinguished from one another. However, determining degrees of noncoaxiality from rock fabrics is a major, longstanding problem in structural

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1996.00015.x

出版商:Blackwell Science Inc    

年代:1996

数据来源: WILEY

摘要:

ABSTRACTThe products of metamorphic fluid flow are preserved in zones within the marbles and metamorphosed semipelites of the Upper Calcsilicate Unit in the granulite portion of the Late Palaeoproterozoic Reynolds Range Group, northern Arunta Block, central Australia. The zones of retrogression, characterized by minerals such as wollastonite, grossular and clinohumite, local resetting of oxygen isotopic compositions and local major element metasomatism, were channelways for water‐rich fluids derived from granulite facies metapelites. U–Th–Pb isotopic ages measured by the SHRIMP ion microprobe on zircon and monazite from a granulite facies semipelite, an early semiconcordant aluminous quartz‐rich fluid‐flow segregation and a late discordant quartz‐rich segregation record some of the extended thermal history of the area. Zircon cores from the semipelite show its likely protolith to be an igneous rock 1812 ± 11 Ma old, itself derived from a source containing zircon as old as 2.2 Ga. Low‐Th/U overgrowths on the zircon grew during granulite facies metamorphism at 1594 ± 6 Ma. Monazite cooled to its blocking temperature at 1576 ± 8 Ma. Zircon cores from the semiconcordant segregation are dominantly>2.3 Ga old, indicating that the source of the fluids was not the particular metamorphosed semipelite studied. Two generations of low‐Th/U overgrowths on the zircon give indistinguishable ages for the older and younger of 1589 ± 8 and 1582 ± 8 Ma, respectively. The monazite age is the same, 1576 ± 12 Ma. Zircon from the late discordant segregation gave 1568 ± 4 Ma. Fluid flow occurred for at least 18 ± 3 (σ) Ma and ended 26 ± 3 (σ) Ma after the peak of metamorphism, suggesting a very slow cooling rate of ∼3°C Ma–1. The last regional high‐grade metamorphism in the Reynolds Range occurred at ∼1.6 Ga, not ∼1.78 Ga as previously thought. The high‐grade event at ∼1.78 Ga is a separate event that affected only

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1996.00029.x

出版商:Blackwell Science Inc    

年代:1996

数据来源: WILEY

摘要:

ABSTRACTIn the Hlinsko region (Variscan Bohemian Massif, Czech Republic) a major extensional shear zone separates low‐grade metasedimentary series (Hlinsko schists) and high‐grade rocks of the Moldanubian terrane (Svratka Crystalline Unit). During late‐Variscan extension, a tonalite intruded syntectonically into the normal ductile shear zone, and caused contact metamorphism of the overlying schists. Concurrent syntectonic sedimentation of a flysch series took place at the top of the hangingwall schists. In order to decipher the detailed petrological evolution of the Hlinsko unit situated in the hangingwall of this tectonic contact, a phase diagram approach and petrogenetic grids, calculated with the thermocalc computer program, were used.The crystallization/deformation relationships and the paragenetic analysis of the Hlinsko schists define aP–Tpath with an initial minor increase in pressure followed by cooling. Calculated pseudosections constrain this anticlockwiseP‐Tevolution to the upper part of the andalusite field between 0.36 and 0.40 GPa for temperatures ranging from 570 to 530°C. A lowaH2Ois required to explain the presence of andalusite‐biotite‐bearing assemblages, and could be related to the presence of abundant graphite.In contrast, the footwall rocks of the Svratka Crystalline Unit record decompression from around 0.8 GPa at a relatively constant temperature, followed by cooling. Thus, the footwall and the hangingwall units display opposite, but convergentP–Thistories. Decompression in the footwall rocks is related to a rapid exhumation. We propose that the inverse, anticlockwiseP–Tpath recorded in the hangingwall pelites is related to the rapid, extension‐controlled sedimentation of the ove

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1996.t01-1-00049.x

出版商:Blackwell Science Inc    

年代:1996

数据来源: WILEY

摘要:

ABSTRACTIon probe traverses across garnets from peridotites of the Caledonides of Norway and the Variscides of Poland show zoning patterns for Y, V, Zr, Cr, Ti and the REE. The complexly zoned patterns of garnets from the Bystrzyca Górna peridotite, Poland, are interpreted in terms of a changingP–Thistory (isobaric cooling followed by decompression and cooling). Weak rimward gradients in REE concentrations in garnets from the Almklovdalen and Sandvika peridotites, Norway, may be relicts of the original growth history of the garnets, but the nearly flat Y, V, Zr, Cr and Ti profiles from the same garnets imply a later period of near‐homogenization at uniformP–T.Crushed garnet separates from each body were separated into three or more fractions on the assumption that density and magnetic susceptibility vary with Fe/Mg ratio, and Fe/Mg ratios change from garnet core to rim. Sm‐Nd garnet–clinopyroxene ‘ages’ were determined for each fraction to determine whether they are also zoned. Four garnet fractions from the Góry Sowie peridotite give nearly the same ages (397–412 Ma) that are believed to span the interval of garnet growth. Garnet fractions from the Norwegian peridotites define scattered ages (816–1350 Ma) that are suspect, but hint at a Sveconorwegian equilibration event. The data indicate the Variscan and Norwegian peridotites had different histories, despite superficial mineralogical and tectonic similarities. Norwegian garnet peridotites had a long pre‐Caledonian history and were extracted from a relatively cold mantle whereas the Variscan garnet peridotites had a comparatively short pre‐ or Eo‐Variscan history and were ex

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1996.00061.x

出版商:Blackwell Science Inc    

年代:1996

数据来源: WILEY

摘要:

ABSTRACTMany thermodynamic calculations relating to metamorphic rocks hinge on the thermodynamic parameters of garnet. Though some models are widely used, it is not clear whether their underlying premise is correct: that a single set of equations can be written for the activities of the end‐members of garnet covering the whole compositional range. A voluminous body of data can be used to constrain the thermodynamics of garnet, namely Fe–Mg exchange experimental data involving garnet and another mineral, particularly clinopyroxene, orthopyroxene and olivine. However, examination of these data reveals inconsistencies, apparently stemming from differences between the thermodynamics of low‐Ca and high‐Ca garnets, with a boundary of aboutXgCa= 0.15. In the two regions, for the highP–Tof the experimental data, the thermodynamics follow the regular model, with values for the interaction parameters in the low Ca region of aboutwgFeMg= 50RandwaFe–wgMgCa=– 1300R, in whichRis the gas constant, and in the high Ca region of aboutwgFeMg= 1100RandwgCaFe–wgMgCa=– 2200R. Using the subregular, rather than the regular, model does not remove the discrepancy. The cause of the discrepancy needs to be identified if reliable calculations on roc

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1996.00075.x

出版商:Blackwell Science Inc    

年代:1996

数据来源: WILEY

摘要:

ABSTRACTParagonite‐bearing amphibolites occur interbedded with a garbenschist‐micaschist sequence in the Austroalpine Schneeberg Complex, southern Tyrol. The mineral assemblage mainly comprises paragonite + Mg‐hornblende/tschermakite + quartz + plagioclase + biotite + ankerite + Ti‐phase + garnet ± muscovite. EquilibriumP–Tconditions for this assemblage are 550–600°C and 8–10 kbar estimated from garnet–amphibole–plagioclase–ilmenite–rutile and Si contents of phengitic muscovites. In the vicinity of amphibole, paragonite is replaced by symplectitic chlorite + plagioclase + margarite +± biotite assemblages. Muscovite in the vicinity of amphibole reacts to form plagioclase + biotite + margarite symplectites. The reaction of white mica + hornblende is the result of decompression during uplift of the Schneeberg Complex. The breakdown of paragonite + hornblende is a water‐consuming reaction and therefore it is controlled by the availability of fluid on the retrogressiveP–Tpath. Paragonite + hornblende is a high‐temperature equivalent of the common blueschist‐assemblage paragonite + glaucophane in Ca‐bearing systems and represents restrictedP–Tconditions just below omphacite stability in a mafic bulk system. While paragonite + glaucophane breakdown to chlorite + albite marks the blueschist/greenschist transition, the paragonite + hornblende breakdown observed in Schneeberg Complex rocks is indicative of a transition from epidote‐amphibolite facies to greenschist facies conditions at a flatterP–Tgradient of the metamorphic path compared to subduction‐zone environments. Ar/Ar dating of paragonite yields an age of 84.5 ± 1 Ma, corroborating an Eoalpine high‐pressure metamorphic event within the Austroalpine unit west of the Tauern Window. Eclogites that occur in the Ötztal Crystalline Basement south of the Schneeberg Complex are thought to be a

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1996.00085.x

出版商:Blackwell Science Inc    

年代:1996

数据来源: WILEY

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1996.00102.x

出版商:Blackwell Science Inc    

年代:1996

数据来源: WILEY