摘要:

The Omeo Metamorphic Complex forms the southern end of the Wagga Metamorphic Belt, which is the main locus of Palaeozoic low‐pressure metamorphism in the Lachlan Fold Belt, south‐eastern Australia. It comprises metamorphosed Ordovician quartz‐rich turbidites originally derived from Precambrian cratonic rocks. Prograde regional metamorphism occurred in the early Silurian, very soon after sedimentation had ceased. The sequence of metamorphic zones, with increasing grade, is: chlorite, biotite, cordierite, andalusite–K‐feldspar and sillimanite–K‐feldspar. Migmatites occur in the sillimanite–K‐feldspar zone, but large bodies of S‐type granite were derived from rocks underlying the exposed Ordovician sequence.PandTestimates for the highest grade rocks areT= 700°C andP= 3.5 kbar, indicating a very highP–Tgradient of 65°C/km.The high heat flow during prograde metamorphism probably resulted from a combination of a thermal anomaly persisting from a pre‐metamorphic back‐arc basin environment, and intrusion of hot, mantle‐derived magmas into the lower and middle crust.Regional retrograde metamorphism coincided with a general reheating of the crust in the Siluro‐Devonian, accompanied by intrusion of many I‐type plutons and resetting of the K–Ar dates of some earlier plutons. The Omeo Metamorphic Complex

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1990.tb00453.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

Porphyroblasts of garnet and plagioclase in the Otago schists have not rotated relative to geographic coordinates during non‐coaxial deformation that post‐dates their growth. Inclusion trails in most of the porphyroblasts are oriented near‐vertical and near‐horizontal, and the strike of near‐vertical inclusion trails is consistent over 3000 km2. Microstructural relationships indicate that the porphyroblasts grew in zones of progressive shortening strain, and that the sense of shear affecting the geometry of porphyroblast inclusion trails on the long limbs of folds is the same as the bulk sense of displacement of fold closures. This is contrary to the sense of shear inferred when porphyroblasts are interpreted as having rotated during folding.Several crenulation cleavage/fold models have previously been developed to accommodate the apparent sense of rotation of porphyroblasts that grew during folding. In the light of accumulating evidence that porphyroblasts do not generally rotate, the applicability of these models to deformed rocks is questionable.Whether or not porphyroblasts rotate depends on how deformation is partitioned. Lack of rotation requires that progressive shearing strain (rotational deformation) be partitioned around rigid heterogeneities, such as porphyroblasts, which occupy zones of progressive shortening or no strain (non‐rotational deformation). Therefore, processes operating at the porphyroblast/matrix boundary are important considerations. Five qualitative models are presented that accommodate stress and strain energy at the boundary without rotating the porphyroblast: (a) a thin layer of fluid at the porphyroblast boundary; (2) grain‐boundary sliding; (3) a locked porphyroblast/matrix boundary; (4) dissolution at the porphyroblast/matrix boundary, and (5) an ellipsoidal porphyroblas

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1990.tb00454.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

Instrumental settings for determination of illite/muscovite ‘crystallinity’(half‐height width of the 10‐Å X‐ray diffraction peak) and the limits of the anchimetamorphic zone adopted by various authors fall into several groups.The variation in the limiting peak widths between the authors that have adopted Kubler's Neuchâtel boundary values of 0.42° and 0.25°Δ2θ can be interpreted in terms of variation in the instrumental settings. The choice of time constants higher than those given by the formula causes peak broadening; this effect is particularly marked at high scan rates. The peak broadening is by constant increments that are virtually independent of the absolute peak width.The differences between the Kubler (Neuchâtel)‐derived limiting values and some other scales are appreciably greater than can be accounted for by these differences in instrumental settings: many of these scales are not equivalent. In particular, the limits adopted by Dunoyer de Segonzac (1969) and subsequent workers at Strasbourg are too broad; their anchizone represents a range of grades of metamorphism lower than that of Kubler, widely overlapping the latter's ‘diagenetic’zone. Those adopted by some other, mainly French, authors are too narrow.The limits of the anchizone should be calibrated by inter‐laboratory standards, and the instrumental settings sho

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1990.tb00455.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

The Narryer Gneiss Complex of the Yilgarn Block is a key segment of the Western Australian Precambrian Shield. It is a regional granulite facies terrain comprised of predominantly quartzo‐feldspathic gneisses derived from granitic intrusionsc. 3.6–3.4 Ga old. Granulite facies metamorphism occurredc. 3.3 Ga ago, and conditions of 750–850°C and 7–10 kbar are estimated for the Mukalo Creek Area (MCA) near Errabiddy in the north. TheP–Tpath of the MCA has been derived from metamorphic assemblages in younger rocks that intruded the gneisses during at least three subsequent events, and this path is supported by reaction coronas in the older gneisses. There is no evidence for uplift immediately following peak metamorphism of the MCA, and a period of isobaric cooling is inferred from the pressures recorded in younger rocks. Pressures and temperatures estimated from metadolerites, which intruded the older gneisses during ‘granite–greenstone’tectonism at about 2.6 Ga and during early Proterozoic thrusting show that the Errabiddy area remained in the lower crust, although it was probably reheated during the younger events. Isothermal uplift to upper crustal levels occurred atc. 1.6 Ga ago, and was followed by further deformation and patchy retrogression of high‐grade assemblages. The effects of younger deformation, cooling and reheating can be discerned in the older gneisses, but as there has been no pervasive deformation or rehydration, the minerals and microstructures formed during early Archaean granulite facies metamorphism for the most part are retained.The MCA remained in the lower crust for about 1700 Ma following peak metamorphism and some event unrelated to the original metamorphism was required to exhume it. Uplift occurred during development of the Capricorn Orogen, when some 30–35 km were added to the crust beneath the Errabiddy area. The recognition of early Proterozoic thrusting, plus crustal thickening, suggests that the Capricorn Orogen is a belt of regional compression which resulted from convergence of the Yilgarn

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1990.tb00456.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

The Anmatjira Range and adjacent Reynolds Range, central Australia, comprise early Proterozoic metasediments and othogneisses that were affected by three, and possibly four, temporally distinct metamorphic events, M1–4, and deformation events, D1–4, in the period 1820–1590 Ma. The north‐western portion of the range, around Mt Stafford, preserves the effects of ±1820 Ma M1‐D1, and shows a spectacular lateral transition from muscovite + quartz‐bearing schists to interlayered andalusite‐bearing migmatites and two‐pyroxene granofelses that reflect extremely low‐pressure granulite facies conditions, over a distance of less than 10 km. Orthopyroxene + cordierite + garnet + K‐feldspar + quartz‐bearing gneisses occur at the highest grade, implying peak conditions of ±750°C and 2.5 ± 0.6 kbar. An anticlockwiseP–Tpath for M1is inferred from syn‐ to late‐D1sillimanite overprinting andalusite, petrogenetic grid considerations and quantitative estimates of metamorphic conditions for inferred overprinting assemblages. The effects of M1have been variably overprinted to the south‐east by ac. 1760 Ma M2–D2event. Much of the central Anmatjira Range, around Ingellina Gap, comprises orthogneiss, deformed during D2, and metapelites that have M1andalusite and K‐feldspar overprinted by M2sillimanite and muscovite. The south‐eastern portion of the range, around Mt Weldon, comprises metasediments and orthogneisses that were completely recrystallized during M2–D2, with metapelitic gneisses characterized by spinel + sillimanite + K‐feldspar + quartz‐bearing assemblages that suggest peak M2conditions of>750°C and 5.5 ± 1 kbar. Overprinting parageneses in metapelitic gneisses imply that D2occurred during essentially isobaric cooling. A third granulite facies event, M3, affected rocks in the Reynolds Range, immediately to the south of the Anmatjira Range, atc. 1730 Ma. A possible fourth event, M4, with a minimum age ofc. 1590 My affected both Ranges, but resulted in only minor overprinting of M1–3assemblages. The superimposed effects of M1–4, mapped for the entire Anmatjira–Reynolds Range area, indicate that only minor or no dislocation of the regional geology occurred during any of the metamorphic and accompanying folding, events. Although the immediate cause of each of the metamorphic events involved advection, the ultimate causes were external to the

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1990.tb00457.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

We present, as a progress report, a revised and much enlarged version of the thermodynamic dataset given earlier (Holland&Powell, 1985). This new set includes data for 123 mineral and fluid end‐members made consistent with over 200P–T–XCO2–fO2phase equilibrium experiments. Several improvements and advances have been made, in addition to the increased coverage of mineral phases: the data are now presented in three groups ranked according to reliability; a large number of iron‐bearing phases has been included through experimental and, in some cases, natural Fe:Mg partitioning data; H2O and CO2contents of cordierites are accounted for with the solution model of Kurepin (1985); simple Landau theory is used to model lambda anomalies in heat capacity and the Al/Si order–disorder behaviour in some silicates, and Tschermak‐substituted end‐members have been derived for iron and magnesium end‐members of chlorite, talc, muscovite, biotite, pyroxene and amphibole.For the subset of data which overlap those of Berman (1988), it is encouraging to find both (1) very substantial agreement between the two sets of thermodynamic data and (2) that the two sets reproduce the phase equilibrium experimental brackets to a very similar degree of accuracy. The main differences in the two datasets involve size (123 as compared to 67 end‐members), the methods used in data reduction (least squares as compared to linear programming), and the provision for estimation of uncertainties with this dataset. For calculations on mineral assemblages in rocks, we aim to maximize the information available from the dataset, by combining the equilibria from all the reactions which can be written between the end‐members in the minerals. For phase diagram calculations, we calculate the compositions of complex solid solutions (together withPandT) involved in invariant, univariant and divariant assemblages. Moreover we strongly believe in attempting to assess the probable uncertainties in calculated equilibria and hence provide a framework for performing simple error propagation in all calculations in thermocalc, the computer program we offer for an effective use of the dataset and the calculatio

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1990.tb00458.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

In a granulite‐facies spinel‐bearing quartzite, corundum, orthopyroxene and sapphirine (and rarely cordierite and sillimanite) form partial rims separating spinel from quartz. Textures indicate the reactions:spinel + quartz = orthopyroxene + corundum, andspinel + quartz = orthopyroxene + sapphirine.Thus, corundum and sapphirine are produced by reactions involving quartz. The low Al‐content of the orthopyroxene (0.5–2.8 wt %) and low values for Mg–Fe distribution coefficient for spinel–sapphirine and spinel–orthopyroxene reflect low‐temperature conditions during formation of the reaction products. Absence of zoning in spinel and a constant Mg–Fe distribution coefficient for spinel–sapphirine and spinel–orthopyroxene, over a compositional range, indicate Mg–Fe equilibration. It is suggested that stable reactions such as spinel + quartz = cordierite or spinel + quartz = garnet + sillimanite were over‐stepped and that metastable reactions give rise to the anomalous juxtaposi

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1990.tb00459.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

A combination of fluid inclusion, stable isotope and geochemical techniques has been used to study the nature of fluids present and their behaviour during Caledonian low‐grade metamorphism of the Harlech Dome, north Wales. Fluid inclusion studies show that in most of the metasedimentary sequence the peak metamorphic fluid was an aqueous Na–K–Cl brine but in the graphitic Clogau Formation and in parts of the overlying Maentwrog Formation immiscible H2O‐rich and CH4‐rich fluids coexisted.Late‐stage inclusions are of calcium‐rich brine and a dilute aqueous fluid. The chemical composition of chlorite in metamorphic veins and rocks varies between different formations and quartz‐oxygen isotopic compositions show considerable variation between different units. Both of these features are taken to indicate that there was little or no pervasive movement of fluid between different units at the peak of metamorphism. After the metamorphic peak there was focused flow of fluid upward through the sequence along fractures, in response to end‐Caledonian uplift and unloading. Where the migrating fluid crossed the graphitic shales, interaction between the fluid and the shales gave rise to the formation of the auriferous veins of the Dolgellau Gold Belt. Subsequent to this mineralizing event there was widespread development of18O‐enriched calcites and micas.In the case of vein minerals it is possible that these crystallized directly from late‐stage fluids at lower temperature than the quartz in the same veins. Alternatively, the original vein minerals may have re‐equilibrated with later18O‐enriched or cooler fluid. In the case of muscovites in the rock matrix it is proposed that the isotopically heavy compositions are the result of re‐equilibration of initially light grains with an introduced fluid, requiring considerable influx of fluid. This event may relate to either of two late‐stage

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1990.tb00460.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1990.tb00461.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY