摘要:

AbstractRegional metamorphic rocks that form Late Palaeozoic subduction complexes in central Queensland, Australia, are products of two metamorphic episodes. Synaccretion metamorphism (M1) gave rise to prehnite‐pumpellyite and greenschist facies rocks, whereas a subsequent episode (M2) at about 250 Ma formed upper greenschist to upper amphibolite facies rocks of both intermediate‐ and low‐pressure type, probably in a compressive arc or back‐arc setting. A similar pattern can be recognized for 1000 km along the New England Fold Belt, although at several localities, where higher grade rocks are exposed, metamorphism was essentially continuous over the M1‐M2 interval, with a rapid rise in geothermal gradient at the end of accretion. Where out‐stepping of tectonic elements has occurred at long‐lived convergent margins elsewhere, similar overprinting of high‐ by lower‐pressure facies series is anticipated, complicating the tectonic interpretation of metamorphism. The discrete character of metamorphic events may be blurred where conditions giving rise to a major episode of accretion and out‐stepping are followed by the subduction of

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1993.tb00150.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractDiscontinuous ultramylonite zones cut Proterozoic granulite facies gneisses in MacRobertson Land, east Antarctica, and preserve evidence of ductile non‐coaxial flow and reverse sense of shear. Cross‐cutting relationships indicate that ultramylonite deformation involved overthrusting to the east, but progressively rotated to involve overthrusting to the north; rotation of the principal compressive stress axes is inferred. Extensive pseudotachylite developed during ultramylonitization, the history of individual ultramylonite zones having involved a single episode of pseudotachylite generation. Neoblastic sillimanite indicates ultramylonitization occurred at>520° C. On the basis of inferred recrystallized granulite facies mineral assemblages ultramylonitization occurred at>700° C, and ≤7.3 ± 0.5 kbar, ataH2O± 0.3 and lowaCO2. Comparison of these values with those suggested by metamorphic assemblages in rocks unaffected by mylonitization indicates that the Rayner Complex experienced a late increase in pressure of 1–2 kbar during ultramylonitization. TheP‐T‐aH2Oconditions of the ultramylonite zones are inferred to have been close to the solidus for minimum melting, pseudotachylite generation having involved a limited pressure drop during brittle fracturing at high strain rates. Most of the pseudotachylite veins are undeformed; the mechanism(s) of fracturing and melting must have caused strain hardening in rocks surrounding the ultramylonite, further strain having been mostly accommodated by a new or subsidiary shear zone. Renewed stress at reduced strain rates, or renewed stress in zones in which the proportion of pseudotachylite was significantly higher, could have led to the rare occurrences of deformed pseudotachylite. The preservation of fine‐grained pseudotachylite is dependent on

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1993.tb00151.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractThe high‐grade metamorphic rocks of southern Brittany underwent a complex tectonic evolution under variousP‐Tconditions (high‐P, high‐T), related to stacking of nappes during Palaeozoic continentcontinent collision.The east to west thrusting observed in the whole belt is strongly perturbed by vertical movements attributed to the ascent of anatectic granites in the high‐Tarea. The field reconstruction of subvertical, closed elliptical structures in gneisses and migmatites, associated with the subhorizontal, doubly radial pattern of stretching lineation in the mica schists, suggests the existence of an elliptical diapiric body buried at depth beneath the present erosion level.Deformation is associated with a complexP‐Tevolution partly recorded in aluminous gneisses (kinzigites, e.g. morbihanites). A chronology of successive episodes of mineral growth at different compositions is established by detailed studies of the mineral‐microstructure relationships inX‐Zsections, using the deformation‐partitioning concept (low‐ and high‐strain zones).Several thermometric and barometric calibrations are applied to mineral pairs either in contact or not in contact but in equivalent microstructiiral positions with respect to the deformation history. This methodology provides a continuous microstructural control ofP‐Tvariations through time and leads to threeP‐T‐t‐dpaths constructed from numerous successiveP‐Testimations. Path 1 is a clockwise retrograde path preserved in low‐strain zones, which records general exhumation movements after crustal thickening. Paths 2 and 3 are clockwise prograde/retrograde paths from high‐strain zones; they are interpreted and discussed in the light of models of crustal anatexis and upward movement of magma (diapirism). Deformation andP‐Teffects induced by diapirism can be distinguished from the general deformation‐metamorphic history of a belt, and would seem to be produced during a late stage of its history.The present microstructural‐petrological approach to defining successive mineral equilibria in relation to progressive deformation steps provides a far more accurate evaluation of the metamorphic evolution than

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1993.tb00152.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractFluid evolution paths in the COHN system can be calculated for metamorphic rocks if there are relevant data regarding the mineral assemblages present, and regarding the oxidation and nitrodation states throughout the entireP‐Tloop. The compositions of fluid inclusions observed in granulitic rocks from Rogaland (south‐west Norway) are compared with theoretical fluid compositions and molar volumes. The fluid parameters are calculated using aP‐Tpath based on mineral assemblages, which are represented by rocks within the pigeonite‐in isograd and by rocks near the orthopyroxene‐in isograd surrounding an intrusive anorthosite massif. The oxygen and nitrogen fugacities are assumed to be buffered by the coexisting Fe‐Ti oxides and Cr‐carlsbergite, respectively. Many features of the natural fluid inclusions, including (1) the occurrence of CO2‐N2‐rich graphite‐absent fluid inclusions near peak M2 metamorphic conditions (927° C and 400 MPa), (2) the non‐existence of intermediate ternary CO2‐CH4‐N2compositions and (3) the low‐molar‐volume CO2‐rich fluid inclusions (36–42 cm3mol−1), are reproduced in the calculated fluid system. The observed CO2‐CH4‐rich inclusions with minor N2(5 mol%) should also include a large proportion of H2O according to the calculations. The absence of H2O from these natural high‐molar‐volume CO2‐CH4‐rich inclusions and the occurrence of natural CH4‐N2‐rich inclusions are both assumed to result from preferential leakage of H2O. This has been previously experimentally demonstrated for H2O‐CO2‐rich fluid inclusions, and has also been theoretically predicted. Fluid‐deficient conditions may explain the relatively high molar volumes, but cannot be used to explain the o

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1993.tb00153.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractDeformed quartz veins in garnet‐zone schist adjacent to the active Alpine Fault, New Zealand, have fluid inclusions trapped along quartz grain boundaries. Textures suggest that the inclusions formed in their present shapes during annealing of the deformed veins. Many of the inclusions are empty, but some contain carbon dioxide with densities that range from 0.16 to 0.80 g cm−3. No water, nitrogen or methane was detected. The inclusions are considerably more CO2‐rich than either the primary metamorphic fluid (<5% CO2) or fluids trapped in fracture‐related situations in the same, or related, rocks (<50% CO2). Enrichment of CO2is inferred to have resulted from selective migration (wicking) of saline water from the inclusions along water‐wet grain boundaries after cooling‐induced immiscibility of a water‐CO2mixture. Inclusion volumes changed after loss of water. Non‐wetting CO2remained trapped in the inclusions until further percolation progressively removed CO2in solution. This mechanism of fluid migration dominated in ductile quartz‐rich rocks near, but below, the brittle‐ductile transition. At deeper levels, hydraulic fracturing is also an important mechanism for fluid migration, whereas at shallower levels advection through open fractures dominates th

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1993.tb00154.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractC‐O‐H fluid produced by the equilibration of H2O and excess graphite must maintain the atomic H/O ratio of water, 2:1. This constraint implies that all thermodynamic properties of the fluid are uniquely determined at isobaric‐isothermal conditions. The O2, H2O and CO2fugacities (fo2,fH2OandfCO2) of such fluids have been estimated from equations of state and fit as a function of pressure and temperature. These fugacities can be taken as characteristic for graphitic metamorphic systems in which the dominant fluid source is dehydration, e.g. pelitic lithologies. Because there are no compositional degrees of freedom for graphite‐saturated fluids produced entirely by dehydration, the variance of the dehydration process is not increased in comparison with that in non‐graphitic systems. Thus, compositional ‘buffering’of C‐O‐H fluids by dehydration equilibria, a common petrological model, requires that redox reactions, decarbonation reactions or external, H/O ± 2, fluid sources perturb the evolution of the metamorphic system. Such perturbations are not likely to be significant in metapelitic environments, but their tendency will be to increase thefO2of the fluid phase. At high metamorphic grades, pyrite desulphidation reactions may cause a substantial reduction offH2Oand slight increases infO2andfCO2relative to sulphur‐free fluid. At low metamorphic grade, sulphur solubility in H/O ± 2 fluids is so low that pyrite decomposition must occur by sulphur‐conserving reactions that cause iron depletion in silicates, a common feature of sulphidic pelites. With increasing temperature and sulphur solubility, pyrite desulphidation may be driven by dehydration reactions or infiltration of H2O‐rich fluids. The absence of magnetite and the assemblages carbonate + aluminosilicate or pyrite + pyrrhotite + ilmenite from most graphitic metapelites is consistent with an H/O = 2 model for GCOH(S) fluid. For graphitic rocks in which such a model is inapplicable, a phase diagram variable that defines the H/O ratio of GCOH(S) fluid is more useful than th

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1993.tb00155.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractDeclining temperatures during decay of a hydrothermal system, or during uplift and erosion, tend to result in veins involving progressive hydration reactions, e.g. veins with laumontite cutting prehnitepumpellyite facies rocks, and stilbite veins cutting laumontite veins.In contrast, examples are described of analcime replacement of heulandite along fractures in heulanditized vitric tuff, of replacement of analcime by albite along fractures in quartz‐analcime rock, of joint‐controlled replacement of heulandite in tuff by laumontite + quartz + (Na, K)‐feldspars, of replacement of laumontite by prehnite + quartz along fractures in alumontitized vitric tuff, and of laumontitebearing feldspathic sandstones cut by vein assemblages of quartz and prehnite ° Calcite. The vein mineral assemblage, sometimes with pumpellyite and/or epidote in the prehnite‐bearing veins, tends to spread as a zone of dehydration into the adjacent country rock. Except perhaps for albite replacement of analcime, and for laumontite replacement of heulandite, these open‐system reactions involve cation activity ratios in the fluid. All involve dehydration. They are favoured by an increase in temperature, and except under certain situations whereP‐Tequilibrium curves have negative slopes, are favoured by a fall inPH2O. Evidence indicates that in at least some cases the triggering mechanism was a drop inPH2O; this may be a widespread phenomenon associated with brittle fracture in the seismogenic upper crust. This may cause fluid pressure to drop from values approaching lithostatic to nearer hydrostatic, and equilibrium may be displaced to yield a less hydrous assemblage that appears as a dehydration vein and vein verge. The dehydration vein assemblage may be diagnostic of a higher grade mineral facies and adds to the mineral complexity attributable to varying permeabilities and fluid pressures in upper crustal strata. Mineral facies are likely to be more uniformly distributed in higher grade rocks from below the brittle‐ductile transition zone.Reactions involving complex solid solutions are inappropriate as fac

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1993.tb00156.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractThe North Shore Volcanic Group in northern Minnesota is part of the Middle Proterozoic Keweenawan sequence, one of the largest plateau lava provinces in the world. The primary geochemistry of the basalts suggests that volcanism occurred in an intracontinental rift environment. The subaerial lava flows, mainly amygdaloidal olivine tholeiites and tholeiites, have undergone low‐grade metamorphism from zeolite to lower greenschist facies. On the basis of alteration phases replacing the primary magmatic minerals, infilling amygdales and veins, and replacing secondary minerals, the following zones have been distinguished: (1) thomsonite‐scolecite‐smectite, (2) heulandite‐stilbite‐smectite, (3) laumontitechlorite‐albite, (4) laumontite‐chlorite‐albite ± prehnite ± pumpellyite and (5) epidote‐chlorite‐albite ± actinolite zone.In addition to the overall zonation based on mineral parageneses, zonations in the composition of the Ab content of the newly formed albite replacing primary Ca‐rich plagioclase and of the newly formed mafic phyllosilicates are observed within the sequence and within single flows. Mafic phyllosilicates in the upper part of the sequence (mainly smectites and mixed‐layer smectite/chlorites) display high Si and Ca + Na + K contents, whereas in the lower part of the sequence the amounts of Si and Ca + Na + K are markedly lower (mainly chlorites and mixed‐layer chlorite/smectites). Similar zonations are observed within the individual flows. The albite content of the newly formed plagioclase is highest, and the Si and Ca + Na + K content of the phyllosilicates lowest in the amygdaloidal flow top while the opposite is true for the massive flow interior.The above features suggest that the overall pattern is one of burial‐type metamorphism associated with extension in the rift setting. In detail, the mineral assemblages are controlled not only by the stratigraphic position but also by the flow morphology controlling permeability whose effect on the assemblages is most pronounced in the stratigraphically upper parts. This suggests that at the first stages of alteration (lowest grade) the patterns of fluid flow were important effects in controlling the assemblages. At greater burial depth, assemblages are more homogeneous, perhaps representative of a more even and pervasive flow pattern.Using the observed assemblages at face value to define grade and/or facies, different conditions would be assigned within the different morphological flow portions. Thus at low‐grade metamorphic conditions it is essential to integrate assemblages from different morphological flow portions in order to define satisfactorily th

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1993.tb00157.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractThe metamorphic history of the Middle to Upper Jurassic volcanic and hypabyssal rocks exposed in the Klamath Mountains and Sierra Nevada of California is related, in part, to the rifting of a volcano‐plutonic arc. The Callovian to Kimmeridgian rocks exposed in the region consist of, from north‐west to south‐east, a back‐arc ophiolite, a rifted volcanic arc and a volcanic arc complex. All of these units have been metamorphosed and contain various combinations of the phases chlorite, amphibole, epidote, prehnite and pumpellyite. Projection of coexisting phases onto the composition plane MgO/(MgO + FeO) and Al2O3+ Fe2O3‐ 0.75 CaO ‐ Na2O through quartz, water, albite and epidote results in consistent mineralogical compatibilities within each region, but crossing tie‐lines between regions. This suggests that the volcanic and hypabyssal rocks from each region have equilibrated under different intensive conditions.The back‐arc ophiolite in the north has suffered subseafloor high‐T/Phydrothermal metamorphism with geothermal gradients on the order of 100° C km−1. The rifted volcanic arc has undergone synchronous burial, hydrothermal and contact metamorphism. Metamorphic field gradients in the region pass through the prehnite‐pumpellyite and greenschist facies suggesting geothermal gradients on the order of 30° C km−1. The southernmost volcanic arc complexes contain metavolcanics of the pumpellyiteactinolite and greenschist facies suggesting moderate‐ to high‐P/Tmetamorphism and geothermal gradients on the order of 20° C km−1.The apparent increase in rifting and calculated geothermal gradients from south‐east to north‐west suggest that the observed very low‐ and low‐grade metamorphism may be a response to enhanced thermal gradients during extension of the volcanic arc. This correlation between the extent of rifting and metamorphism is consistent with a model of diastathermal metamorphism of a propagating rift along the western margin of North America during the Late Jurassic. The plate tectonic setting may be analogo

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1993.tb00158.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractThe Mesozoic Murihiku and Waipapa terranes are two accretionary wedges of linked forearc and trench sediments, respectively, that were juxtaposed in the early Cretaceous.Late Triassic to late Jurassic Murihiku terrane volcaniclastic sediments are folded into a regional syncline and have been diagenetically altered. There is a general relationship between zeolite occurrence, clay mineralogy, vitrinite reflectance and stratigraphic position. Youngest Jurassic sediments contain heulandite, analcime and stilbite, whereas late Triassic to mid‐Jurassic sediments have laumontite and heulandite (in detail the zeolite distribution is complicated). Tuffaceous horizons on the eastern limb of the syncline are calcitized rather than zeolitized. Post‐diagenetic fractures associated with uplift are laumontite‐filled. The inferred geothermal gradient isc.15° C km−1.The Waipapa terrane is an accretionary complex dominated by imbricated terrigenous sediments of Triassic and Jurassic age with enclosed Permian to Jurassic pelagic sediments and basalts. Late Jurassic sediments are massive volaniclastic sandstones. The sediments are non‐foliated, and metamorphic minerals in the massive sandstones have crystallized in specific domains. The observed metamorphic succession of prehnite‐pumpellyite and pumpellyite‐actinolite facies assemblages was overprinted in the imbricated rocks during a thermal event that was late in the deformation sequence and broadly coincident with hydraulic fracturing and veining.The metamorphic successions in the two terranes and their relationships to structural features are in excellent accord with accretionary

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1993.tb00159.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY