摘要:

ABSTRACTA fluid dynamic model for a rigid spherical porphyroblast growing in a viscous fluid matrix undergoing simple shear deformation predicts an arrangement of captured inert inclusions that is remarkably similar to the spiral pattern observed in synkinematic ‘rolled’ garnets. The model assumes only creeping (Stokes) flow of the matrix and a kinetic model for the growth of the porphyroblast of the formrm=kmΩ, whereris the crystal radius, Ω is the amount of shear‐induced rotation, andmandkare kinetic parameters. For rotation less than 18d̀, J. B. Thompson and J. L. Rosenfeld's simple ring‐model provides a surprisingly accurate description of the three‐dimensional geometry of the central inclusion surface in the simulated porphyroblasts. Although marker planes, originally parallel to the shear plane, are deflected around the porphyroblast, their intersections with its surface remain approximately circular for much larger amounts of rotation than anticipated by Rosenfeld. The polar coordinate equation,T(r, θ), of the double spiral formed by the trace of the central inclusion surface in a section through the centre of simulated porphyroblasts, is also surprisingly simple, as follows:rm∼ (6/5k)mθ. This relationship was observed over the complete range of rotation angles investigated, 0‐720d̀, and could form a reasonable basis for estimating the growth‐rotation history of rolled garnets from the shape of their spiral inclusion trails. Two‐dimensional sections through the simulated porphyroblasts, parallel to the rotation axis, yield ‘clamshell’ (Rosenfeld's term) inclusion geometries similar to the controversial ‘millipede’ patterns observed

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1994.tb00044.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTThe Valentine wollastonite skarn in the north‐west Adirondack Mountains, New York, is a seven million ton deposit which resulted from channellized infiltration of H2O‐rich, silica‐bearing fluids. The wollastonite formed by reaction of these fluids with non‐siliceous calcite marble. The skarn formed at the contact of the syenitic Diana Complex and was subsequently overprinted by Grenville‐age granulite facies metamorphism and retrograde hydrothermal alteration during uplift. Calcite marbles adjacent to the deposit have generally high δ18O values (c. 21‰), typical of Grenville marbles which have not exchanged extensively with externally derived fluids. Carbon isotopic fractiona‐tions between coexisting calcite and graphite in the marbles indicate equilibration at 675d̀ C, consistent with the conditions of regional metamorphism. Oxygen isotopic ratios from wollastonite skarn are lower than in the marbles and show a 14‰ variation (‐1‰ to 13‰). Some isotopic heterogeneity is preserved from skarn formation, and some represents localized exchange with low‐δ18O retrograde fluids. Detailed millimetre‐ to centimetre‐scale isotopic profiles taken across skarn/marble contacts reveal steep δ18O gradients in the skarn, with values increasing towards the marble. The gradients reflect isotopic evolution of the fluid as it reacted with high δ18O calcite to form wollastonite. Calcite in the marble preserves high δ18O values to within<5 mm of the skarn contact. The preservation of high δ18O values in marbles at skarn contacts and the disequilibrium fractionation between wollastonite skarn and calcite marble across these contacts indicate that the marbles were not infiltrated with significant quantities of the fluid. Thus, the marbles were relatively impermeable during both the skarn formation and retrograde alteration. Skarn formation may have been episodic and fluid flow was either chaotic or dominantly parallel to lithological contacts. Although these steep isotope gradients resemble fluid infiltration fronts, they actually represent the sides of the major flow system. Because chromatographic infiltration models of mass transport require the assumption of pervasive fluid flow through a permeable rock, such models are not applicable to this hydrothermal system and, by extension, to many other metamorphic systems where low‐permeability rocks restrict fluid migration pathways. Minimum time‐integrated fluid fluxes have been calculated at the Valentine deposit using oxygen isotopic mass balance, reaction progress of fluid buffering reactions, and silica mass balance. All three approaches show that large volumes of fluid were necessary to produce the skarn, but silica mass balance calculations yield the largest minimum

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1994.tb00045.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTThe contact metamorphosed metapelitic and metapsammitic rocks surrounding the Stawell granite, western Victoria, Australia, are divided into three zones: the low‐grade zone, the medium‐grade zone and the high‐grade zone. Detailed petrological study shows consistency of element distributions, implying that equilibrium was widely attained in the rocks, although equilibrium volumes are generally small (millimetre scale) and considerable mineral chemical variations exist between adjacent domains. The metamorphic mineral assemblages are generally of high variance (KFMASH variance ≤ 2). Consequently, the chemical evolution of assemblages is controlled largely by bulk composition and metamorphic temperature, the former factor being more important in most rocks. The chemographic relations of mineral assemblages in low‐ and medium‐ to high‐grade zones are presented in compatibility diagrams projected from biotite, quartz and H2O, and biotite, K‐feldspar and H2O, respectively. These compatibility diagrams have the advantage of showing both quartz‐bearing and quartz‐absent assemblages. The metamorphic reactions are modelled successfully by a calculated petrogenetic grid that combines both KFASH and KMASH equilibria. Based on petrographic observations and with constraints from the calculated petrogenetic grid, the following KFMASH reactions, in the order of increasing metamorphic grade, are responsible for producing the various mineral assemblages in the Stawell rocks:chl + mu + q = bi + cd + V,chl + q + cd = g + V,mu + bi + q = ksp + cd + V,mu + q = ksp + and + cd + V (or KASH mu + q = ksp + and + V),mu + cd = ksp + and + bi + V,mu + bi + and = ksp + sp + V,and + bi = ksp + sp + cd + V,mu + bi = ksp + cor + sp + V,mu = ksp + cor + and + sp + V (or KASH mu = ksp + cor + V),bi + cd + q = g + ksp + V.The combined KFASH and KMASH grid provides constraints on reaction coefficients in the above sequence of reactions and on temperature and press

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1994.tb00046.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTDetailed textural and chemical data for mineral assemblages on a regional scale are presented for the metaandesitic Eocene‐Oligocene Taveyanne greywacke of the Glarus Alps, Eastern Switzerland. Presented data indicate an increase of metamorphic grade from zeolite facies to prehnite‐pumpellyite and pumpellyite‐actinolite facies. Low‐grade outcrops contain laumontite, minor corrensite and pumpellyite (assemblage type 1), whereas outcrops of higher metamorphic grade contain prehnite and two populations of pumpellyite (type 2), prehnite—pumpellyite‐(Al)—white mica (type 3), a single outcrop shows pumpellyite‐actinolite facies (type 4). From the zeolite to prehnite‐pumpellyite/pumpellyite‐actinolite facies there are indications for an increase of the chemical equilibrium domain size for the critical paragenesis from a single detrital grain ≤1 mm) in type 1, to a few millimetres in type 2, and to a whole thin section in type 3. MetamorphicP‐Tconditions were determined by a combination of chlorite thermometry, fluid inclusion and vitrinite reflectance data. Peak temperatures range from 170‐190d̀ C for zeolite facies to 270‐310d̀ C for prehnite‐pumpellyite and pumpellyite‐actinolite facies. For the higher temperature range, pressures of 2‐3 kbar are derived indicating a geothermal gradient of 24‐32d̀ C km‐1. The well‐constrained temperature estimations derived for the assemblages provide a useful test of the different empirical calibrations of chlorite thermometers recently proposed. The best correspondence to the temperatures determined here is for the Cathelineau calibration. In addition, in the lower grade samples differences in textures and calculated temperatures provide a mean to distinguish b

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1994.tb00047.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTDiscordant andalusite‐biotite‐quartz‐bearing veins occur in the contact aureole of the Vedrette di Ries pluton (Italian Eastern Alps), never outside the area of contact metamorphic andalusite development. Andalusite veins are found only within andalusite‐bearing hornfelses, and vein biotite occurs wherever host‐rock garnet is partially replaced by biotite. Veins formed during contact metamorphism, synchronously with the crystallization of andalusite and biotite within host rocks. Their pegmatitic structure and their orientation suggest that vein parageneses crystallized within fluid‐filled cavities that opened by hydraulic fracturing.A mechanism ofsynmetamorphic veiningis proposed to explain rock failure and subsequent mineral deposition within veins. During hydrofracturing induced by dehydration reactions in response to heating in the aureole, fissures were immediately filled with locally derived fluids. The lack of large‐scale flux, together with high fluid pressures required by hydrofracturing, suggest fluid in the cavities was a virtually stagnant, passive medium, and that mass‐transport toward fractures was driven by intergranular diffusion. Because temperature andPfvalues within veins are similar to those in the host rock, vein assemblages are interpreted as the stable, high‐Tside of reactions taking place within pelitic schists, at the time when fractures opened. Once nucleation of product phases occurred, chemical components released by dissolution of reactant minerals were driven to precipitation sites by chemical potential gradients. Since nucleation was favoured at the strained grains of vein walls, andalusite and biotite simultaneously grew in vein and host rock.The proposed genetic model contrasts with generally adopted metasomatic mechanisms for the genesis of Al2SiO5‐bearing veins, in not requiring large fluid/rock ratios or a highly ‘aggressive’ fluid composition. The mechanism of synmetamorphic veining may be particularly useful in the interpretation of vein occurrences in medium‐ and deep‐crustal rocks which have undergone

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1994.tb00048.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTThe Witwatersrand goldfields contain abundant assemblages that include pyrophyllite, chloritoid, chlorite, kaolinite and/or kyanite, with quartz. A chemographic analysis of the system Fe(Mg)‐Al‐Si‐O‐H involving these minerals yields 22 potential phase diagrams. Using orientation criteria and thermodynamic calculations as further constraints, this list has been reduced to three possible diagrams. New thermodynamic data favour one of these in particular.This chemographic analysis demonstrates that formation of chloritoid is not restricted to the breakdown reaction of kaolinite plus chlorite in the F(M)ASH system, as stated by previous studies, but could be from pyrophyllite + chlorite → chloritoid + quartz + H2O.The metamorphic temperature variation between Witwatersrand goldfields exceeded 65d̀ C, based on chlorite and chloritoid compositions. The lower and upper pressure limits are constrained by the andalusite to kyanite, and the sudoite/chlorite to carpholite boundaries, i.e. 1.5–2.8, and 7 kbar, respectively. The widespread pyrophyllite, chlorite and Fe‐chloritoid in all the Witwatersrand goldfields, and the local occurrence of sudoite indicate a consistent low‐pressure environment in which Mg‐chloritoid w

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1994.tb00049.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTMineral and isotope studies were undertaken on migmatites from the Schwarzwald, Moldanubian zone of the Variscan belt. The aims of the study were to date the migmatite formation and to determine the processes involved in migmatization in order to evaluate their influence on isotopic resetting. Textural evidence and the comparison of mineral compositions from leucosomes and mesosomes of two centimetre‐scale migmatite profiles, respectively, suggest that migmatitic textures and mineral assemblages were formed by metamorphic segregation (deformation‐enhanced mass transport) rather than by partial melting (anatexis). The results of Rb‐Sr thin‐slab dating on these profiles indicate that Sr isotopes were not completely reset during migmatization. No true isochron ages, but ages of approximate isotopic homogenization were obtained on the thin slabs by calculating87Sr/86Sr ratios back to various stages in their evolution. The coincidence of these Rb‐Sr data with U‐Pb ages of monazites from migmatites and non‐migmatitic gneisses shows that gneisses and migmatites were formed during the same high‐temperature event in the Carboniferous (330‐335 Ma). The observation that high‐temperature metamorphism failed to equilibrate Sr isotopes on the centimetre‐scale imposes limitations on the use of conventional whole‐rock isochron techniqu

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1994.tb00050.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTPorphyroblast inclusion trails provide important information about the tectonometamorphic evolution of a metamorphic rock. However, there remains considerable controversy over whether porphyroblasts rotate during bulk non‐coaxial deformation.With reference to an area of the Scandinavian Caledonides and utilizing existing data from theoretical and experimental modelling, this study demonstrates that both ‘straight’ and ‘S‐shaped’ inclusion trails are consistent with an interpretation in terms of syndeformational porphyroblast growth in a regime approximating to Newtonian simple shear. At crustal strain rates of 10‐14s‐1and porphyroblast growth times of 0.1–1.0 Ma, it is shown that a maximum of 5d̀‐9d̀ angular rotation would occur during growth. At faster strain rates of 10‐12s‐1(e.g. those in a shear zone) porphyroblast angular rotations of 90d̀ are shown to occur in 0.1–0.25 Ma (i.e. times comparable with or faster than porphyroblastesis). In view of this, ‘S‐shaped’ inclusion trails are to be expected in porphyroblasts growing in active shear zones or other situations of high shear strain, whereas ‘straight’ inclusion trails can be interpreted as static overgrowth of an existing fabric or as syndeformational

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1994.tb00051.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTThe Peripheral Schieferhülle of the Tauern Window of the Eastern Alps represents post‐Hercynian Penninic cover sequences and preserves a record of metamorphism in the Alpine orogeny, without the inherited remnants of Hercynian events that are retained in basement rocks. The temperature‐time‐deformation history of rocks at the lower levels of these cover sequences have been investigated by geochronological and petrographic study of units whoseP‐Tevolution and structural setting are already well understood.The Eclogite Zone of the central Tauern formed from protoliths with Penninic cover affinities, and suffered early Alpine eclogite facies metamorphism before tectonic interposition between basement and cover. It then shared a common metamorphic history with these units, experiencing blueschist facies and subsequent greenschist facies conditions in the Alpine orogeny. The greenschist facies phase, associated with penetrative deformation in the cover and the influx of aqueous fluids, reset Sr isotopes in metasediments throughout the eclogite zone and cover schists, recording deformation and peak metamorphism at 28‐30 Ma.The Peripheral Schieferhülle of the south‐east Tauern Window yields Rb‐Sr white mica ages which can be tied to the structural evolution of the metamorphic pile. Early prograde fabrics pre‐date 31 Ma, and were reworked by the formation of the large north‐east vergent Sonnblick fold structure at 28 Ma. Peak metamorphism post‐dated this deformation, but by contrast to the equivalent levels in the central Tauern, peak metamorphic conditions did not lead to widespread homogenization of the Sr isotopes. Localized deformation continued into the cooling path until at least 23 Ma, partially or wholly resetting Sr white mica ages in some samples.These isotopic ages may be integrated with structural data in regional tectonic models, and may constrain changes in the style of crustal deformation and plate interaction. However, such interpretations must accommodate the demonstrable variation in thermal histories

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1994.tb00052.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1994.tb00053.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY