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1.
Pressure-temperature conditions of pelites in the Ballachulishaureole, Scotland, have been determined from a calibrated petrogeneticgrid and from published geothermometers and geobarometers. Tocalibrate the mineral reactions in the grid, thermodynamic datafor appropriate end members of Ms, Chi, Qtz, And, Sil, Ky, Crn,Crd, Kfs, and Bt were derived from experimental data. This approachwas hampered by the unknown compositions of many of the mineralsused in the experiments, and by apparent inconsistency betweenthe experiments. A best compromise grid that satisfies mostof the data was obtained, which is applicable to the Ballachulishand other contact aureoles. In this grid, the first developmentof sillimanite is constrained to lie between the Richardsonet al. (1969) and Holdaway (1971) andalusite-sillimanite boundaries. A pressure estimate of 3.0 + 0.5 kb is obtained from the calibratedgrid, within 0.3 kb of estimates from geobarometry and fromtwo other independent petrological studies. Temperatures rangedfrom 560?20?C at the first development of cordierite in theassemblage Ms+Qtz+Chl+Crd+Bt to 750–800?C in Grt+Crd+Hyassemblages in pelitic screens within the igneous complex. In graphitic slates, in contrast to non-graphitic pelites, anentire andalusite-bearing subzone is developed, and initialcordierite development occurs further from the igneous contacts.The presence of graphite lowered aH2o in the slaters, expandingthe stability field of the andalusite-bearing assemblage And+Qtz+Bt+Ms+Crdrelative to the assemblage Kfs+Qtz+Bt+Ms+Crd developed in non-graphiticunits. Initial development of cordierite in the assemblage Ms+Qtz+Chl+Crd+Btwas also promoted by reduced aH2o in graphitic slates. The regular sequence and spacing of mineral zones in the aureolesuggests that gross equilibrium was attained during contactmetamorphism, even though the thermal metamorphic pulse is estimatedto have been less than 0.2 Ma (Buntebarth, in press). Thereis no evidence for reaction overstepping in cordierite-producingreactions.  相似文献   

2.
Constraints from P-T pseudosections (MnNCKFMASH system), foliation intersection/inflection axes preserved in porphyroblasts (FIAs), mineral assemblages and textural relationships for rocks containing all three Al2SiO5 polymorphs indicate a kyanite→ andalusite→ sillimanite sequential formation at different times rather than stable coexistence at the Al2SiO5 triple point. All three Al2SiO5 polymorphs grew in the Chl, Bt, Ms, Grt, St, Pl and Crd bearing Ordovician Clayhole Schist in Balcooma, northeastern Australia separately along a looped P-T-t-D path that swaps from clockwise to anticlockwise in the tectono-metamorphic history of the region. Kyanite grew during crustal thickening in an Early Silurian Orogenic event followed by decompression/heating, andalusite and fibrolitic sillimanite growth during Early Devonian exhumation.  相似文献   

3.
Low‐pressure crystal‐liquid equilibria in pelitic compositions are important in the formation of low‐pressure, high‐temperature migmatites and in the crystallization of peraluminous leucogranites and S‐type granites and their volcanic equivalents. This paper provides data from vapour‐present melting of cordierite‐bearing pelitic assemblages and augments published data from vapour‐present and vapour‐absent melting of peraluminous compositions, much of which is at higher pressures. Starting material for the experiments was a pelitic rock from Morton Pass, Wyoming, with the major assemblage quartz‐K feldspar‐biotite‐cordierite, approximately in the system KFMASH. A greater range in starting materials was obtained by addition of quartz and sillimanite to aliquots of this rock. Sixty‐one experiments were carried out in cold‐seal apparatus at pressures of 1–3.5 kbar (particularly 2 kbar) and temperatures from 700 to 840 °C, with and without the addition of water. In the vapour‐present liquidus relations at 2 kbar near the beginning of melting, the sequence of reactions with increasing temperature is: Qtz + Kfs + Crd + Sil + Spl + V = L; Qtz + Kfs + Crd + Spl + Ilm + V = Bt + L; and Qtz + Bt + V = Crd + Opx + Ilm + L. Vapour‐absent melting starts at about 800 °C with a reaction of the form Qtz + Bt = Kfs + Crd + Opx + Ilm + L. Between approximately 1–3 kbar the congruent melting reaction is biotite‐absent, and biotite is produced by incongruent melting, in contrast to higher‐pressure equilibria. Low pressure melts from pelitic compositions are dominated by Qtz‐Kfs‐Crd. Glasses at 820–840 °C have calculated modes of approximately Qtz42Kfs46Crd12. Granites or granitic leucosomes with more than 10–15% cordierite should be suspected of containing residual cordierite. The low‐pressure glasses are quite similar to the higher‐pressure glasses from the literature. However, XMg increases from about 0.1–0.3 with increasing pressure from 1 to 10 kbar, and the low‐temperature low‐pressure glasses are the most Fe‐rich of all the experimental glasses from pelitic compositions.  相似文献   

4.
Metapelites containing muscovite, cordierite, staurolite and biotite (Ms+Crd+St+Bt) are relatively rare but have been reported from a number of low-pressure (andalusite–sillimanite) regional metamorphic terranes. Paradoxically, they do not occur in contact aureoles formed at the same low pressures, raising the question as to whether they represent a stable association. A stable Ms+Crd+St+Bt assemblage implies a stable Ms+Bt+Qtz+Crd+St+Al2SiO5+Chl+H2O invariant point (IP1), the latter which has precluded construction of a petrogenetic grid for metapelites that reconciles natural phase relations at high and low pressure. Petrogenetic grids calculated from internally consistent thermodynamic databases do not provide a reliable means to evaluate the problem because the grid topology is sensitive to small changes in the thermodynamic data. Topological analysis of invariant point IP1 places strict limits on possible phase equilibria and mineral compositions for metamorphic field gradients at higher and lower pressure than the invariant point. These constraints are then compared with natural data from contact aureoles and reported Ms+Crd+St+Bt occurrences. We find that there are numerous topological, textural and compositional incongruities in reported natural assemblages that lead us to argue that Ms+Crd+St+Bt is either not a stable association or is restricted to such low pressures and Fe-rich compositions that it is rarely if ever developed in natural rocks. Instead, we argue that reported Ms+Crd+St+Bt assemblages are products of polymetamorphism, and, from their textures, are useful indicators of P–T  paths and tectonothermal processes at low pressure. A number of well-known Ms+Crd+St+Bt occurrences are discussed within this framework, including south-central Maine, the Pyrenees and especially SW Nova Scotia.  相似文献   

5.
In the Littleton Formation, garnet porphyroblasts preserve three generations of growth that occurred before formation of the Bolton Syncline. Inclusion trails of foliations overgrown by these porphyroblasts are always truncated by the matrix foliation suggesting that garnet growth predated the matrix foliation. In contrast, many staurolite porphyroblasts grew synchronously with formation of the Bolton Syncline. However, local rim overgrowths of the matrix foliation suggest that some staurolite porphyroblasts continued to grow after development of the fold during younger crenulation producing deformations. The axes of curvature or intersection of foliations defined by inclusion trails inside the garnet porphyroblasts lie oblique to the axial plane of the Bolton Syncline but do not change orientation across it. This suggests the garnets were not rotated during the subsequent deformation associated with fold development or during even younger crenulation events. Three samples also contain a different set of axes defined by curvature of inclusion trails in the cores of garnet porphyroblasts suggesting a protracted history of garnet growth. Foliation intersection axes in staurolite porphyroblasts are consistently orientated close to the trend of the axial plane of the Bolton Syncline on both limbs of the fold. In contrast, axes defined by curvature or intersection of foliations in the rims of staurolite porphyroblasts in two samples exhibit a different trend. This phase of staurolite growth is associated with a crenulation producing deformation that postdated formation of the Bolton Syncline. Measurement of foliation intersection axes defined by inclusion trails in both garnet and staurolite porphyroblasts has enabled the timing of growth relative to one another and to the development of the Bolton Syncline to be distinguished in rocks where other approaches have not been successful. Consistent orientation of foliation intersection axes across a range of younger structures suggests that the porphyroblasts did not rotate relative to geographical coordinates during subsequent ductile deformation. Foliation intersection axes in porphyroblasts are thus useful for correlating phases of porphyroblastic growth in this region.  相似文献   

6.
The assumption of oxygen isotope and major element equilibrium during prograde metamorphism was tested using staurolite‐grade pelitic schists that have undergone sequential porphyroblast growth and multiple episodes of recrystallization of matrix minerals and foliation development. Textural relationships are used to infer a metamorphic history that involves garnet growth followed by staurolite growth, with each porphyroblast growth event followed by at least one period of recrystallization of matrix minerals. Conventional geothermobarometry using Qtz–Grt–Pl–Ms–Bt ± St equilibria yields peak P–T conditions of c. 625 °C at 9–11 kbar, consistent with KMnFMASH petrogenetic grid predictions for stability of the assemblage Grt + St + Bt. Qtz–Grt oxygen isotope fractionations yield apparent temperatures of c. 590 °C and Qtz–St fractionations yield an apparent temperature of c. 595 °C. Diffusional modelling indicates that quartz isotopic compositions were reset by c. 30 °C via retrograde isotopic diffusional exchange with micas. The isotopic temperatures appear to be in excellent agreement with one another, and suggest oxygen isotope equilibrium was attained between garnet and staurolite at c. 625 °C. However, the agreement of Qtz–Grt and Qtz–Str isotopic temperatures is not consistent with petrographic observations (garnet grew before staurolite) and petrogenetic grid constraints that predict that garnet grows over a temperature interval of c. 525–550 °C. Given that: (i) oxygen diffusion rates in staurolite and garnet are slow enough to render an individual porphyroblast effectively closed to exchange after it forms; and (ii) matrix minerals are able to exchange isotopes via recrystallization during each period of deformation; garnet and staurolite could not have simultaneously achieved oxygen isotope equilibrium with each other or with minerals in the recrystallized matrix. Thus, the Qtz–Grt fractionations, which yield apparent temperatures that are in apparent agreement with peak metamorphic temperature and apparent temperatures for Qtz–St fractionations, cannot be fractionations resulting from equilibrium isotopic exchange. Instead, they are apparent fractionations between porphyroblasts formed at different temperature and times in the prograde P–T–D path, and quartz that recrystallized and exchanged with micas and plagioclase during several phases of deformation.  相似文献   

7.
The reaction muscovite+cordierite→biotite+Al2SiO5 +quartz+H2O is of considerable importance in the low pressure metamorphism of pelitic rocks: (1) its operation is implied in the widespread assemblage Ms + Crd +And± Sil + Bt + Qtz, a common mineral assemblage in contact aureoles and low pressure regional terranes; (2) it is potentially an important equilibrium for pressure estimation in low pressure assemblages lacking garnet; and (3) it has been used to distinguish between clockwise and anticlockwise P–T paths in low pressure metamorphic settings. Experiments and thermodynamic databases provide conflicting constraints on the slope and position of the reaction, with most thermodynamic databases predicting a positive slope for the reaction. Evidence from mineral assemblages and microtextures from a large number of natural prograde sequences, in particular contact aureoles, is most consistent with a negative slope (andalusite and/or sillimanite occurs upgrade of, and may show evidence for replacement of, cordierite). Mineral compositional trends as a function of grade are variable but taken as a whole are more consistent with a negative slope than a positive slope. Thermodynamic modelling of reaction 1 and associated equilibria results in a low pressure metapelitic petrogenetic grid in the system K2O–FeO–MgO–Al2O3–SiO2–H2O (KFMASH) which satisfies most of the natural and experimental constraints. Contouring of the Fe–Mg divariant interval represented by reaction 1 allows for pressure estimation in garnet‐absent andalusite+cordierite‐bearing schists and hornfelses. The revised topology of reaction 1 allows for improved analysis of P–T paths from mineral assemblage sequences and microtextures in the same rocks.  相似文献   

8.
Abstract Muscovite-poor pelitic schists in the wallrocks of the Proterozoic Annex sulphide deposit, near Prieska, South Africa, contain peak metamorphic assemblages including Crd + Bt + Sil, St + Sil + Bt, Crd + St + Bt and, rarely, Ky + St ° Crd. All rocks include oligoclase, quartz and commonly Fe–Mn garnet, with or without muscovite. Peak assemblages, assigned to M2 regional metamorphism in the Gordonia Belt (Namaqua Province), are syn- to post-kinematic with respect to the main S2 fabric although larger staurolite grains contain S1 inclusion trails. Garnet–biotite thermometry, utilizing corrections for Fe3+, Mn, AlVI and Ti, yields peak temperatures of 571–624°C at pressures of 4.5–6.0 kbar. Consideration of the sympathetic variation of XMn in garnet with XMg in biotite and the preserved zoning patterns in prograde garnets, together with the inferred prograde transition from kyanite to sillimanite, indicates that heating occurred during mild decompression to the M2 metamorphic peak. Sillimanite and cordierite grew last in the prograde sequence, possibly related to a pulse of thermal metamorphism (M3) that is found along the margin of the Keimoes Suite batholith to the north. Retrograde assemblages, including Ms + Ky + Chl + Qtz (after Crd + Bt), Ky + Ms (after Sil) and Chl + Ms (after St) indicate a period of isobaric cooling (M4a) terminated by rehydration in the kyanite stability field at about 500°C. The size difference between prograde (1–2-mm) and retrograde (0.05–0.1-mm) mineral grains indicates substantial undercooling below equilibrium positions of relevant retrograde reactions prior to rehydration, and explains why cordierite that grew during M2 is almost completely destroyed. Post-M4a regrowth of staurolite and garnet (M4b) is spatially linked to sites of M4a rehydration. It reached temperatures of 510–530°C, remaining within the stability field of kyanite. A best fit of the observed textural history to the Namaqua orogenic cycle involves collision and heating (M2/D2) followed by granite intrusion (M3), rifting (M4a) and renewed heating due to crustal loading during volcanism (M4b). The P–T path for the Annex region is consistent with those derived from elsewhere in the Gordonia Belt and, with modification, to that published already for the nearby Prieska Copper Mines.  相似文献   

9.
A sequence of partial melting reactions at Mt Stafford, central Australia   总被引：8，自引：2，他引：6  
Metasedimentary gneisses show a rapid change in grade in a 10  km wide low- P /high- T  regional aureole at Mt Stafford in the Arunta Block, central Australia. Migmatite occurs in all but the lowermost of five metamorphic zones, which grade from greenschist (Zone 1) through amphibolite (Zones 2–3) to granulite facies (Zones 4–5). The sequence of partial melting reactions inferred for metapelitic rocks is dependant upon protolith, temperature and fluid conditions. The metapelite solidus in Zone 2 reflects vapour-present melting at P ≈3  kbar and T  ≈640  °C, melting having initially been controlled by the congruent breakdown of the assemblage Crd–Kfs–Bt–Qtz. At slightly higher temperature, andalusite in leucosome formed via the reaction Kfs+Qtz+Bt+H2O→And+melt; And+melt having been stabilized by the presence of boron. Sillimanite coaxially replaces andalusite in the high-grade portion of Zone 2. In Zone 3, large aluminosilicate aggregates in leucosome are armoured by Spl–Crd±Grt symplectites. Garnet partially pseudomorphs biotite, cordierite or spinel in high-grade portions of Zone 3. Zone 4 Grt–Crd–Opx-bearing metapsammite assemblages and garnet-bearing leucosome reflect T  ≈800  °C and P =2.2±0.9  kbar. In the model KFMASH system the principal vapour-absent melting step reflected significant modal changes related to the breakdown of the As–Bt tie-line and the establishment of the Spl–Crd tie-line; the bulk rock geochemistry of migmatite samples straddle the Spl–Crd tie-line. The aluminous bulk-rock composition of the common bedded migmatite restricted its potential to witness garnet-forming and orthopyroxene-forming reactions, minor textural and modal changes in and above Zone 3 reflecting biotite destablization in biotite-poor assemblages.  相似文献   

10.
Abstract Low-pressure/high-temperature (low-P/high-T) metamorphic rocks of the Cooma Complex, southeastern Australia, show evidence of an anticlockwise pressure-temperature-time-deformation (P-T-t-D) path, similar to those of some other low-P/high-T metamorphic areas of Australia. Prograde paths are reasonably well constrained in cordierite-andalusite schists, cordierite-K-feldspar gneisses and andalusite-K-feldspar gneisses. These paths are inferred to be convex to the temperature axis, involving increase in pressure with increase in temperature. Evidence of the retrograde path is inconclusive, but is consistent with approximately isobaric cooling, as are available isotopic data on the Cooma Granodiorite, which indicate initially rapid cooling following attainment of peak temperatures. The retrograde path is inconsistent with either a clockwise P-T-t-D path involving rapid or even moderate decompression immediately post-dating the peak of metamorphism, or a path in which the retrograde component simply reverses the prograde component, because both these paths should cross reactions forming cordierite from aluminosilicate, for which no evidence has been observed. Determination of the deformational-metamorphic history of the complex is not straightfoward and depends on careful examination of critical samples. Evidence necessary for successful elucidation of the prograde, and part of the retrograde, deformational-metamorphic history in the Cooma Complex includes: (1) sequentially grown porphyroblasts that can be timed relative to surrounding foliations; (2) partial replacement microstructures providing relative timing of metamorphic reactions that cannot be timed relative to foliation development; (3) a tectonic marker foliation (S4 at Cooma) that allows correlation of foliations from one location to another; and (4) single samples containing all of the foliations and all generations of porphyroblast growth within a single metamorphic zone. The latest two or three foliations involve low strain accumulation, allowing relative timing relationships between foliations and porphyroblasts to be more clearly determined. Sequential porphyroblast growth and foliation development in the cordierite-andalusite schists is examined for situations involving rotation and non-rotation of porphyroblasts relative to geographically fixed coordinates. Although the number of foliations developed varies in the rotational situation, depending on the deformation history proposed, the sequential order of porphyroblast growths does not differ from the non-rotational situation. Thus, whether or not porphyroblasts rotated in the Cooma rocks, the sequence of reactions, and therefore P-T-t paths inferred from the relative timing of porphyroblast growths, remain the same, for the deformational histories evaluated.  相似文献   

11.
The Legs Lake shear zone is a crustal‐scale thrust fault system in the western Canadian Shield that juxtaposes high‐pressure (1.0+ GPa) granulite facies rocks against shallow crustal (< 0.5 GPa) amphibolite facies rocks. Hangingwall decompression is characterized by breakdown of the peak assemblage Grt + Sil + Kfs + Pl + Qtz into the assemblage Grt + Crd + Bt ± Sil + Pl + Qtz. Similar felsic granulite occurs throughout the region, but retrograde cordierite is restricted to the immediate hangingwall of the shear zone. Textural observations, petrological analysis using P–T/P–MH2O phase diagram sections, and in situ electron microprobe monazite geochronology suggest that decompression from peak conditions of 1.1 GPa, c. 800 °C involved several distinct stages under first dry and then hydrated conditions. Retrograde re‐equilibration occurred at 0.5–0.4 GPa, 550–650 °C. Morphology, X‐ray maps, and microprobe dates indicate several distinct monazite generations. Populations 1 and 2 are relatively high yttrium (Y) monazite that grew at 2.55–2.50 Ga and correspond to an early granulite facies event. Population 3 represents episodic growth of low Y monazite between 2.50 and 2.15 Ga whose general significance is still unclear. Population 4 reflects low Y monazite growth at 1.9 Ga, which corresponds to the youngest period of high‐pressure metamorphism. Finally, population 5 is restricted to the hydrous retrograded granulite and represents high Y monazite growth at 1.85 Ga that is linked directly to the synkinematic garnet‐consuming hydration reaction (KFMASH): Grt + Kfs + H2O = Bt + Sil + Qtz. Two samples yield weighted mean microprobe dates for this population of 1853 ± 15 and 1851 ± 9 Ma, respectively. Subsequent xenotime growth correlates with the reaction: Grt + Sil + Qtz + H2O = Crd. We suggest that the shear zone acted as a channel for fluid produced by dehydration of metasediments in the underthrust domain.  相似文献   

12.
Abstract Low-pressure prograde metamorphism of pelitic rocks in the Cooma Complex, south-east Australia, has produced cordierite-andalusite schists at intermediate grades. The first foliation (S1) is preserved largely as inclusion trails in cordierite porphyroblasts. Microstructural evidence indicates that the cordierite porphyroblasts grew during the early stages of development of a crenulation-foliation (S2) and that andalusite porphyroblasts grew during the development of a later crenulation-foliation (S3). Microstructural evidence also indicates that the andalusite was a product of the prograde reaction: cordierite + muscovite ± andalusite + biotite + quartz. The occurrence of the products of this reaction in 'beard'structures between cordierite microboudins formed by extension in S3 confirms that the andalusite grew during the development of S3. The investigation shows that porphyroblast-matrix relationships can preserve the orientation of an early S-surface that has been largely obliterated from the matrix, as well as providing relatively direct evidence of sequential mineral growth and metamorphic reactions.  相似文献   

13.
Porphyroblast inclusion trails: the key to orogenesis   总被引：8，自引：0，他引：8  
Detailed microstructural analysis of inclusion trails in hundreds of garnet porphyroblasts from rocks where spiral-shaped inclusion trails are common indicates that spiral-shaped trails did not form by rotation of the growing porphyroblasts relative to geographic coordinates. They formed instead by progressive growth by porphyroblasts over several sets of near-orthogonal foliations that successively overprint one another. The orientations of these near-orthogonal foliations are alternately near-vertical and near-horizontal in all porphyroblasts examined. This provides very strong evidence for lack of porphyroblast rotation.
The deformation path recorded by these porphyroblasts indicates that the process of orogenesis involves a multiply repeated two-stage cycle of: (1) crustal shortening and thickening, with the development of a near-vertical foliation with a steep stretching lineation; followed by (2) gravitational instability and collapse of this uplifted pile with the development of a near-horizontal foliation, gravitational spreading, near-coaxial vertical shortening and consequent thrusting on the orogen margins. Correlation of inclusion trail overprinting relationships and asymmetry in porphyroblasts with foliation overprinting relationships observed in the field allows determination of where the rocks studied lie and have moved within an orogen. This information, combined with information about chemical zoning in porphyroblasts, provides details about the structural/metamorphic ( P-T-t ) paths the rocks have followed.
The ductile deformation environment in which a porphyroblast can rotate relative to geographic coordinates during orogenesis is spatially restricted in continental crust to vertical, ductile tear/transcurrent faults across which there is no component of bulk shortening or transpression.  相似文献   

14.
Abstract Most porphyroblasts never rotate during ductile deformation, provided they do not internally deform during subsequent events, with the exception of relatively uncommon but spectacular examples of spiralling garnets. Instead, the surrounding foliation rotates and reactivates due to partitioning of the deformation around the porphyroblast. Consequently, porphyroblasts commonly preserve the orientation of early foliations and stretching lineations within strain shadows or inclusion trails, even where these structures have been rotated or obliterated in the matrix due to subsequent deformation. These relationships can be readily used to help develop an understanding of the processes of foliation development and they demonstrate the prominent role of reactivation of old foliations during subsequent deformation. They can also be used to determine the deformation history, as porphyroblasts only rotate when the deformation cannot partition and involves progressive shearing with no combined bulk shortening component.  相似文献   

15.
Abstract Reactivation of early foliations accounts for much of the progressive strain at more advanced stages of deformation. Its role has generally been insufficiently emphasized because evidence is best preserved where porphyroblasts which contain inclusion trails are present. Reactivation occurs when progressive shearing, operating in a synthetic anastomosing fashion parallel to the axial planes of folds, changes to a combination of coarse- and finescale zones of progressive shearing, some of which operate antithetically relative to the bulk shear on a fold limb. Reactivation of earlier foliations occurs in these latter zones. Reactivation decrenulates pre-existing or just-formed crenulations, generating shearing along the decrenulated or rotated pre-existing foliation planes. Partitioning of deformation within these foliation planes, such that phyllosilicates and/or graphite take up progressive shearing strain and other minerals accommodate progressive shortening strain, causes dissolution of these other minerals. This results in concentration of the phyllosilicates in a similar, but more penetrative manner to the formation of a differentiated crenulation cleavage, except that the foliation can form or intensify on a fold limb at a considerable angle to the axial plane of synchronous macroscopic folds. Reactivation can generate bedding-parallel schistosity in multideformed and metamorphosed terrains without associated folds. Heterogeneous reactivation of bedding generates rootless intrafolial folds with sigmoidal axial planes from formerly through-going structures. Reactivation causes rotation or ‘refraction’of axial-plane foliations (forming in the same deformation event causing reactivation) in those beds or zones in which an earlier foliation has been reactivated, and results in destruction of the originally axial-plane foliation at high strains. Reactivation also provides a simple explanation for the apparently ‘wrong sense’, but normally observed ‘rotation’of garnet porphyroblasts, whereby the external foliation has undergone rotation due to antithetic shear on the reactivated foliation. Alternatively, the rotation of the external foliation can be due to its reactivation in a subsequent deformation event. Porphyroblasts with inclusion trails commonly preserve evidence of reactivation of earlier foliations and therefore can be used to identify the presence of a deformation that has not been recognized by normal geometric methods, because of penetrative reactivation. Reactivation often reverses the asymmetry between pre-existing foliations and bedding on one limb of a later fold, leading to problems in the geometric analysis of an area when the location of early fold hinges is essential. The stretching lineation in a reactivated foliation can be radically reoriented, potentially causing major errors in determining movement directions in mylonitic schistosities in folded thrusts. Geometric relationships which result from reactivation of foliations around porphyroblasts can be used to aid determination of the timing of the growth of porphyroblasts relative to deformation events. Other aspects of reactivation, however, can lead to complications in timing of porphyroblast growth if the presence of this phenomenon is not recognized; for example, D2-grown porphyroblasts may be dissolved against reactivated S1 and hence appear to have grown syn-D1.  相似文献   

16.
The exceptional andalusite–kyanite–andalusite sequence occurs in Al‐rich graphitic slates in a narrow pelite belt on the hangingwall of a ductile normal fault in NW Variscan Iberia. Early chiastolite is replaced by Ky–Ms–Pg aggregates, which are overgrown by pleochroic andalusite near granites intruded along the fault. Slates plot in AKFM above the chloritoid‐chlorite tie‐line. Their P–T grids are modelled with Thermocalc v2.7 and the 1998 databases in the NaKFMASH and KFMASH systems. The univariant reaction Ctd + And/Ky = St + Chl + Qtz + H2O ends at progressively lower pressure as F/FM increases and A/AFM decreases, shrinking the assemblage Cld–Ky–Chl, and opening a chlorite‐free Cld–Ky trivariant field on the low temperature reaction side. This modelling matches the observed absence of chlorite in high F/FM rocks, which is restricted to low pressure in the andalusite stability field. The P–T path deduced from modelling shows a first prograde event in the andalusite field followed by retrogression into the kyanite field, most likely coupled with a slight pressure increase. The final prograde evolution into the andalusite field can be explained by two different prograde paths. Granite intrusion caused the first prograde part of the path with andalusite growth. The subsequent thermal relaxation, together with aH2O decrease, generated the retrograde andalusite–kyanite transformation, plus chlorite consumption and chloritoid growth. This transformation could have been related to folding in the beginning, and aided later by downthrowing due to normal faulting. Heat supplied by syntectonic granite intrusion explains the isobaric part of the path in the late stages of evolution, causing the prograde andalusite growth after the assemblage St–Ky–Chl. Near postectonic granites, a prograde path with pressure decrease originated the assemblage St–And–Chl.  相似文献   

17.
In the metamorphic cores of many orogenic belts, large macroscopic folds in compositional layering also appear to fold one or more pervasive matrix foliations. The latter geometry suggests the folds formed relatively late in the tectonic history, after foliation development. However, microstructural analysis of four examples of such folds suggests this is not the case. The folds formed relatively early in the orogenic history and are the end product of multiple, near orthogonal, overprinting bulk shortening events. Once large macroscopic folds initiate, they may tighten further during successive periods of sub-parallel shortening, folding or reactivation of foliations that develop during intervening periods of near orthogonal shortening. Reactivation of the compositional layering defining the fold limbs causes foliation to be rotated into parallelism with the limbs.Multiple periods of porphyroblast growth accompanied the multiple phases of deformation that postdated the initial development of these folds. Some of these phases of deformation were attended by the development of large numbers of same asymmetry spiral-shaped inclusion trails in porphyroblasts on one limb of the fold and not the other, or larger numbers of opposite asymmetry spirals on the other limb, or similar numbers of the same asymmetry spirals on both limbs. Significantly, the largest disparity in numbers from limb to limb occurred for the first of these cases. For all four regional folds examined, the structural relationships that accompanied these large disparities were identical. In each case the shear sense operating on steeply dipping foliations was opposite to that required to originally develop the fold. Reactivation of the folded compositional layering was not possible for this shear sense. This favoured the development of sites of approximately coaxial shortening early during the deformation history, enhancing microfracture and promoting the growth of porphyroblasts on this limb in comparision to the other. These distributions of inclusion trail geometries from limb to limb cannot be explained by porphyroblast rotation, or folding of pre-existing rotated porphyroblasts within a shear zone, but can be explained by development of the inclusion trails synchronous with successive sub-vertical and sub-horizontal foliations.  相似文献   

18.
A variety of uncommon garnet-grade assemblages have been foundin rocks from three outcrops in the western part of centralNew Hampshire, and include the associations Grt+MrgCld, Grt+Bt+CldMrg,and Mrg+Cld+HblGrt (all rocks contain Ms, Chl, Ilm, and Qtz).These unusual rocks coexist with more typical Grt+Bt+Chl+Plmetapelites and amphibolites. Rim P–T conditions are {smalltilde}49035C and 5•751•25 kbar. Projection of the assemblages from Qtz, H2O, and Ilm into theCa–Al'–Na–(Fe+Mg) tetrahedron, and from Qtz,Ilm, H2O, and Chl into the Ca–Al'–Fe'–Mn tetrahedronindicates that Ca/(Ca+Na) and Mn differ among the assemblagesin a systematic fashion. Common Grt+Bt+Chl+Pl assemblages arerestricted to relatively high Mn and low Ca/(Ca+Na) values,whereas Cld+Bt+Mrg and Cld+Hbl+Mrg assemblages are stable atlow Mn and high Ca/(Ca+Na). These data suggest that at thisgrade Cld+Bt is more stable than Grt+Chl in the KFMASH system,whereas in the Ca—KFMASH system, Hbl+Cld assemblages arestable. Composition space analysis using the singular value decompositionmethod indicates that compositions of minerals from individualsamples are consistent with local equilibrium, but that differentoutcrops may not have all equilibrated at the same P–T–aH2Oconditions. Thermodynamic analysis suggests that a garnet-zoneprograde sequence of ferromagnesian associations for these bulkcompositions would be Hbl+Cld+Grt+ChlBt+Cld+Grt+ChlBt+Grt+Chl. Staurolite-grade rocks from the same stratigraphic units areexposed across strike, and contain the assemblage Grt+StBtPl(all rocks contain Ms, Qtz, Chl, and Ilm). Margarite is commonlypresent as inclusions in the cores of garnets, but is absentas inclusions near garnet rims and from the matrix; conversely,staurolite inclusions are present towards the rims of the garnets,but are absent from the cores. These inclusion relations suggestthat margarite may react to form staurolite and garnet withincreasing grade via a reaction such as chlorite+margarite=staurolite+garnet+H2O. Biotite is common in the matrix but is not typically abundant,and appears to have been the last phase to join the assemblage.Biotite is inferred to have joined the Grt+St+Chl assemblagesafter margarite breakdown through the reaction Grt+Chl+Ms=St+Bt+H2O. Thus, uncommon margarite assemblages may evolve into commonGrt+Bt+St+Chl assemblages. * Present address: Department of Geology and Geophysics, University of Wisconsin-Madison, Madison, Wisconsin 53706.  相似文献   

19.
过去还无人指出过板块相对运动的方向与缓倾斜叶理、逆断层和断层上的线状指示物有直接关系,这是因为缓倾斜构造上的运动方向只和变厚了的造山地层的重力塌陷有关,它们和俯冲板块传递给仰冲板块的推力没有关系。缓倾斜叶理上的运动方向的线状指示物和斑状变晶中的叶理弯曲或叶理交切轴（FIA）并无直接关系,这是因为FIA的指向受缓倾斜叶理和斑状变晶边缘上产生的、近乎垂直的叶理之间的交切面控制。在班状变晶边缘上形成的、近乎垂直的叶理在基质中的方位可能在较大范围内变动,因为它们会在稍早期间形成的叶理再活化作用影响下发生转动或遭到破坏。斑状变晶边缘上近乎垂直的叶理,与形成于早期或晚期的缓倾斜叶理的交线,在后期的生长中被圈闭在班状变晶里,此交线规定出了FIA的方位,而与叶理上的运动方向无关。从美国佛蒙特州阿巴拉契亚山脉采集的FIA资料指出,在125km×35km的一片地区内,在该地岩层所发生的多次变形中,从未曾使早期形成的FIA组的方位发生变动。这种情况要求：后来的每一代褶皱都是由于渐进的。总体不均匀缩短作用造成的。这种情况表明：FIA保存着原始的运动方向,此方向未因以后的变形而转动。非洲板块与欧洲板块的相对运动方向和由阿尔卑斯期变质岩中叶理交切轴（FIAs）所指示  相似文献   

20.
In a number of recent papers, the theory has been postulated that porphyroblasts as a rule do not rotate with respect to geographical coordinates, and can be used to determine the original orientation of older foliations. Complex inclusion patterns in spiral garnets have even been used to advocate a new model of orogenesis, involving several alternating phases of horizontal shortening and extension. Critical assessment of the assumptions and data used to support the theory of irrotational porphyroblasts reveals numerous flaws. Millipede structures, used as proof for flow partitioning, can also form by other flow geometries. Evidence quoted to support irrotational behaviour of porphyroblasts is unsound. Porphyroblasts do occur in sets with a preferred orientation of the internal foliation trace, but these cannot be shown to represent original orientations. Microstructures which resemble truncation planes in spiral garnets are used as evidence that these structures developed by several phases of deformation and as proof for periodic extension and horizontal shortening in orogenesis. They can, however, also be explained by intermittent growth of a rotating porphyroblast during a single phase of deformation. Finally, porphyroblast sets in which orientation is a function of aspect ratio indicate that porphyroblast rotation with respect to kinematic axes does occur in at least some situations.  相似文献   

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