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1.
Quartz‐rich veins in metapelitic schists of the Sanandaj‐Sirjan belt, Hamadan region, Iran, commonly contain two Al2SiO5 polymorphs, and, more rarely, three coexisting Al2SiO5 polymorphs. In most andalusite and sillimanite schists, the types of polymorphs in veins correlate with Al2SiO5 polymorph(s) in the host rocks, although vein polymorphs are texturally and compositionally distinct from those in adjacent host rocks; e.g. vein andalusite is enriched in Fe2O3 relative to host rock andalusite. Low‐grade rocks contain andalusite + quartz veins, medium‐grade rocks contain andalusite + sillimanite + quartz ± plagioclase veins, and high‐grade rocks contain sillimanite + quartz + plagioclase veins/leucosomes. Although most andalusite and sillimanite‐bearing veins occur in host rocks that also contain Al2SiO5, kyanite‐quartz veins crosscut rocks that lack Al2SiO5 (e.g. staurolite schist, granite). A quartz vein containing andalusite + kyanite + sillimanite + staurolite + muscovite occurs in andalusite–sillimanite host rocks. Textural relationships in this vein indicate the crystallization sequence andalusite to kyanite to sillimanite. This crystallization sequence conflicts with the observation that kyanite‐quartz veins post‐date andalusite–sillimanite veins and at least one intrusive phase of a granite that produced a low‐pressure–high‐temperature contact aureole; these relationships imply a sequence of andalusite to sillimanite to kyanite. Varying crystallization sequences for rocks in a largely coherent metamorphic belt can be explained by P–T paths of different rocks passing near (slightly above, slightly below) the Al2SiO5 triple point, and by overprinting of multiple metamorphic events in a terrane that evolved from a continental arc to a collisional orogen.  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.
Raman spectra of the three Al2SiO5 polymorphs; andalusite, sillimanite and kyanite were recorded as a function of pressure at room temperature. All the Raman active bands which could be observed from the high-pressure cell showed a linear pressure dependence for each of the three Al2SiO5 polymorphs and no phase changes were observed over the pressure ranges used in this study. In andalusite and to a lesser extent in sillimanite, vibrations which could be correlated with internal motions of the SiO4 tetrahedra were generally well separated from the lattice modes and showed a greater pressure dependence than that observed for other modes. The distinct pressure dependence of the internal SiO4 modes is less evident in kyanite, probably due to the lack of continuous tetrahedral chains and the fact that the rigid SiO4 tetrahedra now form an integral part of the structural network. At ambient pressure, kyanite also exhibits two fluorescence bands at 705 and 706.2 nm which are due to small amounts of Cr3+ in the kyanite crystals. These fluorescence bands showed a non-linear frequency shift as pressure was increased.  相似文献   

4.
The distribution and textural features of staurolite–Al2SiO5 mineral assemblages do not agree with predictions of current equilibrium phase diagrams. In contrast to abundant examples of Barrovian staurolite–kyanite–sillimanite sequences and Buchan‐type staurolite–andalusite–sillimanite sequences, there are few examples of staurolite–sillimanite sequences with neither kyanite nor andalusite anywhere in the sequence, despite the wide (~2.5 kbar) pressure interval in which they are predicted. Textural features of staurolite–kyanite or staurolite–andalusite mineral assemblages commonly imply no reaction relationship between the two minerals, at odds with the predicted first development (in a prograde sense) of kyanite or andalusite at the expense of staurolite in current phase diagrams. In a number of prograde sequences, the incoming of staurolite and either kyanite, in Barrovian sequences, or andalusite, in Buchan‐type sequences, is coincident or nearly so, rather than kyanite or andalusite developing upgrade of a significant staurolite zone as predicted. The width of zones of coexisting staurolite and either kyanite, in Barrovian sequences, or andalusite, in Buchan‐type sequences, is much wider than predicted in equilibrium phase diagrams, and staurolite commonly persists upgrade until its demise in the sillimanite zone. We argue that disequilibrium processes provide the best explanation for these mismatches. We suggest that kyanite (or andalusite) may develop independently and approximately contemporaneously with staurolite by metastable chlorite‐consuming reactions that occur at lower P–T conditions than the thermodynamically predicted staurolite‐to‐kyanite/andalusite reaction, a process that involves only modest overstepping (<15°C) of the stable chlorite‐to‐staurolite reaction and which is favoured, in the case of kyanite, by advantageous nucleation kinetics. If so, the pressure difference between Barrovian kyanite‐bearing sequences and Buchan andalusite‐bearing sequences could be ~1 kbar or less, in better agreement with the natural record. The unusual width of coexistence of staurolite and Al2SiO5 minerals, in particular kyanite and andalusite, can be accounted for by a combination of lack of thermodynamic driving force for conversion of staurolite to kyanite or andalusite, sluggish dissolution of staurolite, and possibly the absence of a fluid phase to catalyse reaction. This study represents an example of how kinetic controls on metamorphic mineral assemblage development have to be considered in regional as well as contact metamorphism.  相似文献   

5.
A Nappe system south to southwest of the São Francisco Craton represents the southern extension of the Brasília belt and describes an inverted metamorphic pile of greenschist facies toward amphibolite facies. The Aiuruoca-Andrelândia nappe is one of the nappes of this system. The hind portion of the Aiuruoca-Andrelândia nappe, south of Caxambu and Aiuruoca (MG), consists of a structural-metamorphic domain transported toward the E-NE. There is a metamorphic transition, from the kyanite zone to kyanite and sillimanite coexistence, until the sillimanite zone. Metapelitic rocks preserve high-pressure parageneses (Rt–Ky–Grt–Ms–Bt–Pl–Qtz) and contain retrograde eclogitic rocks. Sil–Pl–Qtz coronitic intergrowths around garnets are common decompressive textures. Kyanite schists register the Pmax of 11 kbar at 660 °C and define a decompressive path until 6–7.5 kbar at 650 °C. These P–T conditions represent the equilibrium in S2 schistosity (amphibolite facies) and the beginning of the cooling path in the Ky–Sil transition. The decompressive path suggests an extrusional process, immediately after burying at about 60 km. Exhumation controlled by convergent events, related to the São Francisco Plate subduction and tectonic erosion, took these units, isothermally, to higher levels (20–33 km). Later, the metamorphic path shifted toward near-isobaric cooling.  相似文献   

6.
We have studied the paleomagnetism of the middle Cretaceous Iritono granite of the Abukuma massif in northeast Japan together with 40Ar–39Ar dating. Paleomagnetic samples were collected from ten sites of the Iritono granite (102 Ma 40Ar–39Ar age) and two sites of its associated gabbroic dikes. The samples were carefully subjected to alternating field and thermal demagnetizations and to rock magnetic analyses. Most of natural remanent magnetizations show mixtures of two components: (1) H component, high coercivity (Bc > 50–90 mT) or high blocking temperature (Tb > 350–560 °C) component and (2) L component, relatively low Bc or low Tb component. H component was obtained from all the 12 sites to give a mean direction of shallow inclination and northwesterly declination (I = 29.9°, D = 311.0°, α95 = 2.7°, N = 12). This direction is different from the geocentric axial dipole field at the present latitude (I = 56.5°) and the typical direction of the Cenozoic remagnetization in northeast Japan. Since rock magnetic properties indicate that the H component of the Iritono granite is carried mainly by magnetite inclusions in plagioclase, this component probably retains a primary one. Thus the shallow inclination indicates that the Abukuma massif was located at a low latitude (16.1 ± 1.6°N) about 100 Ma and then drifted northward by about 20° in latitude. The northwesterly deflection is attributed mostly to the counterclockwise rotation of northeast Japan due to Miocene opening of the Japan Sea. According to this model, the low-pressure and high-temperature (low-P/high-T) metamorphism of the Abukuma massif, which has been well known as a typical location, would have not occurred in the present location. On the other hand, the L component is carried mainly by pyrrhotite and its mean direction shows a moderate inclination and a northwesterly declination (I = 42.8°, D = 311.5°, α95 = 3.3°, N = 9). Since this direction is intermediate between the H component and early Cenozoic remagnetization in northeast Japan, some thermal event would have occurred at lower temperature than pyrrhotite Curie point ( 320 °C) during the middle Cretaceous to early Cenozoic time to have resulted in partial remagnetization.  相似文献   

7.
 The electronic structure of the three polymorphs of Al2SiO5, andalusite, sillimanite, and kyanite, is studied by linearized-augmented-plane-wave (LAPW) calculations using the WIEN code. Total energy calculations verify, in agreement with recent pseudopotential calculations, that andalusite is the most stable phase, followed by sillimanite and kyanite.We determine the electronic charge density distribution and find strong polarizations on all oxygen ions. We identify different polarizations due to Al or Si neighbors which depend on their respective distances to the oxygen atom. The chemical bonding is not purely ionic in nature but has important covalent contributions. Electric field gradients (EFGs) at all sites are calculated and agree well (within 10%) with available experimental data on Al. We identify the origin of the EFGs and demonstrate its relation to the nearest-neighbor coordination and the resulting anisotropy of the electronic charge distribution. Received: 22 March 2000 / Accepted: 3 August 2000  相似文献   

8.
The integration of new and published geochronologic data with structural, magmatic/anatectic and pressure–temperature (P–T) process information allow the recognition of high-grade polymetamorphic granulites and associated high-grade shear zones in the Central Zone (CZ) of the Limpopo high-grade terrain in South Africa. Together, these two important features reflect a major high-grade D3/M3 event at ~ 2.02 Ga that overprinted the > 2.63 Ga high-grade Neoarchaean D2/M2 event, characterized by SW-plunging sheath folds. These major D2/M2 folds developed before ~ 2.63 Ga based on U–Pb zircon age data for precursors to leucocratic anatectic gneisses that cut the high-grade gneissic fabric. The D3/M3 shear event is accurately dated by U–Pb monazite (2017.1 ± 2.8 Ma) and PbSL garnet (2023 ± 11 Ma) age data obtained from syntectonic anatectic material, and from sheared metapelitic gneisses that were completely reworked during the high-grade shear event. The shear event was preceded by isobaric heating (P = ~ 6 kbar and T = ~ 670–780 °C), which resulted in the widespread formation of polymetamorphic granulites. Many efforts to date high-grade gneisses from the CZ using PbSL garnet dating resulted in a large spread of ages (~ 2.0–2.6 Ga) that reflect the polymetamorphic nature of these complexly deformed high-grade rocks.  相似文献   

9.
The Achankovil Zone of southern India, a NW–SE trending lineament of 8–10 km in width and > 100 km length, is a kinematically debated crustal feature, considered to mark the boundary between the Madurai Granulite Block in the north and the Trivandrum Granulite Block in the south. Both these crustal blocks show evidence for ultrahigh-temperature metamorphism during the Pan-African orogeny, although the exhumation styles are markedly different. The Achankovil Zone is characterized by discontinuous strands of cordierite-bearing gneiss with an assemblage of cordierite + garnet + quartz + plagioclase + spinel + ilmenite + magnetite ± orthopyroxene ± biotite ± K-feldspar ± sillimanite. The lithology preserves several peak and post-peak metamorphic assemblages including: (1) orthopyroxene + garnet, (2) perthite and/or anti-perthite, (3) cordierite ± orthopyroxene corona around garnet, and (4) cordierite + quartz symplectite after garnet. We estimate the peak metamorphic conditions of these rocks using orthopyroxene-bearing geothermobarometers and feldspar solvus which yield 8.5–9.5 kbar and 940–1040 °C, the highest P–T conditions so far recorded from the Achankovil Zone. The retrograde conditions were obtained from cordierite-bearing geothermobarometers at 3.5–4.5 kbar and 720 ± 60 °C. From orthopyroxene chemistry, we record a multistage exhumation history for these rocks, which is closely comparable with those reported in recent studies from the Madurai Granulite Block, but different from those documented from the Trivandrum Granulite Block. An evaluation of the petrologic and geochronologic data, together with the nature of exhumation paths leads us to propose that the Achankovil Zone is probably the southern flank of the Madurai Granulite Block, and not a unit of the Trivandrum Granulite Block as presently believed. Post-tectonic alkali granites that form an array of “suturing plutons” along the margin of the Madurai Granulite Block and within the Achankovil Zone, but are absent in the Trivandrum Granulite Block, suggest that the boundary between the Madurai Granulite Block and the Trivandrum Granulite Block might lie along the Tenmalai shear zone at the southern extremity of the Achankovil Zone.  相似文献   

10.
From considerations of relativeG-T surfaces inferred from publishedP-T data and the occurrence of replacement textures of Al2SiO5 polymorphs in rocks, the relative positions of curves representing the following equation in K+ —T — pH ispace on substituting Al2SiO5 different polymorphs are derived.3 Al2SiO5 + 3 SiO2 (quartz) + 2 K+ + 3 H2O 2 KAl2[AlSi3O10](OH)2 (muscovite)+ 2 H+. The curves are different because of the differentG-T values for the polymorphs which, in the field, is borne out by the observation that in a rock containing two or three Al2SiO5 polymorphs, in nearly all instances only one polymorph is replaced by white mica. Instances of textural relations showing the interpreted selective replacement of one Al2SiO5 polymorph by a white mica in the presence of one (or two) other Al2SiO5 polymorph(s) are cited both from the literature and various field examples. The selective replacement of kyanite if sillimanite and/or andalusite is/are present, and of andalusite if only sillimanite is present are interpreted to show that generally during the muscovitization reaction, the field of sillimanite in the above reaction (left hand side) at a particular pH (H+ concentration) and is larger in K+ —T space than that of andalusite which in turn is larger than that of kyanite. Theoretically it is shown that variations to this can exist but the field evidence suggests these only occur under rare geological conditions. Although this is not totally conclusive, the selectiveness of the replacement is interpreted to show that the fluid was buffered with respect to K+ and H+ on or near the curve of the polymorph showing the lowest stability field until that polymorph is totally consumed, after which the fluid composition moves to the next lowest curve for the remaining polymorph(s) present in the rock. The alteration of more than one polymorph by an apparently simultaneous process of alteration is rare and usually occurs at a low grade of metamorphism. This is interpreted to show that the buffering reaction could not keep pace with the influx of fluid and change the composition of this fluid (in most cases).  相似文献   

11.
Oxygen‐isotope compositions of kyanite, andalusite, prismatic sillimanite and fibrolite from the Proterozoic terrane in the Truchas Mountains, New Mexico differ from one another, suggesting that these minerals did not grow in equilibrium at the Al2SiO5 (AS) polymorph‐invariant point as previously suggested. Instead, oxygen‐isotope temperature estimates indicate that growth of kyanite, andalusite and prismatic sillimanite occurred at c. 575, 615 and 640 °C respectively. Temperature estimates reported in this paper are interpreted as those of growth for the different AS polymorphs, which are not necessarily the same as peak metamorphic temperatures for this terrane. Two distinct temperature estimates of c. 580 °C and c. 700 °C are calculated for most fibrolite samples, with two samples yielding clear evidence of quartz‐fibrolite oxygen‐isotope disequilibrium. These data indicate that locally, and potentially regionally, oxygen‐isotope disequilibrium between quartz and fibrolite may have resulted from rapid fibrolite nucleation. Pressures of mineral growth that were extrapolated from oxygen‐isotope thermometry results and calculated using petrological constraints suggest that kyanite and one generation of fibrolite grew during M1 at 5 kbar, and that andalusite, prismatic sillimanite and a second generation of fibrolite grew during M2 at 3.5 kbar. M1 and M2 therefore represent two distinct metamorphic events that occurred at different crustal levels. The ability of the AS polymorphs to retain δ18O values of crystallization make these minerals ideal to model prograde‐growth histories of mineral assemblages in metamorphic terranes and to understand more clearly the pressure–temperature histories of multiple metamorphic events.  相似文献   

12.
The South Indian Craton is composed of low-grade and high-grade metamorphic rocks across different tectonic blocks between the Moyar–Bhavani and Palghat–Cauvery shear zones and an elongated belt of eastern margin of the peninsular shield. The Madras Block north of the Moyar–Bhavani shear zone, which evolved throughout the Precambrian period, mainly consists of high-grade metamorphic rocks. In order to constrain the evolution of the charnockitic region of the Pallavaram area in the Madras Block we have undertaken palaeomagnetic investigation at 12 sites. ChRM directions in 61 oriented block samples were investigated by Alternating Field (AF) and Thermal demagnetization. Titanomagnetite in Cation Deficient (CD) and Multi Domain (MD) states is the remanence carrier. The samples exhibit a ChRM with reverse magnetization of Dm = 148.1, Im = + 48.6 (K = 22.2, α95 = 9.0) and a palaeomagnetic pole at 37.5 °N, 295.6 °E (dp/dm = 7.8°/11.8°). This pole plots at a late Archaean location on the Indian Apparent Polar Wander Path (APWP) suggesting an age of magnetization in the Pallavaram charnockites as 2600 Ma. The nearby St. Thomas Mount charnockites indicate a period of emplacement at 1650 Ma (Mesoproterozoic). Thus the results of Madras Block granulites also reveal crustal evolution similar to those in the Eastern Ghats Belt with identical palaeopoles from both the areas.  相似文献   

13.
Aggregates composed of olivine and magnesiowüstite have been deformed to large strains at high pressure and temperature to investigate stress and strain partitioning, phase segregation and possible localization of deformation in a polyphase material. Samples with 20 vol.% of natural olivine and 80 vol.% of (Mg0.7Fe0.3)O were synthesized and deformed in a gas-medium torsion apparatus at temperatures of 1127 °C and 1250 °C, a confining pressure of 300 MPa and constant angular displacement rates equivalent to constant shear strain rates of 1–3.3 × 10− 4 s− 1. The samples deformed homogeneously to total shear strains of up to γ  15. During constant strain rate measurements the flow stress remained approximately stable at 1250 °C while it progressively decreased after the initial yield stress at the lower temperature. Mechanical data, microstructures and textures indicate that both phases were deforming in the dislocation creep regime. The weaker component, magnesiowüstite, controlled the rheological behavior of the bulk material and accommodated most of the strain. Deformation and dynamic recrystallization lead to grain refinement and to textures that were not previously observed in pure magnesiowüstite and may have developed due to the presence of the second phase. At 1127 °C, olivine grains behaved as semi-rigid inclusions rotating in a viscous matrix. At 1250 °C, some olivine grains remained largely undeformed while deformation and recrystallization of other grains oriented for a-slip on (010) resulted in a weak foliation and a texture typical for pure dry olivine aggregates. Both a-slip and c-slip on (010) were activated in olivine even though the nominal stresses were up to 2 orders of magnitude lower than those needed to activate these slip systems in pure olivine at the same conditions.  相似文献   

14.
Non-steady state deformation and annealing experiments on vein quartz are designed to simulate earthquake-driven episodic deformation in the middle crust. Three types of experiments were carried out using a modified Griggs-type solid medium deformation apparatus. All three start with high stress deformation at a temperature of 400 °C and a constant strain rate of 10− 4 s− 1 (type A), some are followed by annealing in the stability field of α-quartz for 14–15 h at zero nominal differential stress and temperatures of 800–1000 °C (type A + B), or by annealing for 15 h at 900 °C and at a residual stress (type A + C).The quartz samples reveal a very high strength > 2 GPa at a few percent of permanent strain. The microstructures after short-term high stress deformation (type A) record localized brittle and plastic deformation. Statisc annealing (type A + B) results in recrystallisation restricted to the highly damaged zones. The new grains aligned in strings and without crystallographic preferred orientation, indicate nucleation and growth. Annealing at non-hydrostatic conditions (type A + C) results in shear zones that also develop from deformation bands or cracks that formed during the preceding high stress deformation. In this case, however, the recrystallised zone is several grain diameters wide, the grains are elongate, and a marked crystallographic preferred orientation indicates flow by dislocation creep with dynamic recrystallisation. Quartz microstructures identical to those produced in type A + B experiments are observed in cores recovered from Long Valley Exploratory Well in the Quaternary Long Valley Caldera, California, with considerable seismic activity.The experiments demonstrate the behaviour of quartz at coseismic loading (type A) and subsequent static annealing (type A + B) or creep at decaying stress (type A + C) in the middle crust. The experimentally produced microfabrics allow to identify similar processes and conditions in exhumed rocks.  相似文献   

15.
Aluminum silicate triple-point occurrences are common in metamorphicrocks of northern New Mexico. The three polymorphs show extensivesolid solution, with Fe and Mn substituting for Al. Mineraltextures, the spatial distribution of phases, and the systematicpartitioning of Fe and Mn indicate that the kyanite-andalusite-sillimaniteassemblages crystallized in equilibrium. The compositions ofminerals in the three-phase assemblage vary across the studyarea, recording regional variations in the pressures and temperaturesof metamorphism. The highest-pressure rocks, exposed at RioMora, contain kyanite at higher elevations and sillimanite atlower elevations. A sub-horizontal isograd separates the twominerals. Kyanite and sillimanite have nearly identical Fe contentwhich varies systematically with XFe2O3 in hematite or ilmenite.Andalusite occurs only along a single manganiferous layer, incrystals rich in MnAlSiO5 and saturated in FeAlSiO5. Triple-pointassemblages can be found wherever the folded manganiferous layercrosses the unfolded kyanite-sillimanite isograd. The TruchasRange, preserving slightly lower pressures of metamorphism,shows kyanite-andalusite-sillimanite in rocks with titaniferoushematite. Andalusite is enriched in Fe relative to kyanite andsillimanite, but no polymorphs contain Mn. Rocks with lowerXFe2O3 in hematite have kyanite and sillimanite without andalusite,whereas rocks with pure hematite contain only andalusite. Theshallowest erosional levels are preserved in the western PicurisRange where the three polymorphs occur as pure minerals in ilmenite-bearingrocks. Hematitic samples contain only andalusite which showsextensive solid solution of both Fe and Mn. The assemblage kyanite-andalusite-sillimanite is not invariant.Iron and manganese both add degrees of freedom. These transitionmetals have stabilized the three-phase assemblage, in apparentchemical equilibrium, across a P-T interval of 500-540 ?C, 3?8-4?6kb in rocks from New Mexico. The saturation level of FeAlSiO5in andalusite does not vary with Mn content but does vary withpressure and temperature. Calculations indicate that a 2-3 kbdecrease in pressure or a 25-50 ?C increase in temperature resultsin a 1 mole per cent increase in XFeAlSiO5 in iron-saturatedandalusite.  相似文献   

16.
The metamorphic evolution of a key sector of the western Mediterranean internal Alpine orogenic belt (southern Calabrian Peloritani Orogen) is identified and described by means of P–T pseudosections calculated for selected metapelite specimens, showing evidence of multi-stage metamorphism.Attention focused on the two lowermost basement nappes of the Aspromonte Massif (southern Calabria), which were differently affected by poly-orogenic multi-stage evolution. After a complete Variscan orogenic cycle, the upper unit (Aspromonte Peloritani Unit) was involved in a late-Alpine shearing event. In contrast, the several underlying metapelite slices, here grouped together as Lower Metapelite Group, show exclusive evidence of a complete Alpine orogenic cycle.In order to obtain reliable P–T constraints, an integrated approach was employed, based on: a) garnet isopleth thermobarometry; and b) theoretical predictions of the P–T stability fields of representative equilibrium assemblages. This approach, which takes into account the role of the local equilibrium volumes in controlling textural developments, yielded reliable information about P–T conditions from early to peak metamorphic stages, as well as estimates of the retrograde trajectory in the pseudosection P–T space.According to inferred detailed P–T paths, the evolution of the Aspromonte Peloritani Unit is characterised by a multi-stage Variscan cycle, subdivided into an early crustal thickening stage with P–T conditions ranging from 0.56 ± 0.05 GPa at 570 ± 10 °C to 0.63–0.93 GPa at 650–710 °C (peak conditions) and evolving to a later crustal thinning episode in lower P–T conditions (0.25 GPa at 540 °C), as documented by the retrograde trajectory.Conversely, the prograde evolution of the rocks of the Lower Metapelite Group shows evidence of a HP-LT early Alpine multi-stage cycle, with P–T evolving from 0.75–0.90 GPa at 510–530 °C towards peak conditions, with pressure increasing northwards from 1.12 ± 0.02 GPa to 1.24 ± 0.02 GPa, and temperatures of 540–570 °C.A late-Alpine mylonitic overprint affected the rocks of both the Aspromonte Peloritani Unit and the Lower Metapelite Group. This overprint was characterised by an initial retrograde decompression trajectory (0.75 ± 0.05 GPa at 570–600 °C), followed by a joint cooling history, ranging from 0.38 ± 0.14 at temperature from 450 to 520 °C.These inferred results were then used: a) to interpret the Lower Metapelite Group as a single crystalline basement unit exclusively affected by a complete Alpine orogenic cycle, according to the very similar features of P–T paths, comparable petrography and analogous structural characteristics; b) as a tool for more reliable correlations between the Aspromonte Massif, the other Calabrian terranes and the north African Orogenic Complexes. They may therefore consider a contribution to the geodynamic modelling of the western Mediterranean.  相似文献   

17.
The unit-cell dimensions and crystal structure of sillimanite at various pressures up to 5.29 GPa have been refined from single-crystal X-ray diffraction data. As pressure increases, a and b decrease linearly, whereas c decreases nonlinearly with a slightly positive curvature. The axial compression ratios at room pressure are βa:βb:βc=1.22:1.63:1.00. Sillimanite exhibits the least compressibility along c, but the least thermal expansivity along a (Skinner et al. 1961; Winter and Ghose 1979). The bulk modulus of sillimanite is 171(1) GPa with K′=4 (3), larger than that of andalusite (151 GPa), but smaller than that of kyanite (193 GPa). The bulk moduli of the [Al1O6], [Al2O4], and [SiO4] polyhedra are 162(8), 269(33), and 367(89) GPa, respectively. Comparison of high-pressure data for Al2SiO5 polymorphs reveals that the [SiO4] tetrahedra are the most rigid units in all these polymorphic structures, whereas the [AlO6] octahedra are most compressible. Furthermore, [AlO6] octahedral compressibilities decrease from kyanite to sillimanite, to andalusite, the same order as their bulk moduli, suggesting that [AlO6] octahedra control the compression of the Al2SiO5 polymorphs. The compression of the [Al1O6] octahedron in sillimanite is anisotropic with the longest Al1-OD bond shortening by ～1.9% between room pressure and 5.29 GPa and the shortest Al1-OB bond by only 0.3%. The compression anisotropy of sillimanite is primarily a consequence of its topological anisotropy, coupled with the compression anisotropy of the Al-O bonds within the [Al1O6] octahedron.  相似文献   

18.
The Ross of Mull pluton consists of granites and granodioritesand intrudes sediments previously metamorphosed at amphibolitefacies. The high grade and coarse grain size of the protolithis responsible for a high degree of disequilibrium in many partsof the aureole and for some unusual textures. A band of metapelitecontained coarse garnet, biotite and kyanite prior to intrusion,and developed a sequence of textures towards the pluton. InZone I, garnet is rimmed by cordierite and new biotite. In ZoneII, coarse kyanite grains are partly replaced by andalusite,indicating incomplete reaction. Coronas of cordierite + muscovitearound kyanite are due to reaction with biotite. In the higher-gradeparts of this zone there is complete replacement of kyaniteand/or andalusite by muscovite and cordierite. Cordierite chemistryindicates that in Zone II the stable AFM assemblage (not attained)would have been cordierite + biotite + muscovite, without andalusite.The observed andalusite is therefore metastable. Garnet is unstablein Zone II, with regional garnets breaking down to cordierite,new biotite and plagioclase. In Zone III this breakdown is welladvanced, and this zone marks the appearance of fibrolite andK-feldspar in the groundmass as a result of muscovite breakdown.Zone IV shows garnet with cordierite, biotite, sillimanite,K-feldspar and quartz. Some garnets are armoured by cordieriteand are inferred to be relics. Others are euhedral with Mn-richcores. For these, the reaction biotite + sillimanite + quartz garnet + cordierite + K-feldspar + melt is inferred. Usinga petrogenetic grid based on the work of Pattison and Harte,pressure is estimated at 3·2 kbar, and temperature atthe Zone II–III boundary at 650°C and in Zone IV asat least 750°C. KEY WORDS: contact metamorphism; disequilibrium  相似文献   

19.
Characteristic patterns in the preferential replacement of Al2SiO5 polymorphs by white mica have been interpreted by Kwak (1971) as indicating that, for alkali equilibria, the fields in order of decreasing stability are: kyanite + soln < andalusite + soln < sillimanite + soln Alternatively, preferential replacement patterns may be interpreted as reflecting the repetition of certain types of Pressure-Temperature-time paths in various metamorphic episodes.I am grateful to J. S. Fox for commenting on this comment.  相似文献   

20.
“Hard” carbon-based Pennsylvania anthracite was deformed in the steady-state at high temperatures and pressures in a series of coaxial and simple shear experiments designed to constrain the role of shear strain and strain energy in the graphitization process. Graphitization did not occur in coaxially deformed anthracite. Nonetheless, dramatic molecular ordering occurs at T 700°C, with average bireflectance values (%) increasing from 1.68 at 700°C to 6.36 at 900°C. Romin is lowest and bireflectance is highest in zones of high strain (e.g., kink bands) at all temperatures.In anthracite samples deformed in simple shear over the 600°–900°C range at 1.0 GPa, average Romax (%) values increase up to 11.9, whereas average bireflectance (%) values increase up to 10.7. Bireflectance increases with increasing shear strain and locally exceeds 12.5%. Graphitization occurs in several anthracite sample deformed in simple shear at 900°C. X-ray diffraction and transmission electron microscopy confirms the presence of graphite with d002=0.3363 nm. These data strongly suggest that shear strain is the dominant factor responsible for the natural transformation of anthracite to graphite at temperatures far below the 1600°C required for graphitization of other hard carbons in earlier hydrostatic heating experiments at 0.5 GPa pressure.  相似文献   

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