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1.
Petrology and phase equilibria of rocks from two profiles inEastern Nepal from the Lesser Himalayan Sequences, across theMain Central Thrust Zone and into the Greater Himalayan Sequencesreveal a Paired Metamorphic Mountain Belt (PMMB) composed oftwo thrust-bound metamorphic terranes of contrasting metamorphicstyle. At the higher structural level, the Greater HimalayanSequences experienced high-T/moderate-P metamorphism, with ananticlockwise P–T path. Low-P inclusion assemblages ofquartz + hercynitic spinel + sillimanite have been overgrownby peak metamorphic garnet + cordierite + sillimanite assemblagesthat equilibrated at 837 ± 59°C and 6·7 ±1·0 kbar. Matrix minerals are overprinted by numerousmetamorphic reaction textures that document isobaric coolingand re-equilibrated samples preserve evidence of cooling to600 ± 45°C at 5·7 ±1·1 kbar.Below the Main Central Thrust, the Lesser Himalayan Sequencesare a continuous (though inverted) Barrovian sequence of high-P/moderate-Tmetamorphic rocks. Metamorphic zones upwards from the loweststructural levels in the south are: Zone A: albite + chlorite + muscovite ± biotite; Zone B: albite + chlorite + muscovite + biotite + garnet; Zone C: albite + muscovite + biotite + garnet ± chlorite; Zone D: oligoclase + muscovite + biotite + garnet ± kyanite; Zone E: oligoclase + muscovite + biotite + garnet + staurolite+ kyanite; Zone F: bytownite + biotite + garnet + K-feldspar + kyanite± muscovite; Zone G: bytownite + biotite + garnet + K-feldspar + sillimanite+ melt ± kyanite. The Lesser Himalayan Sequences show evidence for a clockwiseP–T path. Peak-P conditions from mineral cores average10·0 ± 1·2 kbar and 557 ± 39°C,and peak-metamorphic conditions from rims average 8·8± 1·1 kbar and 609 ± 42°C in ZonesD–F. Matrix assemblages are overprinted by decompressionreaction textures, and in Zones F and G progress into the sillimanitefield. The two terranes were brought into juxtaposition duringformation of sillimanite–biotite ± gedrite foliationseams (S3) formed at conditions of 674 ± 33°C and5·7 ± 1·1 kbar. The contrasting averagegeothermal gradients and P–T paths of these two metamorphicterranes suggest they make up a PMMB. The upper-plate positionof the Greater Himalayan Sequences produced an anticlockwiseP–T path, with the high average geothermal gradient beingpossibly due to high radiogenic element content in this terrane.In contrast, the lower-plate Lesser Himalayan Sequences weredeeply buried, metamorphosed in a clockwise P–T path anddisplay inverted isograds as a result of progressive ductileoverthrusting of the hot Greater Himalayan Sequences duringprograde metamorphism. KEY WORDS: thermobarometry; P–T paths; Himalaya; metamorphism; inverted isograds; paired metamorphic belts  相似文献   

2.
The Kelly's Mountain gneiss complex of Cape Breton Island, Nova Scotia, is a migmatitic paragneiss dominated by biotite- and cordierite-bearing assemblages. Metamorphic grade throughout the complex is in the upper amphibolite facies, with garnet absent and only retrograde muscovite present. In the high grade core of the complex the reaction biotite+andalusite+quartz=cordierite+K-feldspar+sillimanite+ilmenite+H2O is preserved. The pelitic migmatites contain cordierite- and K-feldspar-rich leucosomes and biotite-rich melanosomes. Minor clinopyroxene-bearing amphibolite in the complex does not show migmatitic textures. The migmatites are interpreted as in situ peraluminous partial melts on the basis of phase relations and textural criteria. Retrograde metamorphism under conditions of high fluid pressure locally produced muscovite after K-feldspar and muscovite+green biotite+chlorite after cordierite in paragneiss, and sphene after ilmenite in amphibolite. Peak metamorphic conditions of 1–3.5 kb and 580–700° C are estimated. The high geothermal gradient inferred from these conditions was probably caused by the intrusion of diorites associated with the gneiss complex. The Kelly's Mountain complex represents a rare example of migmatites formed in the low-pressure facies series, and illustrates some of the reactions involving melting in high grade pelitic rocks.  相似文献   

3.
The compositions of biotite and muscovite were examined in terms of the paragenesis and the metamorphic grade in low- to medium-grade pelitic rocks of the Ryoke metamorphism in the Yanai district, southwest Japan. The biotite and muscovite that coexist with K-feldspar have a higher K component in an A'KF diagram than those in rocks lacking K-feldspar. This fact reflects an increase in the K2O content in muscovite, but in biotite it reflects an increase of not only the K2O content but also of the octahedral vacancy.
At higher metamorphic grade beyond the cordierite isograd, where cordierite coexists with neither chlorite nor K-feldspar, the biotite shows an increase in illite, K Aliv □xii−1 Si−1, and Tschermak components, Alvi Aliv R+−1 Si−1, where □xii and R+ denote the interlayer vacancy and (Fe+Mg+Mn), respectively. A reaction to define the cordierite isograd is proposed by treating this chemical change as being responsible for the first appearance of cordierite, i.e. K,Al-poor biotite+phengitic muscovite=K,Al-rich biotite+cordierite+quartz+water .By treating this as a key reaction in medium-grade metamorphism, a set of reaction in a progressive metamorphism is established for the Ryoke metamorphism, a typical low-pressure type metamorphism. Some textures in one of the high-grade areas, the K-feldspar-cordierite zone, suggest that a further two prograde reactions have taken place, i.e. andalusite+biotite+quartz=cordierite+K-feldspar+water
and   andalusite=sillimanite.quartz=cordierite+K-feldspar+water
This implies that this zone probably has a P–T  path involving isobaric heating.  相似文献   

4.
The Leverburgh Belt and South Harris Igneous Complex in South Harris (northwest Scotland) experienced high-pressure granulite facies metamorphism during the Palaeoproterozoic. The metamorphic history has been determined from the following mineral textures and compositions observed in samples of pelitic, quartzofeldspathic and mafic gneisses, especially in pelitic gneisses from the Leverburgh Belt: (1) some coarse-grained garnet in the pelitic gneiss includes biotite and quartz in the inner core, sillimanite in the outer core, and is overgrown by kyanite at the rims; (2) garnet in the pelitic gneiss shows a progressive increase in grossular content from outer core to rims; (3) the AlVI/AlIV ratio of clinopyroxene from mafic gneiss increases from core to rim; (4) retrograde reaction coronas of cordierite and hercynite+cordierite are formed between garnet and kyanite, and orthopyroxene+cordierite and orthopyroxene+plagioclase reaction coronas develop between garnet and quartz; (5) a P–T path is deduced from inclusion assemblages in garnet and from staurolite breakdown reactions to produce garnet+sillimanite and garnet+sillimanite+hercynite with increasing temperature; and (6) in sheared and foliated rocks, hydrous minerals such as biotite, muscovite and hornblende form a foliation, modifying pre-existing textures. The inferred metamorphic history of the Leverburgh Belt is divided into four stages, as follows: (M1) prograde metamorphism with increasing temperature; (M2) prograde metamorphism with increasing pressure; (M3) retrograde decompressional metamorphism with decreasing pressure and temperature; and (M4) retrograde metamorphism accompanied by shearing. Peak P–T conditions of the M2 stage are 800±30 °C, 13–14 kbar. Pressure increasing from M1 to M2 suggests thrusting of continental crust over the South Harris belt during continent–continent collision. The inferred P–T path and tectonic history of the South Harris belt are different from those of the Lewisian of the mainland.  相似文献   

5.
ABSTRACT The Darjeeling-Sikkim region provides a classic example of inverted Himalayan metamorphism. The different parageneses of pelitic rocks containing chlorite, biotite, garnet, staurolite, kyanite, sillimanite, plagioclase and K-feldspar are documented by a variety of textures resulting from continuous and discontinuous reactions in the different zones. Microprobe data of coexisting minerals show that XMg varies in the order: garnet < staurolite < biotite < chlorite. White mica is a solid solution between muscovite and phengite. Garnet is mostly almandine-rich and shows normal growth zoning in the lower part of the Main Central Thrust (MCT) zone, and reverse zoning in the upper part of the zone. Chemographical relations and inferred reactions for different zones are portrayed in AFM space. In the low-grade zones oriented chlorites and micas and rolled garnets grew syntectonically, and were succeeded by cross-cutting chlorites and micas and garnet rims. In the upper zones sillimanite, kyanite and staurolite crystallized during a static inter-kinematic phase. P-T contitions of metamorphism, estimated through different models of geothermobarometry, are estimated to have been 580°c for the garnet zone to a maximum of 770°c for the sillimanite zone. The preferred values of pressure range from 5.0 kbar to 7.7 kbar. Models to explain the inverted metamorphism include overthrusting of a hot high Himalayan slab along a c. 5 km wide ductile MCT zone and the syn- or post-metamorphic folding of isograds.  相似文献   

6.
Metamorphic zones in the Chinese Altai orogen have previously been separated into the kyanite- and andalusite-types, the andalusite-type being spatially more extensive. The kyanite-type involves a zonal sequence of biotite, garnet, staurolite, kyanite, sillimanite and, locally, garnet–cordierite zones. The andalusite-type zonal sequence is similar: it includes biotite, garnet and staurolite zones at lower-T conditions and sillimanite and garnet–cordierite zones at higher-T conditions, but additionally contains staurolite–andalusite and andalusite–sillimanite zones at intermediate-T conditions. As relic kyanite-bearing assemblages commonly persist in the staurolite–andalusite, andalusite–sillimanite and sillimanite zones, it is not clear that the distinction is valid. On the basis of a reevaluation of phase relations modelled in KMnFMASH and KFMASH pseudosections, kyanite and andalusite-bearing rocks of the Chinese Altai orogen record, respectively, the typical burial and exhumation history of the terrane. Mineral assemblages distributed through the various zones reflect a mix of portions of the ambient P–T array and the effects of evolving P–T conditions. The comparatively low-T biotite, garnet and staurolite zones mostly preserve kyanite-type peak assemblages that only experienced minor changes during exhumation. Rocks in the comparatively high-T sillimanite and garnet–cordierite zones are dominated by mineral assemblages of a transitional sillimanite type, having formed by the extensive modification of earlier higher pressure assemblages during exhumation. Only rocks in the intermediate-T kyanite and probably some lower sillimanite zones were clearly recrystallized by late stage andalusite metamorphism, producing the staurolite–andalusite and andalusite–sillimanite zones. This andalusite metamorphism could not reach an equilibrium state because of limited fluid availability.  相似文献   

7.
A quantitative petrogenetic grid for pelitic schists in the system KFMASH that includes the phases garnet, chlorite, biotite, chloritoid, cordierite, staurolite, talc, kyanite, andalusite, sillimanite, and pyrophyllite (with quartz, H2O and muscovite or K-feldspar in excess) is presented. The grid is based on thermodynamic data of Berman et al. (1985) and Berman (1988) for endmember KFASH and KMASH equilibria and natural Fe-Mg partitioning for the KFMASH system. Calculation of P-T slopes and the change in Fe/(Fe+Mg) along reactions in the KFMASH system were made using the Gibbs method. In addition, the effect on the grid of MnO and CaO is evaluated quantitatively. The resulting grid is consistent with typical Buchan and Barrovian parageneses at medium to high grades. At low grades, the grid predicts an extensive stability field for the paragenesis chloritoid+biotite which arises because of the unusual facing of the reaction chloritoid+biotite + quartz+H2O = garnet+chlorite+muscovite, which proceeds to the right with increasing T in the KFMASH system. However, the reaction proceeds to the left with increasing T in the MnKFASH system so the assemblage chloritoid + biotite is restricted to bulk compositions with high Fe/(Fe+Mg+Mn). Typical metapelites will therefore contain garnet+chlorite at low grades rather than chloritoid + biotite.  相似文献   

8.
Connemara pelites show progressive metamorphism from stauroliteto upper sillimanite zones and possess low Mg/(Fe + Mg) values,typically 0.30 to 0.35 from about 100 analyses. As a consequenceof their composition, many sillimanite zone pelites lack bothmuscovite and K-feldspar. Staurolite, garnet, biotite, muscovite,feldspars and iron ores have been microprobe analysed in 48samples. Assemblages, textures and mineral compositions indicatethat metamorphism followed a sequence of continuous and discontinuousreactions with systematic variations in mineral Mg/(Mg + Fe)as predicted by theory. Contrary to some common assumptions,most reaction takes place along divariant equilibria; univariantreactions are seldom reached because reactants such as chloriteor muscovite are first consumed along divariant curves. Pelitepetrogenetic grids showing univariant curves can only indicatelimits to natural assemblages; they typically do not show whichreactions have actually taken place. Physical conditions of metamorphism have been calculated bya variety of means; temperatures range from 550° for thestaurolite zone to 650° for the upper silimanite zone, withthe first appearance of sillimanite near 580°. An earlykyanite-staurolite metamorphism at pressures above about 5 kbwas followed by a steepening of the thermal gradient leadingto regional cordierite and andalusite. This was probably accompaniedby uplift with pressures of around 4 kb for roeks near the sillimanite-inisograd.  相似文献   

9.
Abstract Biotite and cordierite occur in a 1-km wide zone of pelitic hornfelses around the McGerrigle pluton. These phases display systematic changes in X Fe that can be attributed to continuous reactions involving chlorite or andalusite in the system KFMASH. Through much of the zone biotite and cordierite were products of the 'breakdown'of chlorite. Close to the pluton this continuous reaction was terminated by a discontinuous reaction that introduced andalusite. Pelites which interdigitate with apophyses of the intrusive at the pluton margin contain assemblages that record a continuous reaction between biotite, cordierite, andalusite, muscovite, and quartz or, alternatively, the discontinuous breakdown of muscovite and quartz to K-feldspar and andalusite.
The mole fraction of Fe in biotite and cordierite increased significantly with the progress of the first continuous reaction and apparently decreased during the second continuous reaction. The K D of Fe-Mg between the minerals decreased and apparently increased, respectively, during the two reactions.
Biotite-cordierite-chlorite assemblages are interpreted to have been stable at temperatures between 525° C and 615° C and biotite-cordierite-andalusite assemblages stable at temperatures between 615° C and 635° C. The confining pressure was estimated to have been < 2 kbar.
The results of this study suggest that the K D of Fe-Mg between biotite and cordierite is a function of temperature, the Fe-Mg exchange characteristics of the controlling continuous reaction and non-ideal mixing of Fe and Mg.  相似文献   

10.
The Sauwald Zone, located at the southern rim of the Bohemian Massif in Upper Austria, belongs to the Moldanubian Unit. It exposes uniform biotite + plagioclase ± cordierite paragneisses that formed during the post-collisional high-T/low-P stage of the Variscan orogeny. Rare metapelitic inlayers contain the mineral assemblage garnet + cordierite + green spinel + sillimanite + K-feldspar + plagioclase + biotite + quartz. Mineral chemical and textural data indicate four stages of mineral growth: (1) peak assemblage as inclusions in garnet (stage 1): garnet core + cordierite + green spinel + sillimanite + plagioclase (An35–65); (2) post-peak assemblages in the matrix (stages 2, 3): cordierite + spinel (brown-green and brown) ± sillimanite ± garnet rim + plagioclase (An10–45); and (3) late-stage growth of fibrolite, muscovite and albite (An0–15) during stage 4. Calculation of the P–T conditions of the peak assemblage (stage 1) yields 750–840°C, 0.29–0.53 GPa and for the stage 2 matrix assemblage garnet + cordierite + green spinel + sillimanite + plagioclase 620–730°C, 0.27–0.36 GPa. The observed phase relations indicate a clockwise P–T path, which terminates below 0.38 GPa. The P–T evolution of the Sauwald Zone and the Monotonous Unit are very similar, however, monazite ages of the former are younger (321 ± 9 Ma vs. 334 ± 1 Ma). This indicates that high-T/low-P metamorphism in the Sauwald Zone was either of longer duration or there were two independent phases of late-Variscan low-P/high-T metamorphism in the Moldanubian Unit.  相似文献   

11.
Medium grade metapelites of the Torrox unit (Betic-Rif Belt, S Spain) contain mineral assemblages consisting of garnet (Grt), staurolite (St), cordierite (Crd), biotite, kyanite, sillimanite, andalusite, muscovite (Ms) and quartz (Qtz) and record complex reaction processes of cordierite growth through garnet and staurolite decomposition. The reaction textures, the chemical composition of the reactant and product phases, including Fe-Mg-Mn partitioning, and the results of equilibrium thermodynamic calculations indicate that these cordierite-bearing assemblages are largely deviated from equilibrium. Furthermore, the actual cordierite-forming reactions, as estimated from the assemblage and associated textures, conflict with the predictions of thermodynamically based petrogenetic grids for the model pelite system KFMASH, either those that predict the stable coexistence of cordierite + muscovite plus garnet or staurolite or those that do not foresee a field of stability for these types of assemblages. This conflict is explained in terms of cordierite growth (at ca. 575 °C and 2.5 kbar) through metastable reactions whose operation was conditioned by the relict persistence of higher pressure phases (garnet and staurolite) and phase compositions (e.g. muscovite and biotite) after fast decompression. This interpretation militates against the existence of a wide P-T range of stable coexistence at low P of Crd + Ms + Qtz ± Grt ± St in medium grade metapelites of normal composition (i.e. poor in Zn and/or Mn). The triggering of metastable cordierite-forming reactions and the preservation of even subtle disequilibrium features associated to them indicate that the rocks underwent fast near-isothermal decompression from ca. 12 kbar down to 2–3 kbar, then rapid cooling. These inferences agree with independent evidence indicating that termination of alpine metamorphism in the western Betic-Rif Belt was related to the extensional collapse of thickened crust and that the latter had consisted of a single, continuous event. Received: 6 August 1998 / Accepted: 9 February 1999  相似文献   

12.
The Priest pluton contact aureole in the Manzano Mountains, central New Mexico preserves evidence for upper amphibolite contact metamorphism and localized retrograde hydrothermal alteration associated with intrusion of the 1.42 Ga Priest pluton. Quartz–garnet and quartz–sillimanite oxygen isotope fractionations in pelitic schist document an increase in the temperatures of metamorphism from 540 °C, at a distance of 1 km from the pluton, to 690 °C at the contact with the pluton. Comparison of calculated temperature estimates with one‐dimensional thermal modelling suggests that background temperatures between 300 and 350 °C existed at the time of intrusion of the Priest pluton. Fibrolite is found within 300 m of the Priest pluton in pelitic and aluminous schist metamorphosed at temperatures >580 °C. Coexisting fibrolite and garnet in pelitic schist are in oxygen isotope equilibrium, suggesting these minerals were stable reaction products during peak metamorphism. The fibrolite‐in isograd is coincident with the staurolite‐out isograd in pelitic schist, and K‐feldspar is not observed with the first occurrence of fibrolite. This suggests that the breakdown of staurolite and not the second sillimanite reaction controls fibrolite growth in staurolite‐bearing pelitic schist. Muscovite‐rich aluminous schist locally preserves the Al2SiO5 polymorph triple‐point assemblage – kyanite, andalusite and fibrolite. Andalusite and fibrolite, but not kyanite, are in isotopic equilibrium in the aluminous schist. Co‐nucleation of fibrolite and andalusite at 580 °C in the presence of muscovite and absence of K‐feldspar suggests that univariant growth of andalusite and fibrolite occurred. Kyanite growth occurred during an earlier regional metamorphic event at a temperature nearly 80 °C lower than andalusite and fibrolite growth. Quartz–muscovite fractionations in hydrothermally altered pelitic schist and quartzite are small or negative, suggesting that late isotopic exchange between externally derived fluids and muscovite, but not quartz, occurred after peak contact metamorphism and that hydrothermal alteration in pelitic schist and quartzite occurred below the closure temperature of oxygen self diffusion in quartz (<500 °C).  相似文献   

13.
A petrogenetic grid is presented for the system KFMASH (K2O-FeO-MgO-Al2O3-SiO2-H2O), including biotite, muscovite, K-feldspar, chlorite, chloritoid, staurolite, cordierite, garnet, orthoamphibole, orthopyroxene, spinel, andalusite, sillimanite, kyanite, quartz and corundum with H2O in excess, which was calculated using the computer program THERMOCALC and the Powell and Holland internally consistent thermodynamic dataset. By removing the normal constraint of having quartz in excess, both quartz-bearing and quartz-absent equilibria are shown. Quartz-absent equilibria are particularly relevant at high- T and low- P conditions, because of their common occurrence at these conditions. The calculated mineral assemblage and mineral compositional variations in terms of FeMg-1 and (Fe, Mg)SiAl-2 exchange vectors are broadly compatible with observations on natural rocks, particularly when non-KFMASH components are taken into account.  相似文献   

14.
Grandidierite, kornerupine, and tourmaline occur in high-grade pelitic gneisses from southeastern Ontario, Canada. The kornerupine occurs in quartz-bearing layers associated with biotite, cordierite, garnet, ilmenite, K-feldspar, magnetite, quartz, and, less commonly, sillimanite. Grandidierite is found in quartz-poor, cordierite+sillimanite segregations in contact with biotite, cordierite, ilmenite, K-feldspar, magnetite, sillimanite, and, more rarely, garnet. Tourmaline is sporadically distributed in all compositional layers, but is not in contact with the other borosilicates. There is no textural evidence for a reactive relationship among the three borosilicates. Neither chemical or textural equilibrium has been achieved on the scale of a thin section.It is proposed that the granite, K-feldspar-rich leucosomes, and different borosilicate assemblages in adjacent compositional layers evolved along a path of decreasing pressure and increasing temperature. The P-T path intersected a series of dehydration and melting reactions. This P-T path indicates that uplift had occurred before cooling had started and before the maximum temperature was reached. Corona and symplectite textures developed at various times during uplift both before and after cooling had started.  相似文献   

15.
During prograde metamorphism of the Connemara pelites, sillimanite first develops in biotite immediately adjacent to, or replacing, garnet. In some rocks, breakdown of garnet+muscovite and staurolite +muscovite+quartz leads to the development of fibrolite pseudomorphs after garnet. The textures indicate a constant volume replacement of garnet with movement of Al from staurolite and muscovite in the matrix towards the few, widely scattered, garnet sites. The complex ionic reaction patterns are the result of the strong preference of sillimanite to grow on biotite that is replacing garnet, and this pattern of preferred nucleations is taken to indicate that the equilibrium conditions for the reaction were only overstepped by the minimum required for initial sillimanite nucleation. Chemical movements were controlled by the heterogeneous nucleation pattern, not by intrinsic properties of the moving species. In order for extensive reaction to occur under near-equilibrium conditions, the rate at which the total thermal reaction proceeded must have been controlled by the supply of heat to the rocks rather than by diffusion or local reaction steps.  相似文献   

16.
Kyanite replaces andalusite in a belt of Ordovician and Silurian pelitic rocks that form a narrow synform pinched between high-grade antiforms in NW Variscan Iberia. Kyanite occurs across the belt in Al-rich, black pelites in assemblages I: kyanite–chloritoid–chlorite–muscovite and II: kyanite–staurolite– chlorite–muscovite. In I, kyanite occurs in the matrix and in kyanite–muscovite aggregates that pseudomorph earlier andalusite porphyroblasts. The aggregates are found across the belt and can still be recognized in assemblage II and even in III: andalusite–staurolite–biotite–muscovite, this latter being a hornfelsic Silurian schist where kyanite is relic and staurolite occurs in the matrix, and is resorbed inside new massive pleochroic andalusite. KFMASH and MnKFMASH pseudosections have been constructed using Thermocalc for Al-rich and Al-poorer compositions from the belt. Chloritoid zoning in Al-rich rocks containing assemblage I, plus chloritoid–chlorite thermometry complemented with garnet–chlorite thermometry in Al-poorer lithologies, mean that the path is one of increasing pressure and temperature. Conditions prior to assemblage I, with earlier andalusite stable, are those of the andalusite–chloritoid– chlorite field as testified by chloritoid enclosed in andalusite porphyroblast rims. The passage from assemblage I to II implies a prograde path within the kyanite field. Assemblage III represents peak conditions, indicating a prograde staurolite-consuming reaction across a KFMASH field, leading eventually to a locally found andalusite–biotite–muscovite hornfels. The lowest pressure stages are recorded by cordierite–biotite in Al-poor pelites. Garnet-bearing MnKFMASH assemblages in Al-poorer pelites record conditions similar to assemblages II and III. The replacement of andalusite by kyanite in assemblage I is attributed to downdragging of andalusite-bearing rocks into a synform as testified by the strained andalusite porphyroblasts affected by a subvertical crenulation cleavage. Prograde metamorphism in the eastern contact of the belt is due to heat transferred to the belt from the ascending high grade antiform across the Vivero fault.  相似文献   

17.
R. H. Vernon 《Lithos》1978,11(4):283-289
High-grade metapelitic gneisses rich in cordierite and K-feldspar at Cooma, N.S.W., Australia, show microstructural evidence of partial to complete replacement of cordierite grains with biotite inclusions by fine-grained, symplectic intergrowths of andalusite, biotite and quartz. The replacement occurred during the later stages of the history of the Cooma Complex, under retrograde conditions. An equation for the local reaction: cordierite+red-brown biotite+H2O+K+ andalusite+green-brown biotite+quartz+H+ can be balanced approximately, using compositions obtained from microprobe analyses of the minerals involved. The volumes occupied by the solid reactants and solid products are nearly identical. The potassium and water were derived from an unknown external source, possibly connected with the replacement of K-feldspar by muscovite in nearby rocks.  相似文献   

18.
A natural example of phengite that had undergone partial thermal decomposition at a pressure of about 0.5 kbar and a temperature of about 680° C in a contact aureole was exmined in the transmission electron microscope (TEM). Partially pseudomorphed phengites were found to consist of combinations of phengite, biotite, K-feldspar, mullite, sillimanite, spinel and cordierite. Different areas within individual, partially pseudomorphed, phengite grains show various degrees of reaction and different reaction products; the cores are the least reacted and the margins have reacted most. In the cores the assemblage Al-, Mg-enriched phengite+biotite +K-feldspar+mullite±spinel has formed, whereas the assemblage K-feldspar+mullite+sillimanite+spinel +biotite+cordierite has formed at the edges. According to our thermodynamic calculations, the breakdown of phengite should have produced cordierite+spinel +corundum+K-feldspar in regions isolated from the influx of SiO2 and cordierite+andalusite+quartz+K-feldspar in regions near the edge of the grains that were essentially saturated with SiO2. Chemical equilibrium was not achieved in any part of the partially pseudomorphed phengites on a micron scale or larger. Breakdown theoretically should have been complete by about 550° C; the reaction temperature was overstepped by at least 130° C for 20–25 years. The variations in the degree and type of reaction are probably due partly to the availability of suitable nucleation sites in different regions, partly to the need to remove H2O from reaction sites and partly to the influence of SiO2, which diffused into the grains during metamorphism. The presence of SiO2 lowers the equilibrium temperatures. Thus there is a higher driving force for breakdown near the grain boundaries than in the cores. Most of the products show an orientation relationship with the parent phengite and have consistent habit planes; they have their closest-packed planes and closest-packed directions parallel to one another and to those of phengite. Such relationships minimize the strain and surface energies at nucleation and favour most rapid nucleation and growth of the reaction products. The great structural similarity of biotite to phengite resulted in its having the highest rate of nucleation and growth of any product and it occurred in all areas of the phengite pseudomorphs studied. Mullite and sillimanite were produced metastably. Mullite has more rapid nucleation kinetics than other aluminosilicates because it is structurally disordered. Sillimanite formed rather than andalusite in regions of the partially pseudomorphed phengites where the reaction reached an advanced stage, because the reaction from phengite to andalusite requires an energetically unfavourable change in aluminium co-ordination state.  相似文献   

19.
Metapelites from the southern aureole of the Vedrette di Ries tonalite (eastern Alps) were variably overprinted by contact and earlier regional metamorphic events during pre-Alpine and Alpine metamorphic cycles. In these rocks, starting from a primary garnet mica-schist (garnet stage), a complex sequence of transformations, affecting the site of the garnet, has been recognized. In the outermost part of the aureole, the primary garnet sites are occupied by nodules of kyanite (kyanite stage). Closer to the tonalite, kyanite is replaced by staurolite (staurolite stage), which in turn is pseudomorphed by muscovite (muscovite stage). The aggregates of kyanite do not overgrow garnet directly; they post-date a stage (fibrolite stage) represented by the pseudomorphic alteration of garnet into fibrolitic sillimanite plus biotite. A further sericite stage is likely to have occurred between the fibrolite and kyanite stages. Preservation of the sub-spherical garnet shape during all these transformations and persistence of mineralogical and textural relicts from earlier stages were favoured by the very low strain experienced by the rocks since the garnet stage. The textural sequence is in agreement with the metamorphic history of this part of the Austroalpine basement of the Eastern Alps: the garnet and fibrolite stages, and the coeval main foliation of the samples, are referred to the high-grade Hercynian metamorphism; the kyanite stage to the Eo-Alpine metamorphism; the staurolite and muscovite stages to the Oligocene contact metamorphism. It is suggested that kyanite growth as microgranular aggregates took place in polymetamorphic rocks where static, high- P /low- T  metamorphism overprinted high- T  assemblages that contained sillimanite or andalusite.  相似文献   

20.
Rocks of the Snake Creek Anticline are mainly pelitic schists, psammitic schists and quartzites that were metamorphosed during multiple high‐T/low‐P events extending from D1 to D5, with the metamorphic peak occurring late to post‐D3. Albitites are widespread, but are concentrated in five areas. They are typically fine‐ to medium‐grained, and consist of albite, with or without combinations of quartz, biotite, staurolite, cordierite, garnet, andalusite, sillimanite, kyanite, gedrite and tourmaline. From the presence or absence of albite inclusions in porphyroblasts, the albitites are interpreted as forming early in the D3 event as a result of infiltration of external fluids. Psammitic schists and quartzites were preferentially altered, but pelitic schists were also albitized in localities where the alteration was more extreme, with the replacement of muscovite total and the replacement of quartz and biotite variable. Structural controls on albitization include fracturing and syn‐D3 shear zones in fold hinges. Biotite schists with abundant porphyroblasts (combinations of staurolite, garnet, andalusite and cordierite) occur adjacent to albitites, and it is argued that they formed by the addition of Fe and Mg sourced from the albitites. In several albitite‐rich areas, cordierite grew early in D3 and was partly or entirely replaced during or after D3 by combinations of biotite, andalusite, tourmaline, staurolite and sillimanite. A postulated P–T–d path involved an increase in pressure (with or without a decrease in temperature) subsequent to early D3 albitization, followed by an increase in temperature up to the metamorphic peak (late D3 to early D4. The metamorphism was contemporary in part with the emplacement of the Williams Batholith (c. 1550–1500 Ma), which probably supplied the Na‐rich fluids.  相似文献   

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