The Metamorphism of Fe-rich Pelites from Connemara, Ireland
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Connemara pelites show progressive metamorphism from staurolite to upper sillimanite zones and possess low Mg/(Fe + Mg) values, typically 0.30 to 0.35 from about 100 analyses. As a consequence of their composition, many sillimanite zone pelites lack both muscovite and K-feldspar. Staurolite, garnet, biotite, muscovite, feldspars and iron ores have been microprobe analysed in 48 samples. Assemblages, textures and mineral compositions indicate that metamorphism followed a sequence of continuous and discontinuous reactions with systematic variations in mineral Mg/(Mg + Fe) as predicted by theory. Contrary to some common assumptions, most reaction takes place along divariant equilibria; univariant reactions are seldom reached because reactants such as chlorite or muscovite are first consumed along divariant curves. Pelite petrogenetic grids showing univariant curves can only indicate limits to natural assemblages; they typically do not show which reactions have actually taken place. Physical conditions of metamorphism have been calculated by a variety of means; temperatures range from 550° for the staurolite zone to 650° for the upper silimanite zone, with the first appearance of sillimanite near 580°. An early kyanite-staurolite metamorphism at pressures above about 5 kb was followed by a steepening of the thermal gradient leading to regional cordierite and andalusite. This was probably accompanied by uplift with pressures of around 4 kb for roeks near the sillimanite-in isograd.Keywords:
Sillimanite
Staurolite
Pelite
Andalusite
Muscovite
Cordierite
Isograd
Sillimanite
Staurolite
Andalusite
Cordierite
Isograd
Muscovite
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Staurolite
Sillimanite
Pelite
Isograd
Greenschist
Syncline
Phyllite
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Connemara pelites show progressive metamorphism from staurolite to upper sillimanite zones and possess low Mg/(Fe + Mg) values, typically 0.30 to 0.35 from about 100 analyses. As a consequence of their composition, many sillimanite zone pelites lack both muscovite and K-feldspar. Staurolite, garnet, biotite, muscovite, feldspars and iron ores have been microprobe analysed in 48 samples. Assemblages, textures and mineral compositions indicate that metamorphism followed a sequence of continuous and discontinuous reactions with systematic variations in mineral Mg/(Mg + Fe) as predicted by theory. Contrary to some common assumptions, most reaction takes place along divariant equilibria; univariant reactions are seldom reached because reactants such as chlorite or muscovite are first consumed along divariant curves. Pelite petrogenetic grids showing univariant curves can only indicate limits to natural assemblages; they typically do not show which reactions have actually taken place. Physical conditions of metamorphism have been calculated by a variety of means; temperatures range from 550° for the staurolite zone to 650° for the upper silimanite zone, with the first appearance of sillimanite near 580°. An early kyanite-staurolite metamorphism at pressures above about 5 kb was followed by a steepening of the thermal gradient leading to regional cordierite and andalusite. This was probably accompanied by uplift with pressures of around 4 kb for roeks near the sillimanite-in isograd.
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Staurolite
Pelite
Andalusite
Muscovite
Cordierite
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Previous studies of metapelitic rocks from the core of the southern Brittany metamorphic belt suggest a complex clockwise P–T evolution. We use pseudosections calculated for an average subaluminous metapelite composition in the MnNCKFMASH system and average P–T calculations to investigate in more detail the metamorphic evolution of these rocks. For migmatites, sequential occurrence of kyanite, kyanite + staurolite and sillimanite suggests that a prograde evolution to P > 8 kbar at T ≈ 625°C was followed by a P decrease to around 6 kbar at 650°C. Subsequent heating and burial led to melting as a result of the incongruent breakdown of first muscovite then biotite; at the metamorphic peak (P ≈ 8 kbar, T ≈ 800°C), around 25 mol % melt and >20 mol % garnet are predicted. The retrograde evolution began with conductive cooling, allowing crystallization of melt and retrograde replacement of garnet by reaction to biotite and sillimanite. Subsequent near-isothermal decompression from around P ≈ 6 kbar to P ≈ 4 kbar was associated with widespread development of cordierite and a second episode of melt generation. Microstructural relations in upper amphibolite-facies metapelites from the unit structurally overlying the migmatites are consistent with growth of staurolite, andalusite and white mica along the retrograde path. On the basis of phase relations that account for the quantity of H2O required to saturate a given assemblage, an influx of an H2O-rich volatile phase is implied, which we infer to have been derived from crystallizing melt in underlying granites. The close correspondence of predictions from pseudosections to thermobarometric results is encouraging, and the complementary approach used has allowed tighter constraints than before to be placed on the P–T–X evolution of this sector of the Variscides.
Staurolite
Andalusite
Sillimanite
Muscovite
Migmatite
Cordierite
Geothermobarometry
Isograd
Diachronous
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Metamorphic reactions related to isograds derived from aluminum silicate-bearing pelitic schists were studied in an area of Grenville Province adjacent to Southern Province rocks near Sudbury, Ontario. Progressing from the northwest to southeast of the area, the meaningful hand-drawn isograds are: (1) sillimanite first occurrence, (2) the last occurrence of staurolite when associated with the entire assemblage, (3) K-feldspar first occurrence, (4) staurolite last occurrence as inclusions in garnet, (5) muscovite last occurrence, and (6) kyanite last occurrence. Whole-rock chemical analysis of 14 representative pelitic schist hand specimens in the area were collected and used to show that metamorphic factors, and not chemical differences, were responsible for the metamorphic isograds. The entire area lies thermally above the melting of rocks of granitic composition. Breakdown curves of the minerals related to the isograds have been used to imply a gradient of 670 °C to 750 °C and 6.3 to 7.3 kilobars, across the area, but the equations for these breakdowns are not entirely substantiated by the modal abundance and textural data.To a first approximation, the rocks may be considered homochemical, but many deviations (due partly to metasomatic change) from this exist. The ionic breakdown of kyanite to muscovite has been shown and an explanation as to why muscovite selectively replaces kyanite and not sillimanite is given. The breakdown of muscovite at the higher grades has been inferred to form K-feldspar, but not sillimanite. Near the kyanite isograd, textures showing the thermal breakdown of kyanite (left over after the partial ionic breakdown of the mineral) to sillimanite are shown. The rocks must have had at least K and possibly Fe added metasomatically to account for the textures shown. From generalized modal abundance surfaces (trend surface analysis), general equations representing the difference in modal abundance of minerals across various isograds were determined and from these, specific equations explaining the breakdown of a particular mineral at its isograd were derived. The most significant of these reactions is the first staurolite isograd, where it is inferred to breakdown in the following way, in the area studied:[Formula: see text]The dissolved Al and Si forms the fibrolite (sillimanite) lenses common in adjacent pelitic rocks
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Staurolite
Muscovite
Isograd
Andalusite
Pelite
Main Central Thrust
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Andalusite
Sillimanite
Staurolite
Isograd
Geothermobarometry
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Hornblende
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The Orrs Island-Harpswell Neck area on the southern Maine coast experienced one major low-pressure metamorphic event, which was synchronous wilh D2 deformation. Metamorphic grade increases from southwest to northeast toward plutons outside the area. Isograds in pelitic rocks divide the area into garnet, staurolite, staurolite-andalusite, staurolite-sillimanite, and sillimanite zones. At the sillimanite isograd, andalusite began to be replaced by coarse-grained muscovite to form prograde pseudomorphs, while sillimanite formed in the matrix. At slightly higher grade, staurolite was replaced by muscovite pseudomorphs. and additional sillimanite formed.
Isograd reactions deduced from changes in AFM topology were located in pressure-temperature space on the basis of experimental and thermodynamic data. The resulting petrogenetic grid constrains the pressure during metamorphism to between 2.3 and 3.0 kbars (230 and 300 MPa). Estimated temperature ranges for each of the metamorphic zones are as follows: garnet zone, <500°C; staurolite zone, 500 to 530°C; stauroli te-andalusite zone, 530 to 570°C; staurolite-sillimanite zone, 570 to 590°C; and sillimanite zone, >590°C. Metamorphism in the Orrs Island-Harpswell Neck area is similar to M3 metamorphism in west-central Maine, because both events occurred at pressures where staurolite gave way to sillimanite at high grade, and both were related to plutons. Metamorphism in the study area differs from M3 in west-central Maine in diat it produced andalusite at intermediate grade.
RÉSUMÉ
Sur la cote méridionale du Maine, la région d'Orrs Island-Harpswell Neck subit un épisode majeur de métamorphisme à basse pression synchrone de la déformation D2. Le degré de métamorphisme augmente du sud-ouest au nord-est en direction des plutons hors de la région. Les isogrades des pélites définissent dans cette région des zones à grenat, staurotide, staurotide-andalousite, staurotide-sillimanite, et a sillimanite. A l'isograde de la sillimanite, l'andalousite commença, à être remplacée par une muscovite grossiere pour former des pseudomorphes pro grades, alors que lasillimanite se forma dans la malrice. A un degré un peu plus éleveé des pseudomorphes de muscovite remplacerent la staurotide et d'autre sillimanite apparût.
On situa dans l'espace pression-température les réactions d'isogrades déduiles des changements dans la topologie AFM à partir de données expérimentales et thermodynamiques. Le champ pétrogénéique résultant ceme la pression durant le métamorphisme entre 2.3 et 3.0 kbars (230 et 300 MPa). Voici l’e'tendue estimée des températures pour chaque zone métamorphique: zone à grenat <500°C, zone a staurotide 500 à 530°C, zone a staurolide-andalousite 530 à 570°C, zone à staurt'tide-sillimanite 570 à 590°C et zone a sillimanite 590°C. Le métamorphisme dans la région d'Orrs Island-Harpswell Neck rappelle le métamorphisme M3 du centre et de l’ouest du Maine en ce que l'un el l'autre épisodes prirent place à des press ions auxquelles la sillimanite se substitua à la staurotide a un haut degré et que tous deux sont lies à des plutons. Le métamorphisme dans la région étudiée différede M3 du centre étde l'ouestdu Maine par la production d'andalousite à un degré intermédiate.
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【The Janggun mine area is occupied by the Proterzoic and the Paleozoic meta-pelites, which are intruded by the Jurassic Chunyang granite. The metamorphic terrain is divided into four zones of progressive metamorphism on the basis of mineral assemblages. The zones are chlorite zone, staurolite zone, andalusite zone, sillimanite zone ascending order. Boundary lines between the zones resemble outline of the Chunyang granite mass. Isograd reactions are chlorite+chloritoid+muscovite=staurolite+biotite+quartz+water, staurolite+chlorite+muscovite+quartz=andalusite+biotite+water, and staurolite+muscovite+quartz=andalusite+biotite+garnet+water between the chlorite zone and the staurolite zone, the staurolite zone and the andalusite zone, and the andalusite zone and the sillimanite zone, repectively. They are univariant reactions in KFMASH component system. Metamorphic conditions estimated from garnet-biotite geothermometers and phase equlibria are $530^{\circ}C$ and lower than 4 kb.】
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Pelitic metasediments immediately southwest of Yale, British Columbia contain mineral assemblages characteristic of staurolite through sillimanite zones of the Barrovian facies series. Microprobe analyses of major constituent phases in the pelites are combined with linear regression techniques to formulate probable kyanite- and sillimanite-forming reactions.A zone some 3 km wide contains the assemblage staurolite–kyanite–garnet–biotite–muscovite–quartz–plagioclase-ilmenite-rutile, which is univariant in AFM projection. Within precision limits of microprobe analysis, this assemblage is also univariant using linear regression techniques. Univariant relations are possible if [Formula: see text] with the composition of the fluid phase being buffered by the progressing reaction. This assemblage emphasizes the need for precise analyses when using the regression method, since minor components are often within permissible error limits rather than being balanced.Pelitic and calc-silicate assemblages from the metasediments restrict estimates of pressure–temperature conditions during regional metamorphism to 6–8 kbar and 550–770 °C. Pseudomorphs after andalusite indicate that contact metamorphism preceded regional upgrading of the pelites.
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