An attempt to correlate non-predicted variations of distribution coefficients with mineral grain internal inhomogeneity using a field example studied near Sudbury, Ontario
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Keywords:
Staurolite
Muscovite
Sillimanite
Almandine
Tourmaline
Abstract The distribution and textural features of staurolite–Al 2 SiO 5 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 Al 2 SiO 5 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.
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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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Relict staurolite was newly found, together with metastable kyanite with or without Zn-rich spinel, in the sillimanite+K-feldspar zone grade quartz-rich Takanuki pelitic gneisses at three localities near Takanuki. It occurs as anhedral grains, up to 0.2mm in diameter, completely included in plagioclase single crystals, and is much Zn-richer, containing up to 2 wt. % ZnO, than staurolites in the nearby silica-undersaturated metalateritic rocks. The relict staurolites in the pelitic gneisses do not show any breakdown texture to the garnet+sillimanite+spinel assemblage, which is conspicuous in the metalateritic rocks. This may be due mainly to the high partial pressure of H2O locally prevailed around relict staurolites within plagioclase single crystals in pelitic gneisses, that resulted from dehydration of a trace amount of staurolite. On the other hand, kyanite associated with Zn-rich spinel may have been formed after staurolite which was located at the grain boundary of host plagioclase crystals, because H2O released from staurolite could escape along the grain boundary. This, in turn, suggests that some kyanite was formed as a product of staurolite breakdown in the absence of quartz.
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High-grade metamorphic rocks in the Burke area include staurolite-, andalusite-staurolite-, sillimanite-andalusite-staurolite-, sillimanite-garnet-staurolite-, sillimanite-garnet-potash feldspar (rare)-, kyanite-sillimanite-staurolite-, and kyanite-sillimanite-andalusite-staurolite-bearing assemblages. These rocks are interpreted as having been formed from low-grade schist and phyllite under load conditions higher than those characterizing normal hornfels aureoles. The various assemblages indicate the metamorphic trends produced as a result of increasing temperature. The one occurrence of a kyanite-sillimanite-andalusite-bearing assemblage suggests that these polymorphs were probably formed under closely similar conditions during thermal metamorphism but not necessarily simultaneously. Change of temperature is easier to envisage than change of pressure during the crystallization of this assemblage. Temperature and pressure conditions must have hovered near to the assumed triple point of the alumino-silicates.
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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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Regionally metamorphosed, muscovite-bearing quartzites from Sivrihisar, Turkey, contain coexisting andalusite, kyanite, and sillimanite. Kyanite is the most abundant polymorph and defines a lineation along with prismatic sillimanite, andalusite, staurolite, and elongate quartz. Andalusite is the most Fe-rich of the polymorphs (0.9-1.6 wt% Fe2O3, compared with 0.6-0.9 wt% for kyanite and sillimanite), and was ductilely deformed. Staurolite has partially pseudomorphed kyanite, and occurs intergrown with sillimanite. Garnet occurs in some metaquartzites and interlayered mica schists. Mica schists lack Al2SiO5 polymorphs. Porphyroblasts in mica schists are chloritoid, chloritoid + staurolite ± garnet, or staurolite ± garnet with inclusions of chloritoid and staurolite.
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Bounded by mylonite zones and imbrications the Zone of Erbendorf-Vohenstrauss is built up by medium- and low-pressure amphibolite-facies metapelites associated with amphibolites and biotite–hornblende gneisses, and intruded by the Leuchtenberg granite. Pelitic index assemblages define the staurolite– kyanite-, the garnet–kyanite-, the garnet–sillimanite- and the garnet–kyanite–sillimanite zone of medium pressure metamorphism, which is estimated at P=5–9.6 kbar and T=580–730 °C, and the cordierite zone of subsequent low-pressure metamorphism, which is estimated at P<4.5 kbar and T≤700 °C. Both facies series probably are tectonically juxtaposed. Staurolite–kyanite- and garnet zones are related by the reaction: staurolite+muscovite+quartz=garnet+ aluminium-silicate+biotite+H2O. This relation defines the only observable trace of a field gradient in the ZEV. An early sillimanite generation (sillimanite I) is found in all mineral zones as mineral inclusions in garnet rims. Post-P–Tmax evolution is indicated by the inversion kyanite⇒sillimanite (sillimanite II), the reaction: garnet+muscovite⇒sillimanite (sillimanite III)+biotite+quartz and by XFe decreasing in garnet rims. Garnet is resorbed under the conditions of stable kyanite+sillimanite+muscovite+quartz. Some staurolite–kyanite zone garnets show two stages separated by resorption which may be ascribed to published Ordovician and Late Devonian radiometric ages. The second garnet generation formed near 9.7 kbar/ 620–660 °C. No evidence for high-pressure metamorphism was found. Comparison of the medium-pressure part of the ZEV with the Zone of Tepla-Domazlice (Czech Republic) and with garnet–kyanite rocks at the northern edge of the Black Forest seems possible. The low-pressure cordierite rocks are similar to metapelites framing the Winklarn eclogites towards southeast of the ZEV, but are in contrast to the Moldanubian-type cordierite gneisses.
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