Staurolite-forming reactions in the eastern Dalradian rocks of Scotland

Data on mineral and rock compositions along with textural relations are used to deduce the staurolite-forming reactions in eastern Dalradian rocks in Scotland. Initially staurolite is formed as a product of the breakdown of the assemblage chloritoid +quartz in iron-rich metapelites. With increase in grade the iron-rich rocks are succeeded by more magnesium-rich ones and staurolite is formed as a product of the breakdown of the assemblage chloritoid +chlorite+muscovite.     

Chemical mineralogy and illite crystallinity in low grade metasediments,Zarouchla Group,northern Peloponnesus,Greece

Summary Low grade metasediments from the Zarouchla Group of the Phyllite-Quartzite series in northern Peloponnesus have been investigated. Mineralogically, there is a clear distinction between the lowermost and the overlying formations. Rocks of the former contain characteristic minerals such as chloritoid or garnet whereas the other formations contain the assemblage muscovite + chlorite + qz ± paragonite ± paragonite/muscovite. Illite crystallinity values are low to middle anchizone in the uppermost formation and increase progressively through upper anchizone values in the intermediate formations, reaching low epizone values in the lowermost formation. Pumpellyite-actinolite facies metabasic rocks are sandwiched between metaclastites with upper anchizone or anchizone-epizone illite crystallinity values; and chloritoid bearing quartzites with low epizone illite crystallinity values. Although geothermometric data obtained from metasediments of the lowermost formation do not support a simple burial-related pattern of metamorphism, illite crystallinity data point to a progressive increase in metamorphic grade with stratigraphic depth.

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Chemische Mineralogie und Illitkristallinität in niedriggradigen Metasedimenten, Zarouchla, Gruppe, nördlicher Peleponnes, Griechenland

Zusammenfassung In Metasedimenten niedrigen Metamorphosegrades aus der Zarouchla Gruppe der Phyllit-Quarzit-Serien im nördlichen Peleponnes bestehen klare mineralogische Unterschiede zwischen der tiefsten und den darüber-liegenden Formationen. Gesteine der ersteren enthalten als charakteristische Minerale Chloritoid oder Granat, während die darüberliegenden Formationen weithin durch die Paragenese Muskovit + Chlorit + Quarz + Paragonit + Paragonit/Muskovit gekennzeichnet werden. Die Werte der Illit-Kristallinität entsprechen in der obersten Formation der unteren bis mittleren Anchizone und nehmen graduell zu: Werte der oberen Anchizone wurden in den dazwischenliegenden Formationen registriert um schließlich Werte der unteren Epizone zu erreichen. Metabasite in Pumpellyit-Aktinolith-Fazies treten zwischen Metaklastiten mit Illit-Kristallinitätwerten der oberen Anchizone oder des Bereiches Anchizone-Epizone und chloritoid-führenden Quarziten mit Illit-Kristallinität der niedrigen Epizone auf. Obwohl geothermometrische Daten, die von Metasedimenten der untersten Formation gewonnen wurden nicht ein einfaches Schema der Überlagerungs-Metamorphose unterstützen, weist die Illit-Kristallinität doch auf eine progressive Zunahme des metamorphen Grades mit stratigraphischer Tiefe hin.

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With 6 Figures     

Distribution of elements between coexisting phengite and chlorite from low grade rocks from Skiros Island,Greece

Summary The distribution of Fe²⁺ and Mg between coexisting phengite and chlorite has been examined in a suite of low-grade rocks. The distribution coefficientK _D(Mg) is influenced in part by the content of tetrahedral or octahedral Al of either chlorite or phengite and in part byX _Mg of phengite. The tie lines for the mineral pairs on a SAF triangular diagram show a subparallel trend.

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Elementarverteilung zwischen koexistierenden Phengit und Chlorit aus niedrig metamorphen Gesteinen von der Insel Skiros, Griechenland

Zusammenfassung Es wird untersucht, wie Fe²⁺ und Mg zwischen koexistierenden Phengit und Chlorit aus einer Folge schwach metamorpher Gesteine verteilt sind. Der VerteilungskoeffizientK _D(Mg) wird teils durch den Gehalt an tetraedrischem bzw. oktaedrischem Al in Chlorit oder in Phengit und teils durch dasX _Mg vom Phengit beeinflußt. Die Konoden zwischen den Mineralpaaren in einem SAF-Dreieckdiagramm zeigen subparallelen Verlauf.

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With 4 Figures     

Retrogression of staurolite schists and the sources of infiltrating fluids during metamorphism

Staurolite-kyanite schists from the Dalradian of Scotland and Ireland show two types of retrograde alteration. In the Irish examples staurolite and kyanite are replaced by muscovite and paragonite, often with margarite and sometimes chloritoid. Plagioclase in these samples may be albitized. In contrast one Scottish sample shows replacement of both staurolite and plagioclase by muscovite. Retrogression resulted from infiltration of fluid, but while in some samples the large number of retrograde phases internally buffered the fluid composition and cation metasomatism cannot be proven, in others the fluid composition was externally controlled and there was cation metasomatism indicating more extensive infiltration. It is demonstrated by the use of activity diagrams showing the relationships between Al-silicate, white mica and alkali feldspar that wholesale alteration to muscovite was most likely caused by fluids moving down-temperature from a granitic or arkosic source. In contrast, if growth of retrograde albite is actually accompanied by Na-metasomatism then the infiltrating fluids are likely to have been moving up-temperature through pelitic lithologies. Large fluid: rock ratios of around 101 are needed to achieve significant alkali metasomatism except in extreme situations of large temperature changes and highly concentrated fluids.