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Metabasite parageneses in south-west England.

Douglas Robinson, Claudio Mazzoli, Tim J. Primmer

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Abstract

basites with diagnostic chlorite ± pumpellyite ± actinolite assemblages are sporadically developed in the Variscan belt of southEngland. Consideration of these phases in the NCFMASH system [SiO2-(Al2O3+Fe2O3)FeO-MgO-CaO-Na2O-H2O], using the epidote projection, shows in most cases sub-parallel tielines between coexisting chlorite pumpellyite and chlorite – actinolite assemblages. These relationships are suggestive of equilibrium relationships in the majority of samples and indicate prehnite-pumpellyite and low greenschist facies metamorphism. This is compatible with previously described anchizone and epizone conditions based on illite crystallinity. Application of the chlorite geothermometer suggests temperatures of c. 270 and 310°C for the two faciees respectively. The paragenetic relationships do not support the previously defined higher pressure pumpellyite-actinolite faciees for the Roseland area.

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What this paper is about

basites with diagnostic chlorite ± pumpellyite ± actinolite assemblages are sporadically developed in the Variscan belt of southEngland. Consideration of these phases in the NCFMASH system [SiO2-(Al2O3+Fe2O3)FeO-MgO-CaO-Na2O-H2O], using the epidote projection, shows in most cases sub-parallel tielines between coexisting chlorite pumpellyite and chlorite – actinolite assemblages. These relationships are suggestive of equilibrium relationships in the majority of samples and indicate prehnite-pumpellyite and low greenschist facies metamorphism. This is compatible with previously described anchizone and epizone conditions based on illite crystallinity. Application of the chlorite geothermometer suggests temperatures of c. 270 and 310°C for the two faciees respectively. The paragenetic relationships do not support the previously defined higher pressure pumpellyite-actinolite faciees for the Roseland area.

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Available abstract

basites with diagnostic chlorite ± pumpellyite ± actinolite assemblages are sporadically developed in the Variscan belt of southEngland. Consideration of these phases in the NCFMASH system [SiO2-(Al2O3+Fe2O3)FeO-MgO-CaO-Na2O-H2O], using the epidote projection, shows in most cases sub-parallel tielines between coexisting chlorite pumpellyite and chlorite – actinolite assemblages. These relationships are suggestive of equilibrium relationships in the majority of samples and indicate prehnite-pumpellyite and low greenschist facies metamorphism. This is compatible with previously described anchizone and epizone conditions based on illite crystallinity. Application of the chlorite geothermometer suggests temperatures of c. 270 and 310°C for the two faciees respectively. The paragenetic relationships do not support the previously defined higher pressure pumpellyite-actinolite faciees for the Roseland area.

Key concepts: Metamorphism, Greenschist, Chlorite, Actinolite, Geology, Geochemistry, Epidote, Mineralogy

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