The metamorphic history of the Borrowdale Volcanic Group: a petrographic study

Abstract

SUMMARY Petrographic studies of the secondary mineralogy of the Borrowdale Volcanic Group of north-west England suggest that these rocks were affected by at least four metamorphic events during the Palaeozoic. The first event was hydrothermal metamorphism which probably occurred during volcanism and resulted in deposition of chalcedony in veins and vesicles. Celadonite may be a result of this hydrothermal event or later burial metamorphism. The chalcedony was subsequently overprinted by contact and burial metamorphism. Contact metamorphism is defined by the presence of biotite and/or hornblende with or without hornfelsing and extends up a few km from the batholith/volcanic interface. Burial metamorphism is defined by two assemblages: calc-silicate hosted and calcite hosted, where their distribution is interpreted as being controlled by a change in the X CO 2 . content of the (predominantly hydrous) fluid associated with burial metamorphism. A fluid with lower X CO 2 was responsible for formation of the calc-silicate assemblage, whereas the fluid associated with the calcite hosted assemblage had a relatively higher X CO 2 . Burial metamorphic minerals replaced and infilled both hydrothermal chalcedony and primary igneous features (crystals, matrix and vesicles) and reached a maximum grade of prehnite-actinolite to actinolite-pumpellyite facies in the Devonian prior to the Acadian Orogeny. The last metamorphic event is regional metamorphism which is associated with the Acadian Orogeny and resulted in the formation of tectonically aligned white mica. Pervasive carbonate associated with the cleavage may be a result of regional metamorphism or a later post-metamorphic event. Post-metamorphic carbonate veins in the Lake District were probably coeval with post-Brockram carbonate veins in Nirex borehole volcanic sequences.