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

Petrographic studies of the secondary mineralogy of the Borrowdale Volcanic Group of north-west England suggest that these rocks were affected by at l east four metamorphic events during the Palaeozoic. The first event was hyd rothermal metamorphism which probably occurred during volcanism and resulte d in deposition of chalcedony in veins and vesicles. Celadonite may be a re sult of this hydrothermal event or later burial metamorphism. The chalcedon y was subsequently overprinted by contact and burial metamorphism. Contact metamorphism is defined by the presence of biotite and/or hornblende with o r without horn-felsing 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 bein g controlled by a change in the X-CO2 content of the (predominantly hydrous ) fluid associated with burial metamorphism. A fluid with lower X-CO2 was r esponsible 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 chal cedony and primary igneous features (crystals, matrix and vesicles) and rea ched a maximum grade of prehnite-actinolite to actinolite-pumpellyite facie s in the Devonian prior to the Acadian Orogeny. The last metamorphic event is regional metamorphism which is associated with the Acadian Orogeny and r esulted in the formation of tectonically aligned white mica. Pervasive carb onate 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 t he Nirex borehole volcanic sequences.