APPINITIC INTRUSIONS IN THE ENGLISH LAKE DISTRICT

Within the Skiddaw Group, which underlies the mid-Ordovician volcanic are of the English Lake District, are minor intrusions of calc-alkalin e, hornblende-augite rich, dioritic to picritic bodies reminiscent of the appinite suite of the Scottish Caledonides. Representative of thes e is the Scawgill Bridge microdiorite, a variable, hornblendic body wh ich includes augite-phyric meladiorite and pyroxenite; others include the 'picrite' (augite-meladiorite or cortlandtite) plugs at Dash and B arkbeth. They are distinct from tholeiitic intrusions of similar age r epresented by the Embleton Diorite, and also from late Silurian biotit e-lamprophyres. Despite alteration, primary variation trends indicated by immobile trace elements are also expressed by major elements inclu ding Mg, Fe and Ca, Differentiation at Scawgill Bridge is modelled as fractionation of olivine, augite, hornblende and chromite, and a simil ar process is inferred for the meladiorite intrusions. It is concluded these bodies represent primitive, calc-alkaline magmas related to the Lower Borrowdale Volcanic Group, which fractionated under confined co nditions of high P-(H2O). In contrast, the Lower Borrowdale Volcanic G roup itself is dominated by plagiophyric rocks which resulted from fra ctionation at lower P-(H2O).