GEOCHEMICAL VARIATIONS WITHIN METAVOLCANIC ROCKS OF THE DALRADIAN FARRAGON BEDS AND ADJACENT FORMATIONS

A detailed petrological and geochemical study of metavolcanic schists of the Dalradian Argyll Group of the southern Highlands of Scotland re veals the activity of five main centres of volcanicity. Volcanic activ ity started with the localized eruption of low HFSE tholeiites compris ing the Laoigh Metabasites during the deposition of the Ben Lawers Sch ist protolith in the Daldhu-Fearnate area. Subsequent deposition of th e Farragon Beds was characterized by simultaneous eruption from three centres: low HFSE tholeiitic lavas and tuffs were produced in the Ben Vrackie area and in the Farragon area to the SW, while more fractionat ed high HFSE tholeiites were erupted in the intervening Pitcastle-Derc ulich area. A final volcanic episode occurred in the NE of the area, w here transitional to alkaline basalts were produced during deposition of the Queen's Hill Gneiss protolith. The composition of these metabas altic rocks is consistent with volcanism in an extensional tectonic re gime. The low HFSE contents of some magmas may result from their conta mination during a residence time in the lower crust, while the later p hase of more alkaline magmatism is consistent with current models of v olcanism associated with extensional regimes.