Geochemical variations within metavolcanic rocks of the Dalradian Farragon Beds and adjacent formations

Abstract

Synopsis A detailed petrological and geochemical study of metavolcanic schists of the Dalradian Argyll Group of the southern Highlands of Scotland reveals the activity of five main centres of volcanicity. Volcanic activity started with the localized eruption of low HFSE tholeiites comprising the Laoigh Metabasites during the deposition of the Ben Lawers Schist protolith in the Daldhu–Fearnate area. Subsequent deposition of the 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 fractionated high HFSE tholeiites were erupted in the intervening Pitcastle–Derculich area. A final volcanic episode occurred in the NE of the area, where transitional to alkaline basalts were produced during deposition of the Queen’s Hill Gneiss protolith. The composition of these metabasaltic’ rocks is consistent with volcanism in an extensional tectonic regime. The low HFSE contents of some magmas may result from their contamination during a residence time in the lower crust, while the later phase of more alkaline magmatism is consistent with current models of volcanism associated with extensional regimes.