Isotopic and chemical constraints on the development of peraluminous Caledonian and Acadian granites.

Peraluminous magmas may be generated by 1) melting of diopside-normative parental materials; 2) fractional crystallization of amphibole; 3) anatectic melting of peraluminous parental materials; 4) assimilation of peraluminous materials by metaluminous magmas. The use of ⁸⁷Sr/⁸⁶Sr and ¹⁸O/¹⁶O ratios, together with information on petrology, chemistry and geological relationships, is necessary in order to distinguish between these mechanisms. Interpretation of the origins of the Scottish Caledonian granites of the NE Grampian region, the Strontian and Foyers complexes and granites of the Southern Uplands and the Acadian South Mountain batholith of Nova Scotia were made using these criteria. The characteristics of the plutons are best explained if they originated through fusion or assimilation of crustal rocks. Vapour-phase transfer of components of the melt and fractional crystallization may explain local variations in the Al₂O₃/(CaO + Na₂O + K₂O) ratio. -S.A.K.