The Northern Plutonic Complex of Guernsey is a calc-alkaline intrusion emplaced within a metamorphic complex composed mainly of the Icart Gneiss and the Perelle Gneiss. The Northern Plutonic Complex is made up of four distinct bodies: the St Peter Port Gabbro, the Bordeaux Diorite Group, the L’Ancresse Granodiorite and the Cobo Granite, which are roughly contemporaneous, emplaced at the end of the Cadomian orogeny, at around 550–560 Ma. The St Peter Port Gabbro and the Bordeaux Diorite Group are relicts of a magma chamber, characterized by layered rocks along the margins, and heterogeneous rock associations in the core of the chamber resulting from stirring by convection. Oxygen isotopic data are consistent with melting of the country-rock (possibly the Perelle Gneiss) in response to the intrusion of the gabbro-diorite magma chamber. The resulting acidic melts mixed with crystallizing gabbro-diorite magma to yield the Cobo Granite and the L’Ancresse Granodiorite. This mechanism of acid-magma genesis can probably be extended to other late Cadomian magmatic events in the Northern Armorican Massif, suggesting that basic plutonism in this area may be more important than previously considered.