The Miocene igneous rocks in the Basal Unit of the Lavrion area form part of the granitoid province of the central Aegean. Undeformed, subvertical dykes of quartz-syenite to granodiorite and granite porphyries, and a little deformed but variably altered granodiorite stock intrude metamorphic rocks of the Basal Unit. A 9.4 ± 0.3 Ma K–Ar age on feldspar for a dyke rock provides a minimum age for the igneous activity in the Basal Unit. East–west orientation of porphyry dykes is indicative of a regional extensional stress field with roughly north–south direction. Substantial extension in the Basal Unit after granodiorite emplacement is evident from widespread quartz veining associated with hydrothermal alteration of the granodiorite and the occurrence of mineralized tension gashes cutting the hydrothermally altered hornfelses. Final emplacement of the Blueschist Unit over the Basal Unit by extensional detachment post-dates contact metamorphism of the rocks surrounding the granodiorite. Geochemical diagrams show a continuous range of compositions from the dykes to the granodiorite. Radiogenic isotope compositions are compatible with a common magmatic source for the two lithologies. Elemental variations, as well as the considerable geochemical similarity of the dyke rocks to the Hercynian paragneiss of the central Cyclades, indicate that crustal melts were significant components during the evolution of the igneous rocks with fractional crystallization as an important process during later stages of evolution. The granodiorite displays geochemical signatures indicative of a significant mafic mantle-derived magma component.