Pleistocene and Holocene peralkaline rhyolites from Torfajo¨ kull (South Iceland Volcanic Zone) and Ljo´ sufjo¨ ll central volcanoes and trachytes from Snæfellsjo¨ kull (Snæfellsnes Volcanic Zone) allow the assessment of the mechanism for silicic magma genesis as a function of geographical location and crustal geothermal gradient. The low d18O (2.4&) and low Sr concentration (12.2 ppm) measured in Torfajo¨ kull rhyolites are best explained by partial melting of hydrated metabasaltic crust followed by major fractionation of feldspar. In contrast, very high 87Sr/86Sr (0.70473) and low Ba (8.7 ppm) and Sr (1.2 ppm) concentrations measured in Ljo´ sufjo¨ ll silicic lavas are best explained by fractional crystallisation and subsequent 87Rb decay. Snæfellsjo¨ kull trachytes are also generated by fractional crystallisation, with less than 10% crustal assimilation, as inferred from their d18O. The fact that silicic magmas within, or close to, the rift zone are principally generated by crustal melting whereas those from off-rift zones are better explained by fractional crystallisation clearly illustrates the controlling influence of the thermal state of the crust on silicic magma genesis in Iceland.