The composition of the lava domes suggests that they were formed by degassed magma left behind by the caldera-forming eruption. The pre-caldera lava domes were generated either directly from a common magma chamber or indirectly through secondary chambers. The lead isotope ratios are consistent with the volcano having formed at the edge of an area of granite and rhyolite of Paleozoic age. Incapillo magmas probably formed as adakitic high-pressure mafic magmas derived from the crust, either directly by anatexis or indirectly by dragged-down crustal fragments. The magmas are then modified by crustal contamination and fractional crystallization. As the subducting slab shallowed, crustal garnet-containing lherzolite and granulite-eclogite, contributed both from the crustal basis and forearc rocks that were dragged down by the subducting slab, became an increasingly important component of erupted magmas. Eventually, the Incapillo magma chamber was disconnected from the mantle and lower crust.