 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 . 
