Assimilation of Plutonic Roots, Formation of High-K ‘Exotic’ Melt Inclusions and Genesis of Andesitic Magmas at Volcán De Colima, Mexico

Abstract

Phenocryst-hosted melt inclusions from the 1998^2005 andesite eruptions of Volcan de Colima (Mexico) show broad ranges of major and trace element contents that do not overlap with the bulk- rock compositions and indicate that melt inclusions can be formed by and record a range of processes involved in the genesis of andesites. The melt inclusions that demonstrably record the evolution of the melt feeding the eruption indicate low-pressure (130^10 MPa) crys- tallization of a dacitic melt despite the monotonous bulk andesitic composition of historical magmas at Volcan de Colima. Mingling of dacite melt with gabbroic fragments in the shallow sub-volcanic system is the process responsible for generating the bulk andesitic composition of the magmas. A significant proportion of the melt inclusions have distinctive high large ion lithophile element (LILE) signatures. These 'exotic' high-K melt inclusions in pyrox- enes are thought to result from incongruent melting of interstitial bio- tite during assimilation of gabbroic fragments in the dacitic melt. A second group of exotic high-K melt inclusions found in plagioclase are likely to result from dissolution of higher-pressure (4200 MPa) amphibole, plagioclase, magnetite and biotite cumulates during assimilation in the ascending dacitic melt. Although they are not volumetrically abundant, high-K melts formed during assimilation of plutonic fragments and crystal cumulates made a significant con- tribution to the LILE contents of the magmas and represent a poten- tial source for this group of elements. The range of melt inclusion compositions in Volcan de Colima magmas emphasizes the impor- tance of mixing between ascending evolved melts and crystal popula- tions formed during previous episodes of magmatism over a range of pressures, temperatures and volatile contents. Cannibalization of plutonic roots appears to be a fundamental process in the genesis of andesite magmas and melt inclusions at continental arc volcanoes.