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

Phenocryst-hosted melt inclusions from the 1998–2005 andesite eruptions of Volcán 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) crystallization of a dacitic melt despite the monotonous bulk andesitic composition of historical magmas at Volcán 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 pyroxenes are thought to result from incongruent melting of interstitial biotite 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 (>200 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 contribution to the LILE contents of the magmas and represent a potential source for this group of elements. The range of melt inclusion compositions in Volcán de Colima magmas emphasizes the importance of mixing between ascending evolved melts and crystal populations 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.