Adakitic Magmas: Amphibole Composition and Geobarometric Constraints

Adakites are calc-alkaline magmatic rocks showing distinctive features which suggest that they are derived from a basaltic source in the garnet and amphibole stability field. Three genetic models have been proposed: (1) the subducted oceanic crust melts at depths of 40 to 90 km. The adakitic liquids migrate upward and pond in a shallow reservoir where they evolve by fractional crystallization; (2) basalts are injected continuously into the base of arc crust. This lower crust thickens until it reaches the garnet and amphibole stability field. Melting of this lower crust produces adakitic magmas; and (3) hydrated basaltic magmas, derived from the melting of metasomatised arc mantle, fractionate in a deeper reservoir and residual liquids are injected in a shallower one. These models are tested for the generation of Baja California and Philippines adakites by studying amphibole phenocrysts in allows us to constrain adakite genetic model(s). Amphiboles crystallize early in water-rich magmas, and their composition reflects equilibration pressures. Both high and low pressure amphiboles are observed in this study. They are identified following their Al content. High pressure amphiboles show high content in Al, while low pressure amphiboles show low content in Al. These observations are in agreement with the third hypothesis. Furthermore, magma compositions in the shallow reservoir are likely to be adakitic, whereas it is more likely to be basaltic in the deep reservoir. Thus, we propose the following petrogenetic model for the genesis of Philippine and Baja California adakites: Hydrous basaltic magmas derived from melting metasomatised arc mantle are stored in a deep reservoir (30-35 km), where they start to crystallize high pressure minerals. The residual adakitic melts migrate upwards and pond in a shallow storage zone (7-14 km) where they resume their crystallization before being erupted as adakites.