A-types granites and rhyolites: Is A for ambiguous?



A symposium entitled “;A-Type Granites and Rhyolites: Is A for Ambiguous?,” held last May at the AGU 1992 Spring Meeting in Montreal, Canada, provided a forum for continued discussion and scientific debate of the complexities inherent in A-type granites. The concept of A-type granites [Loiselle and Wones, 1979] originally distinguished granites that are relatively anhydrous, with alkaline affinities, and which are formed in anorogenic tectonic settings, commonly after a previous episode of crustal melting. These granites were noted for their distinctive mineralogy and chemistry. A-type granites commonly contain sodic pyroxenes and amphiboles, late crystallizing iron-rich amphiboles and biotites, fluorite, and exotic accessory minerals (for example, chevkinite). They have high SiO2, Na2O+K2O, Fe/Mg, F, and low CaO and Sr. They are enriched in high field strength elements (for example, zirconium, niobium, gallium, yttrium) and REEs except europian.