Peralkaline and metaluminous mixed-liquid ignimbrites of the Guadalajara region, Mexico

Abstract

Two widespread ignimbrites, the 4.8-Ma San Gaspar ignimbrite and the 3.3-Ma Guadalajara ignimbrite, are distinctive units in the Guadalajara region, Mexico. Both ignimbrites contain fiamme of two distinct compositions and in some fiamme two glasses are intricately intermixed, indicating that two magmas were erupted simultaneously. The metaluminous San Gaspar ignimbrite is characterized by high-K dacite fiamme containing abundant phenocrysts of andesine, augite, hypersthene, hornblende, and biotite, and greatly subordinate shards and small aphyric fiamme of colorless rhyodacitic glass. Geothermometry based on coexisting mafic phenocrysts indicates pre-eruptive magma temperatures of about 1000°C. Conversion of common hornblende to oxyhornblende at the top of the ignimbrite, revesiculation of larger fiamme, and dense welding throughout the ignimbrite are indicative of high emplacement temperature. The Guadalajara ignimbrite contains in nearly equal proportions aphyric, peralkaline rhyolite and sparsely porphyritic, peralkaline, low-silica rhyolite. Several Plio-Pleistocene occurrences of peralkaline volcanism in the western portion of the dominantly calc-alkaline Mexican Neovolcanic Belt suggest that locally this zone accommodates extension, which may be related to opening of the Gulf of California.