Volcanic stratigraphy of the Guadalajara area, Mexico

Abstract

Two ignimbrites of petrologic interest provide distinctive time horizons in the volcanic succession near Guadalajara. The older of the two, referred to as San Gaspar ignimbrite, is approximately 4.8 million years old and has the composition of dacitic andesite. The younger ignimbrite, called Guadalajara ignimbrite because of its use as the characteristic building stone in the city, is more siliceous and approximately 3 .3 million years old. In the formation ofboth ignimbrites, two contrasting magmas were erupted simultaneously, for each rock contains glass fragments of two distinct compositions. In sorne fiamme, the two glasses are intricately intermixed. The San Gaspar ignimbrite is widespread, thin, firmly welded throughout, and characterized by numerous fiamme of porphyritic dark glass containing abundant phenocrysts of plagioclase, augite, hypersthene, hornblende and biotite. Dark glass (R. l.= 1.521) is the predominant phase in the rock, but all samples contain shards and small fiamme of colorless glass (R.l. = l.510) that contains approximately 5 percent more SiO2 and significantly less CaO, MgO, and FeO than the dark glass. Microprobe analyses of ferromagnesian phenocrysts indicate pre-eruptive magma temperatures of about l000ºC. Conversion of common hornblende to basaltic hornblende at the top of the ignimbrite signifies an emplacement temperature greater than 800ºC. Late vesiculation of the larger fiamme, following compaction and welding, is characteristic. The Guadalajara ignimbrite is characterized by abundant fiamme of two distinct compositions. Vitric facies consist of two different glasses in nearly equal proportions, one colorless and aphyric, the other dark-colored with sparse phenocrysts of alkali feldspar. Generally, however, the ignimbrite has been completely devitrified, the light-colored fiamme being axiolitic and the darker porphyritic fiamme cryptocrystalline and vesicular.East and southeast of Guadalajara, the terrane beneath the San Gaspar ignimbrité consists largely of basaltic flows, but to the northwest rhyolitic rocks are widespread. An olivine basalt distinguished by megaphenocrysts of plagioclase overlies the ignimbrite north ofGuadalajara and provides a recognizable horizon about 4 million years old. To the west, the younger Guadalajara ignimbrite is overlain by siliceous ash flows and dacitic lavas about 3 million years old.Younger volcanism has been concentrated along a northwest-trending zone where activity has continued since late Pliocene time, culminating at Sierra La Primavera during the last 140 000 years with eruptions of high-silica rhyolite. Elsewhere the young eruptions have produced flows of basalt and basal tic andesite.