Vol. 24 No. 1 (1985)
Articles

VOLCANIC STRATIGRAPHY OF THE GUADALAJARA AREA, MEXICO

C. M. Gilbert
Department of Geology and Geophysics, University of Caiifornia, Berkeley, California, USA 94 720
I. S. E.Carmichael.
Department of Geology and Geophysics, University of Caiifornia, Berkeley, California, USA 94 720
G. A. Mahood.
Department of Geology, Stanford University, Stanford, California, USA 94305

Published 1985-01-01

How to Cite

Gilbert, C. M., E.Carmichael., I. S., & Mahood., G. A. (1985). VOLCANIC STRATIGRAPHY OF THE GUADALAJARA AREA, MEXICO. Geofísica Internacional, 24(1), 169-191. https://doi.org/10.22201/igeof.00167169p.1985.24.1.1069

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 yearsold. In the formation of both ignimbrites, two contrasting magmas were erupted simultaneously, for each rock contains glass fragments of two distinct compositions. In some ï¬amme, the two glasses are intricately intermixed.

The San Gaspar ignimbrite is widespread, thin, firmly welded throughout, and characterized by numerous ï¬amme of porphyritic dark glass containing abundant phenocrysts of plagioclase, augite, hypersthene, hornblende and biotite. Dark glass (R. I.= 1.521) is the predominant phase in the rock, but all samples contain shards and small ï¬amme of colorless glass (R.l_.= 1.510) that contains approximately 5 percent more SÍOâ‚‚ and signiï¬cantly less CaO, MgO, and FeO than the dark glass. Microprobe analyses of ferromagnesian phenocrysts indicate pre-eruptive magma temperaturas of about 1000°C. Conversion of common hornblende to basaltic hornblende at the top of the ignimbrite signifies an emplacement temperature greater than 800°C.
Late vesiculation of the larger ï¬amme, following compaction and welding, is characteristic.

The Guadalajara ignimbrite is characterized by abundant ï¬amme 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. devitriï¬ed, the light-colored ï¬amme being axiolitic and the darker porphyritic ï¬amme cryptocrystalljne and vesicular.

East and southeast of Guadalajara, the terrane beneath the San Gaspar ignimbrite 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 of Guadalajara and provides a recognizable horizon about 4 million years old. To the west, the younger Guadalaiara 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 flow of basalt and basaltic andesite