A preliminary report on the comenditic dome and ash flow complex of Sierra La Primavera, Jalisco

Resumen

The late Pleistocene La Primavera volcanic complex, which makes up the Sierra La Primavera, west of Guadalajara, consists of comenditic (peralkaline-rhyolitic) lavas, ash flows, air-fall pumice, and associated lacustrine and fluvial deposits. The domes and flows, which were erupted along two arcuate zones, are divided into two map units on the basis of topographic form and phenocryst content. These two groups, the porphyritic and the aphyric, are also distinct with respect to major and trace element chemistry· and K-A ages.

The oldest lavas contain 5-15% phenocrysts (sodic sanidine ≥ quartz >> ferrohedenbergite > fayalite > ilmenite), and are characterized by greater peralkalinity than the younger aphyric units. They began to erupt approximately·100,000 years ago along the northerly fracture zone, forming domes 1-4 km in diameter. The last porphyritic lavas were erupted aproximately 60-80,000 years ago.