Contrasting volcanism in the Michoacan-Guanajuato Volcanic Field, central Mexico: shield volcanoes vs. cinder cones

Abstract

The Michoacan-Guanajuato Volcanic field (40 000 km2) of the western Mexican Volcanic Belt contains both small- and medium-sized volcanoes, but lacks large composite volcanoes. The small-sized volcanoes include 900 cinder cones and 100 other volcanoes such as lava cones, lava domes, thick lava flows not associated with cones, and maars. In contrast, the medium-sized volcanoes include over 300 shield volcanoes, and a few lava domes and rare composite volcanoes. Both groups of volcanoes coexist in time and space. Cinder cone lavas have a wide compositional range from 47 to 65% SiO2, with abundant calc-alkaline olivine basalts and basaltic andesites. Shield lavas are all calc-alkaline andesites, which show a limited SiO2 range (mostly 55%-61%) with common occurrence of orthopyroxene phenocrysts. Because these lavas are more fractionated than, but plot on the same compositional trend as the calc-alkaline cinder cone lavas, they may be products of fractional crystallization of primitive calc-alkaline basalts which are found in some of the cinder cones. -from Authors