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

Abstract

The Michoacán-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 cones lavas have a wide compositional range from 47 to 65% SiO₂, with abundant calc-alkaline olivine basalt and basaltic andesite. They also contain a few alkaline rocks, Shield lavas are all calc-alkaline andesites, which show a limited SiO₂ range(mostly 55%-61%) with common occurrence of orthopyroxene phenocrysts. Similar composition are found for the lava flows not associated with cones. These and Shield lavas represent effusive and less explosive type of eruptions. Shield volcanoes have more extensive lava flow that are not associated with cones, indicating a greater effusion rate and a greater magma supply. Because their 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.