A widespread mafic volcanic unit at the base of the Mexican Volcanic Belt between Guadalajara and Querétaro

Abstract

In central Mexico, large basaltic plateaux, mainly emplaced between 10 and 6 Ma, form the basal part of the Plio-Quaternary Volcanic of the Mexican Volcanic Belt (MVB). In the Guadalajara and Querétaro areas these planteaux are separated from the late-Oligocene to early-Miocene Sierra Madre Occidental ignimbrites and andesites by a period of volcanic quiescence marked by various erosional deposits, whereas in the areas surrounding the Chapala and Cuitzeo lakes they overlie andesitic sequences of middle-to late-Miocene age. The plateau lavas are relatively primitive (Mg# 66-52), did not undergo significant low pressure fractionation, and display clear calc-alkaline affinities. They represent the first widespread and uniform volcanic event in MVB history, Geological and geochemical data suggest that these basalts were emplaced during the first extensional tectonic phase that accompanied the development of the MVB.