Radiometric dating (40Ar/39Ar and 14C), compositions, and erupted volumes of volcanoes of the Valle de Santiago area (Michoacán-Guanajuato Volcanic Field, Mexico)

Abstract The Valle de Santiago (VS) area is located in the Michoacán-Guanajuato Volcanic Field (MGVF), within the Trans-Mexican Volcanic Belt (TMVB). By geological mapping of an ~2800 km2 quadrangle, 40Ar/39Ar and radiocarbon dating, morphometry, and whole-rock chemical and petrographic analyses of the volcanic products, we established the stratigraphy and eruptive history of the VS area. A total of 118 volcanic landforms was identified, including 61 scoria/spatter cones, 21 phreatomagmatic volcanoes, 20 medium-sized shield volcanoes, eight lava domes, and eight fissure-fed lava flows and plateaus. Volcanic activity in the VS area began ca. 8 Ma, persisting until the late Pleistocene, ca. 11 ka. During the Miocene to middle Pleistocene, volcanism was distributed throughout the VS area, and was predominantly effusive, giving rise to voluminous medium-sized shield volcanoes (15–47 km3). In the late Pleistocene, phreatomagmatic eruptions dominated the western sector of VS along an NNW-SSE–oriented strip. This phreatomagmatism was facilitated by fractured aquifers on the slopes of shield volcanoes (basaltic trachyandesitic and basaltic andesitic lava flows) and the humid climatic conditions that existed during the late Pleistocene, which guaranteed water-saturated conditions. Erupted products are mainly basaltic andesites/basaltic trachyandesites and andesites, and lesser basalts/trachybasalts, trachyandesites, and rhyolites. Their chemical affinity also shifted over time, transitioning from solely sub-alkaline to sub-alkaline and alkaline by the late Pleistocene. The VS area holds one of the largest phreatomagmatic clusters in the TMVB, offering an excellent opportunity to study the conditions favoring phreatomagmatism and providing records of intense volcanic activity during the Plio-Pleistocene in the MGVF.