Giant silicic ignimbrites and comagmatic domes at the southeastern end of the Sierra Madre Occidental, México Article uri icon

abstract

  • The Sierra Madre Occidental (SMO) ignimbrite province includes giant ignimbrites related to large silicic calderas and dome complexes. Previous work in the SMO has paid little attention to the ignimbrites and domes exposed in the southeasternmost portion of the volcanic province, in the Guanajuato and Querétaro States. This work focuses on two widespread Oligocene ignimbrite sheets and numerous lava domes of this part of the SMO, and provides data on their distribution, stratigraphy, geochronology, textures, and geochemistry. Two large ignimbrite sheets and several domes were identified, which according to the stratigraphy they were classified in pre- and post-ignimbrites. We have named the ignimbrites as Victoria I and Victoria II. Victoria I is purple to light grey, with strong welding, horizontal foliation, and well-developed columnar jointing. It contains quartz, plagioclase and sanidine; fiamme is a common feature. Victoria II ignimbrite conformably overlies Victoria I. It is dark pink to brown, densely welded but less welded than Victoria I; in most locations it has developed columnar jointing. Victoria II%27s mineral assemblage includes biotite and hornblende in addition to quartz, plagioclase and sanidine; it has a higher content of lithics fragments and fiamme than Victoria I. A black vitrophyre forms base of each ignimbrite and marks the contact between them. These ignimbrites are stratigraphic markers due to their wide distribution and distinctive field and textural characteristics. We have obtained zircon U–Pb ages of 31.65 ± 0.17 to 30.57 ± 0.14 Ma (1σ) for Victoria I ignimbrite and 31.31 ± 0.21 to 29.73 ± 0.18 Ma for Victoria II ignimbrite. Silicic lava domes identified in the study area are either pre- or post-ignimbrite; the former are dacitic, and the latter are rhyolitic or rhyodacitic. We focus on the post-ignimbrite domes with the hypothesis that they were comagmatic with the Victoria ignimbrites. The post-ignimbrite domes are devitrified rocks with phenocrysts of plagioclase, quartz, sanidine, biotite and hornblende; and zircon U–Pb ages yielded 30.81 ± 0.21 Ma to 28.02 ± 0.19 Ma. © 2021 Elsevier Ltd

publication date

  • 2022-01-01