Margarito Tristán-González

Tectono-volcanic control of fissure type vents for the 28 Ma Panalillo Ignimbrite in the Villa de Reyes graben, San Luis Potosí, México.

The volcano-tectonic events at the Villa de Reyes Graben (VRG), in the southern Sierra Madre Occidental, Mexico, include 1) a regional NNE fault system developed before 32 Ma, 2) this pre-32 Ma faulting controlled the emplacement of 31.5 Ma dacitic domes, 3) NE faulting at 28 Ma that displaced the 31.5 Ma dacitic domes and formed the VRG, as well as the oblique grabens of Bledos and Enramadas oriented NW, 4) emplacement of Panalillo ignimbrite at 28 Ma filling the VRG and erupting from fissures related to the oblique grabens, and eruption of Placa basalt apparently also from fault-controlled vents.

Geologic setting of the Peña de Bernal Natural Monument, Querétaro, México: An endogenous volcanic dome

Pena de Bernal is a natural monument located near the town of Bernal, in Queretaro State, central Mexico. It is one of the tallest monoliths of the world, with a maximum height of 433 m. Pena de Bernal was recently declared Intangible Cultural Heritage of Humanity Patrimony by United Nations Educational, Scientific, and Cultural Organization (UNESCO). In spite of being both a natural and cultural monument, little is known about its origin, physical characteristics, and chemical composition. It is a leucocratic-igneous rock intruding marine Mesozoic sedimentary rocks and has been misinterpreted as a pluton of Eocene or older age. However, this study shows that Pena de Bernal is a dacitic dome with SiO 2 = 67 wt% and an age of 8.7 ± 0.2 Ma. The complete Pena de Bernal body includes three plugs that crop out in an ∼3.5 × 1.5 km area elongated N40°E. Texture of the rock is porphyritic, nearly holocrystalline (80 vol% crystals and 20 vol% glass), with a mineral assemblage of pyroxene, hornblende, biotite, plagioclase, and quartz, plus accessory apatite and zircon. Pena de Bernal dacite is a spine-type endogenous dome that was forcefully intruded through the Mesozoic sequence practically as a solid plug.