Geologic setting of the Peña de Bernal Natural Monument, Querétaro, México: An endogenous volcanic dome
Abstract
Peña de Bernal is a natural monument located near the town of Bernal, in Querétaro State, central Mexico. It is one of the tallest monoliths of the world, with a maximum height of 433 m. Peña 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 Peña de Bernal is a dacitic dome with SiO2 = 67 wt% and an age of 8.7 ± 0.2 Ma. The complete Peña 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. Peña de Bernal dacite is a spine-type endogenous dome that was forcefully intruded through the Mesozoic sequence practically as a solid plug.