The Amazcala caldera, Queretaro, Mexico. Geology and geochronology

The Amazcala caldera is located 30 km NE of Querétaro City, near Amazcala, state of Querétaro. This caldera is the northernmost caldera within the central sector of the Mexican Volcanic Belt (MVB). It has a 11×14 km2 elliptical shape, and was formed 7.3–6.6 Ma ago. All caldera products are rhyolites. The oldest caldera unit, the Ezequiel Montes pumice (EMP), is a widespread pumice fallout emplaced around 7.3 Ma ago. An isopach map of the EMP shows two dispersal axes, oriented to the SW and SE with respect to the caldera. The EMP is 5 m thick 40 km to the SW and 35 km to the SE of the caldera. An isopleth map of the EMP shows that pumice fragments increase in size toward the caldera, from 1 cm at 40 km to 25 cm near the caldera rim. The EMP is a regional stratigraphic marker. The Colón ignimbrite, dated at 7.3±0.5 Ma, is stratigraphically above the EMP. It consists of several ash-flow units interbedded with minor pumice fall lapilli and ash, with a minimum thickness of about 80 m at Colón. The caldera rim is occupied by several rhyolite lava domes and flows, some of which extend 10 km from the rim. These domes contain parts of fresh, aphyric obsidian. The last caldera event is an intracaldera rhyolitic dome near the NE rim at about 6.6 Ma. The dome is 4×2 km2 and is elongated in the NE direction. The Amazcala caldera is 480 km from the Middle America Trench and represents the farthest inland caldera in the central sector of the MVB. Its age of 7.3–6.6 Ma indicates that it is the oldest caldera of the MVB so far reported. This confirms the general view that the volcanic activity of the MVB initiated at its northern margin, and then migrated southward in time.