Volcanic evolution of the Amealco caldera, central Mexico

The Amealco caldera is a well-preserved Pliocene volcanic center, 11 km in diameter, located in the central part of the Mexican Volcanic Belt. It is one of seven calderas known in the belt. Compared to those of the other calderas, the Amealco products are less evolved, and include only a minor volume of rhyolite. According to the stratigraphic record and K-Ar data, the inferred volcanic history of the Amealco caldera is as follows. Caldera-related activity started ca. 4.7 Ma with eruptions of pumice fallout and pyroclastic flows apparently of Plinian type. These events were followed by eruption of far-reaching surges and pyroclastic flows that deposited three widespread ignimbrites named Amealco I, Amealco II, and Amealco III. By about 4.7 Ma at least 77 km 3 (Dense rock equivalent, DRE) of trachyandesitic-trachydacitic magma were evacuated from the magma chamber and caused caldera collapse. After this climatic stage, pyroclastic activity continued, probably as tephra fountains from ring-fracture vents, that erupted pumice flows and fallouts that were accompanied by mud flows forming deposits of local extent. Both tephra and mud-flow deposits make up a DRE volume of 2.35 km 3. This was followed by 4.3 Ma trachyandesitic lava domes that were emplaced through several ring-fracture vents, making up a DRE volume of 3.8 km 3; the domes form the caldera’s present topographic rim. At about 4.0 Ma, a modest-sized volcano had formed on the western flank of the caldera that erupted several trachyandesitic lava flows and fallout tephra (both lava and tephra deposits = 0.8 km 3). Between 3.9 and 3.7 Ma, 10 intracaldera lava domes were emplaced, accompanied by tephra eruptions that produced relatively small deposits (volume not quantified) that were later reworked and redeposited as lake deposits within the caldera; five of these lava domes are trachyandesitic (4.3 km3) and five are rhyolitic (2.4 km 3). The central lava domes are interpreted here as the viscous, gas-poor magma that usually erupts at the end of a caldera cycle, and thus may mark the end of the volcanic evolution of the Amealco caldera. Volcanic activity continued adjacent to the caldera for at least 1.6 m.y. after the emplacement of the central lava domes. These events include bimodal volcanism at 3.7 Ma of basaltic-andesite lava from a volcano just north of the caldera rim (Hormigas volcano) and emplacement of several rhyolitic lava domes to the southwest of the caldera (Coronita rhyolite). At 2.9 Ma a rhyolite (obsidian) lava dome complex was