Mineralogy and major element modelling of volcanic rocks from Amealco caldera, Queretaro, Mexico

Amealco caldera is a quasi-circular structure of 10 km x 11 km in diameter, located in the central part of the Mexican Volcanic Belt, about 130 km NW of Mexico City. The rocks from this caldera vary from basalt to rhyolite an d belong to the calc-alkaline and high-K calc-alkaline series. Common miner als have been analyzed by electron microprobe in samples from all volcanic units. Olivines are forsterite-rich; orthopyroxenes vary from bronzite to h ypersthene; clinopyroxenes show two groups of augites, one of which tends t owards ferroaugites; amphiboles present in some evolved rocks are brown hor nblendes; plagioclases vary from bytownite to andesine; opaque minerals are ilmenites and magnetites. Several common geothermometers are used to infer the temperature regime of the volcanic units. Major element mass-balance e stimates indicate that fractional crystallization (FC) alone cannot account for the geochemical diversity of magmas erupted in Amealco caldera.