BEHIND-THE-FRONT RHYOLITIC VOLCANISM IN SOUTHEASTERN GUATEMALA

Quaternary volcanoes behind the volcanic front in southeastern Guatemala are dominantly basaltic, monogenetic cinder cones. However, a number of rhyolitic domes are also present making this a setting of bimodal activity much like the Medicine Lake Highlands of northern California and the Newberry area of central Oregon as first pointed out by Williams and others (1964). The largest of the rhyolitic domes is Ixtepeque volcano, a mound of largely obsidian which covers an area of approximately 12 km². There are also two separate smaller domes lying just northeast of Ixtepeque on an alignment of vents that also includes basaltic cinder cones and the Laguna de Obrajuelo volcano. In addition, there is a series of domes about 2-5 km north of Laguna de Obrajuelo.

Although all of the domes erupted high-Si rhyolite, at least two distinct end-member compositions can be identified. The first, with higher Ti, P, Zr, Hf, slightly lower Si and distinctly lower ¹⁴³Nd/¹⁴⁴Nd, was erupted exclusively from Ixtepeque. The second, with lower Ti, P, Zr, Hf and higher ¹⁴³Nd/¹⁴⁴Nd, was erupted from the Pino Redondo dome immediately adjacent to Ixtepeque. All of the remaining rhyolites appear to be mixtures between these two end-member compositions. Given their isotopic distinctions it is clear that the rhyolitic end members cannot be related by fractional crystallization alone. A genetic kinship via fractional crystallization coupled with assimilation of putative crustal lithologies is also not viable. Thus, at least two unrelated rhyolitic magmas fed the domes of southeastern Guatemala.

These rhyolitic melts are not easily related to the contemporaneous and contiguous basaltic magmas via magmatic differentiation and instead appear to represent individual crustal melts. The lower Nd isotopic ratios for the Ixtepeque rhyolites indicates that their crustal source cannot have been recently added, mantle-derived mafic compositions as has been suggested for the entire Central American volcanic front (Vogel and others, 2006).