Magma Mixing in the 1100 AD Montaña Reventada Composite Lava Flow: Interaction of Rift Zone and Central Complex Magmatism

Abstract

Zoned eruption deposits frequently show a lower felsic and an upper mafic member, thought to reflect eruption from a large, stratified magma chambers. In contrast, however, the Montaña Reventada composite flow in Tenerife consists of a lower basanite and a much thicker upper phonolite. A sharp interface separates the basanite and phonolite, and a chilled margin at this contact indicates the basanite was still hot upon emplacement of the phonolite, i.e. the two magmas erupted in very quick succession. Three types of mafic to intermediate inclusions are found in the phonolite, which comprise foamy quenched ones, inclusions with chilled margins and those that are physically mingled, reflecting progressive mixing with a decreasing temperature contrast between the end-member magmas involved. Analysis of basanite, phonolite and inclusions for majors, traces and Sr, Nd and Pb isotopes show the inclusions to be derived from binary mixing of basanite and phonolite end-members in ratios of 2:1–4:1. Although basanite and phonolite magmas were erupted in quick succession, contrasting 206Pb/204Pb ratios show them to be genetically distinct. The Montaña Reventada basanite and phonolite first came into contact just prior to eruption and had seemingly limited interaction time. Montaña Reventada erupted from the transition zone between two plumbing systems, the phonolitic Teide-Pico Viejo complex and the basanitic Northwest rift zone. A rift zone basanite dyke most likely intersected a previously emplaced phonolite magma pocket, leading to eruption of geochemically and texturally unaffected basanite, followed by inclusion-rich phonolite that exploited the already established conduit.