Mafic enclaves and granitic cargo in a plinian high-Si rhyolite: insights into the pre-eruptive magma reservoir at Laguna del Maule

Abstract:

A suite of mafic cauliflower-shaped comagmatic ejecta (bulk SiO2 ~53–62 wt%) from the dominantly crystal-poor high-Si rhyolite plinian deposits of the rhyolite of Laguna del Maule (LdM) indicate mingling of three distinct melts with the nearly aphyric high-Si rhyolite melt (unit rdm, bulk SiO2 ~76–77 wt. %) just prior to eruption. Evidence for mingling is seen in hand samples and confirmed by electron microprobe analyses of phenocrysts. Textures of chilled mafic enclaves indicate that basaltic and andesitic melts coexisted with rhyolitic melt, but the narrow range of rhyolite composition shows that they did not mix thoroughly. The mafic melts may be magmatically related, but each has a unique fractionation and mixing history.

Primary postglacial pyroclastic rdm deposits, exposed radially around the long-lived Andean rear-arc LdM volcanic field, contain three chemically distinct types of mafic clasts now recognized as comagmatic melts—olivine basaltic andesite, hornblende + apatite andesite, and pumiceous hornblende andesite. These clast types are also represented as quenched mafic enclaves within rhyolitic pumice. They overlap in bulk composition with the main mafic array of the extensive Quaternary LdM volcanic field (Hildreth et al. 2010, Boletín). Mafic clasts are volumetrically minor (< 1%) but conspicuous throughout the otherwise wholly rhyolitic eruption—an estimated ~20 km3. The three distinct mafic clast types represent thermal and mass recharge in the voluminous silicic system. Their occurrence in the pyroclastic deposits thus provides a snapshot of magmatic interactions just prior to eruption.

Subordinate in volume to the mafic clasts, granitic clasts are also found ubiquitously in the pyroclastic deposits, and as fragments engulfed within the mafic enclaves. They vary compositionally (bulk SiO2 ~59–76 wt. %), and are typically high in alkalis (8–9%); several have miarolitic textures. Notably, granitic rocks are not exposed anywhere in the drainage basin of LdM; the nearest granite exposure is ~15 km to the northwest. The granite fragments were entrained by the rhyolite, which erupted at the center of the LdM silicic system. Microbeam work in progress is clarifying the age of the granitic clasts and their compositional relation to development of the large silicic system.