Evolution of Holocene dacite and compositionally zoned magma, Volcán San Pedro, Southern Volcanic Zone, Chile

Abstract

Volcán San Pedro in the Andean Southern Volcanic Zone (SVZ) Chile, comprises Holocene basaltic to dacitic lavas with trace element and strontium isotope ratios more variable than those of most Pleistocene lavas of the underlying Tatara-San Pedro complex. Older Holocene activity built a composite cone of basaltic andesitic and silicic andesitic lavas with trace element ratios distinct from those of younger lavas. Collapse of the ancestral volcano triggered the Younger Holocene eruptive phase including a sequence of lava flows zoned from high-K calc-alkaline hornblende-biotite dacite to two-pyroxene andesite. Notably, hornblende-phlogopite gabbroic xenoliths in the dacitic lava have relatively low 87Sr/86Sr ratios identical to their host, whereas abundant quenched basaltic inclusions are more radiogenic than any silicic lava. The latest volcanism rebuilt the modern 3621 m high summit cone from basaltic andesite that is also more radiogenic than the dacitic lavas. We propose the following model for the zoned magma: (1) generation of hornblende-biotite dacite by dehydration partial melting of phlogopite-bearing rock similar to the gabbroic xenoliths; (2) forceful intrusion of basaltic magma into the dacite, producing quenched basaltic inclusions and dispersion of olivine and plagioclase xenocrysts throughout the dacite; (3) cooling and crystallization-differentiation of the basalt to basaltic andesite; (4) mixing of the basaltic andesite with dacite to form a small volume of two-pyroxene hybrid andesite. The modern volcano comprises basaltic andesite that developed independently from the zoned magma reservoir. Evolution of dacitic and andesitic magma during the Holocene and over the past 350 kyr reflects the intrusion of multiple mafic magmas that on occasion partially melted or assimilated hydrous gabbro within the shallow crust. The chemical and isotopic zoning of Holocene magma at Volcán San Pedro is paralleled by that of historically erupted magma at neighboring Volcán Quizapu. Consequently, the role of young, unradiogenic hydrous gabbro in generating dacite and contaminating basalt may be underappreciated in the SVZ.