Geochemistry and Petrology of the Most Recent Deposits from Cotopaxi Volcano, Northern Volcanic Zone, Ecuador

Abstract

Cotopaxi volcano is located in the Northern Volcanic Zone of the South American Andes. Pyroclastic deposits and lava flows from Cotopaxi comprise basaltic andesites, andesites and rhyolites that have erupted since 13 200 years BP. Nine rhyolite eruptions were produced in at least five separate events, punctuated by intermittent andesite eruptions. High La/Yb (>5) and 230Th excesses in the andesites are consistent with equilibration of magma with garnet-bearing lower crust or mantle, and numerical models show that lower crustal assimilation–fractional crystallization involving crystallization of amphibole and plagioclase is sufficient to create the observed variations in trace elements. The Cotopaxi andesites contain intergrowths of plagioclase + pyroxene, and at least four populations of plagioclase crystals indicate pervasive magma mixing. Sr, Nd and Pb isotopes are consistent with 5–20% assimilation of radiogenic crust, but higher levels of mafic crust (cryptic assimilation) and/or source contamination by volcanogenic sediments are also likely. The Cotopaxi rhyolites formed from compaction and extraction of high-SiO2 melt from an andesitic or dacitic crystal mush. On the basis of U-series data, the residence time of the rhyolitic melts is of the order of 74 kyr. Temporal variations in MgO and Sr of the Cotopaxi andesites reflect frequent recharge of the Cotopaxi system, variable recharge composition and a system that has not become more evolved over time.