Geochemical evolution of the Quaternary Chachimbiro Volcanic Complex (frontal volcanic arc of Ecuador)

Abstract

The Chachimbiro volcanic complex (CVC) is a composite volcano located in the frontal part of the Ecuadorian Quaternary Arc.Four periods of activity can be differentiated in Chachimbiro ∼400 ka-long volcanic history, named CH1 (405.7 ± 20.0–298.6 ± 32.9 ka), CH2 (121.75 ± 23.2–36.08 ± 2.8 ka), CH3 (36080 ± 280–22730 ± 120 ybp) and CH4 (5.760 ± 30 ybp) respectively. The magmatic suite ranges in composition from andesites to rhyodacites, and displays systematic increases of SiO2, Sr/Y, La/Yb, incompatible elements (e.g., LREE, MREE, LILE) and depletion of HREE with time. These geochemical features, coupled with radiogenic isotope data and textural observations (e.g., several resorption and overgrowth cycles in plagioclase phenocrysts), suggest an open system evolution for CVC including typical crustal magmatic processes such as recharge, assimilation and fractional crystallisation.To better constrain the magmatic crustal processes of the CVC, we use a multi-model approach including three open system models: RFC (Recharge, Fractional Crystallisation), AFC (Assimilation, Fractional Crystallisation), and EC-RAFC (Energy Constrained – Recharge Assimilation, Fractional Crystallisation). Different simulations have been developed to best fit CVC’s geochemistry (including trace elements, Sr/Y ratios and Sr isotopes), which suggest predominant fractional crystallisation and variable assimilation as the main processes explaining the bulk of the geochemical data of CVC. This is consistent with an overall thermal decline of the magmatic system through time.The genetic model here presented involves the input of a magmatic recharge component matured in the low-mid crust through dominant fractionation of amphibole (±garnet), into intermediate crustal reservoir(s) where magma undergoes extensive FC.