Contrasting sources and conditions of shallow magmatic reservoirs of the Fui Group small eruptive centres associated with the Liquiñe-Ofqui Fault Zone (Chilean Andes)

Abstract

Along the Southern Volcanic Zone of the Andes, the Liquiñe Ofqui Fault Zone controls the occurrence and distribution of small eruptive centres and stratovolcanoes. The Fui Group are clusters of Holocene small eruptive centres within the vicinity of ∼5.5 km west of the Liquiñe Ofqui Fault Zone and 8–16 km ENE from the Mocho-Choshuenco Volcanic Complex. Although separated ∼2 km from each other, the Fui Norte (north) volcanic products are olivine-rich basalts (49–52 wt% SiO2) and enriched in incompatible elements, whilst the Fui Sur (south) materials are basaltic andesites to andesites (55–58 wt% SiO2) with less amount of olivine but more plagioclase and clinopyroxene and depleted in incompatible elements. Crystal clots of olivine, plagioclase and clinopyroxene are abundant in all the studied samples. We found two compositional zones of olivine and plagioclase: High-Fo, High-An (Zone 1, Fo78-89 and An76-93) and Low-Fo, Low-An (Zone 2, Fo60-78 and An57-73); only Zone 2 are in equilibrium with the groundmass (representing the melt) compositions. We determined shallow magmatic reservoir conditions (2–8 km and 6–19 km in the Fui Norte and Fui Sur eruptive centres, respectively) for Zone 2 crystallisation using geothermobarometers and thermodynamic modelling by rhyolite-MELTS v.1.0.2 freeware. Additionally, we determined water content dissolved in melt of 1–2.5 wt% and 2–3.8 wt% for Zone 2 compositions of the Fui Norte and Fui Sur small eruptive centres, respectively, and oxidation conditions for Zone 2 of QFM, QFM-1 and NNO. The relatively more primitive compositions of Zone 1 and the common occurrence of crystal clots with compositions of Zone 1 crystals, suggest an earlier stage of crystallisation in a mushy zone of the shallow reservoirs. Although their geographical proximity, we propose that the magmatic origins of Fui Norte and Fui Sur small eruptive centres are independent due to their differing petrographic and geochemical features and the likely genetic relation of Fui Sur magmas with those erupted from Mocho-Choshuenco Volcanic Complex, where the structures associated to the Liquiñe Ofqui Fault Zone aids the magmatic ascent and emplacement of the volcanic products.