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Contrasting Sources and Conditions of Shallow Reservoirs of the Fui Group Small Eruptive Centres Associated with the Liquiñe-Ofqui Fault Zone (Chilean Andes)

2022; RELX Group (Netherlands); Linguagem: Inglês

10.2139/ssrn.4082318

1556-5068

Francisca Mallea-Lillo, Miguel A. Parada, Eduardo Morgado, Claudio Contreras, Darío Hübner,

Geological formations and processes

Along the Southern Volcanic Zone (SVZ) of the Andes, the Liquiñe Ofqui Fault Zone (LOFZ) controls the occurrence and distribution of small eruptive centres (SECs) and stratovolcanoes. The Fui Group are clusters of Holocene SECs <5.5 km west of the LOFZ and 8 – 16 km ENE from the Mocho-Choshuenco Volcanic Complex (MCVC). Although separated ~2 km from each other, the Fui Norte (north) volcanic products are olivine-rich basalts (49 – 52 wt% SiO 2 ) and enriched in incompatible elements, whilst the Fui Sur (south) materials are basaltic andesites to andesites (55 – 58 wt% SiO 2 ) 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, Fo 78-89 and An 76-93 ) and Low-Fo, Low-An (Zone 2, Fo 60-78 and An 57-73 ); only Zone 2 are in equilibrium with the groundmass (representing the melt) compositions. We determined shallow reservoir conditions (up to 7.6 and 18.9 km in the Fui Norte and Fui Sur SECs, 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 SECs, respectively, and oxidation conditions for Zone 2 of QFM, QFM-1 and NNO. The relative more primitive compositions and the common occurrence as part of crystal clots 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 SECs are independent due to their differing petrographic and geochemical features and the likely genetic relation of Fui Sur SECs magmas with those erupted from MCVC, where the structures associated to the LOFZ aids the magmatic ascent and emplacement of the volcanic products.