Insights on the Plumbing System of a Stratovolcano and its Interaction with the Local Tectonics, based on the Petrology, Geochemistry and Textural Analysis of Satellite Scoria Cones. Llaima Volcano, Chile.

Abstract:

The plumbing system of stratovolcanoes (i.e. emplacement of magmatic feeder dikes) mainly depends on the state of stress in the crust and the regional tectonics; yet, the effect of local tectonic deformation on the plumbing system is poorly understood. Scoria cones are considered proxies of magmatic feeder dikes; hence, their study provides insights on the factors controlling their emplacement. Here, we investigate the satellite scoria cones at Llaima volcano, Chile, and their relation to the local tectonic deformation generated by the Liquiñe-Ofqui Fault Zone (LOFZ). Based on petrologic characteristics, two groups of scoria cones have been recognized. The first group exhibits glomeroporphyritic textures (ol, cpx, pg), kinked Crystal Size Distributions (CSD) and calculated mineral residence-times up to 20 years. Conversely, the second group exhibits pilotaxitic textures (pg, cpx), log-linear CSDs and residence-times up to 4 years. Whole-rock chemistry compositions are less evolved (50.0-53.5 wt.% SiO₂; 4.0-6.0 wt.% MgO; 10-110 ppm Cr) in the first group, than in the second group (53.5-60.0 wt.% SiO₂; 2.0-4.0 wt.% MgO; 10-50 ppm Cr). The spatial location of cones is consistent with local tectonic deformation generated by coeval antithetic and synthetic faults, at the northern-end of the LOFZ. The pilotaxitic cones seem to be associated to extensive regimes; while the glomeroporphyritic cones seems to be controlled by the regional stress. Zones of crustal compression do not display scoria cones. We conclude that at Llaima, both the location and composition of scoria cones are the result of the interplay between magmatic processes and local tectonic deformation. Magma recharge and volatile exsolution generated overpressure at the chamber, which controlled the emplacement of the glomeroporphyritic cones. In contrast, local tectonic extension controlled the emplacement of the pilotaxitic cones, which were fed by a fractionating magma that underwent sudden decompression.