The interplay of magmatism and tectonics : an example based on the satellite scoria cones at Llaima volcano, Chile

Abstract

We present a geochemical and mineralogical study of the satellite scoria cones at Llaima volcano, Chile, providing insights on magmatic processes and how these are affected by the local and regional tectonics. At Llaima, we identify two different and distinctive groups of satellite scoria cones with a common magmatic source but different petrological evolutions. Cones from group 1 (the “glomeroporphyritic group”), predominantly occur in the NE flank of the volcano, are characterized by basaltic to basaltic andesitic compositions (51–55 wt% SiO2) and have glomeroporphyritic textures. We have interpreted these cones as the product of magma replenishment, followed by an unimpeded propagation of their feeder dikes controlled mainly by the regional stress. As a result, dikes are emplaced parallel to the regional σ1, and perpendicular to the axis of local crustal extension. Cones from the group 2 (the “pilotaxitic group”), appear at the NE, NW and SW flanks of the volcano, they have more evolved compositions (55–60 wt% SiO2) and textures lacking glomerocrysts. Their location and morphometric parameters suggest that their feeder dike emplacement is mainly controlled by the stress exerted by the load of the volcanic edifice, where the compressional stress hinders magma ascent, allowing fractionation and volatile overpressure buildup. The characteristic texture of these cones is interpreted as fractionation, followed by a single crystallization event upon eruption. The lack of scoria cones at the SE flank is attributed to the local horizontal compressional stress caused by the local tectonic deformation associated to the Liquiñe-Ofqui Fault Zone.