The emplacement of satellite scoria cones at Llaima Volcanic System, Chile; The interaction between magmatic overpressure and the local tectonics
Schonwalder Angel, Dayana A.
MetadataShow full item record
Satellite scoria cones in volcanic systems are the surface representation of dike emplacement. Current models establish that dike propagation and emplacement depends of the state of stresses generated by the magmatic source (i.e. magma chamber) and the regional state of stresses. However, in areas where volcanic systems are associated to strike-slip faults, the relationship between local-scale crustal deformation and dike emplacement has not been addresses yet. This dissertation investigates the petrogenesis of the scoria cones at Llaima Volcanic System and its relation with the local-scale crustal deformation generated by the Liquiñe-Ofqui dextral, strike-slip, regional fault. Two groups of scoria cones were recognized. The products from the cones located at the NE flank of Llaima stratovolcano are characterized by glomeroporphyritic textures and the presence of phenocrysts with and without disequilibrium features. They have relatively low whole-rock silica content (50.0-53.5 wt. % SiO 2 ) and higher Forsterite (Fo 60-80 ) and Anorthite (up to An 90 ) contents. The Crystal Size Distribution (CSD) trends of the main mineral phases from this group are generally curved and kinked. The maximum sizes for crystals in this group can reach up to 0.8 mm. Conversely, the products from the scoria cones located at the NW flank exhibit pilotaxitic textures and phenocrysts are rare. They have higher silica (53.5-60.0 wt. % SiO 2 ), and lower Forsterite (Fo 60 ) and Anorthite (An 50-70 ) contents. Their CSD trends are mainly log-linear, with crystal sizes up to 0.2 mm. The location of the satellite scoria cones agress with the crustal deformation generated by coeval antithetic and synthetic faults (i.e. SC-kinematic), at the northern termination of the Liquiñe-Ofqui fault. The scoria cones at the NW flank are located in the zone of major crustal extension, while the absence of scoria cones at the SE flank coincides with the zones of crustal compression. The scoria cones at the NE flank exhibit a trend in agreement to the regional state of stresses. The fundamental contribution of this dissertation is to show that a comprehensive model of the plumbing of complex volcanic systems, associated to strike-slip faults, can be attained by integrating information from different research fields (e.g. petrography, geochemistry, mineral chemistry, tectonic and structural analysis). This approach provides evidence that both magmatic and tectonic processes affect plumbing systems. I show that in complex volcanic systems there is a variation regarding the petrogenesis of satellite scoria cones. Both, the location and composition of scoria cones products, are the result of the interplay between the factors controlling the plumbing system. At Llaima, the scoria cones located at the NE flank result from processes of magma recharge in the chamber. Influx of new magma disrupts the crystal mush and changes the melt conditions (i.e. composition and temperature) in the chamber. The basic composition of the recharge magma can be documented by the whole-rock and the mineral chemistry of the samples from this group. When magma recharge occurs, exolution of volatiles from the new magma and vigorous convection between the two magmas, causes overpressure at the chamber and dikes initiate radially from the chamber, as they propagate they re-orientate towards the NE according to the regional state of stresses. The emplacement of the feeder dikes for the scoria cones at the NW flank were controlled by local-scale crustal extension. It is inferred that crustal deformation causes magma chamber rupture and dike initiation, which are fed by the magma that had been fractionating at the chamber. The sudden decompression of the magma as it ascend to the surface induced crystallization due to degassing.