Andean Southern Volcanic Zone Research Articles

Volcanism and erosion during the past 930 k.y. at the Tatara–San Pedro complex, Chilean Andes

Geologic mapping, together with 73 new K-Ar and 40Ar/39Ar age determinations of 45 samples from 17 different volcanic units, plus paleomagnetic orientations, geochemical compositions, and terrestrial photogrammetry are used to define the chronostratigraphy of the Tatara–San Pedro complex, an eruptive center at 36°S on the volcanic front of the Andean southern volcanic zone. The Tatara–San Pedro complex preserves ≈55 km3 of lavas that erupted from at least three central vent regions. Remnant, unconformity-bound sequences of lavas are separated by lacunae that include significant periods of erosion. Quaternary volcanism commenced ca. 930 ka with eruption of voluminous dacitic magma, followed 100 k.y. later by the only major rhyolitic eruption. From 780 ka onward, more than 80% of the preserved volume is basaltic andesite (52%–57% SiO2), but petrographically and geochemically diverse dacitic magmas (63%–69% SiO2) erupted sporadically throughout this younger, dominantly mafic phase of activity. A few basaltic lavas (49%–52% SiO2) are present, mainly in portions of the complex older than 230 ka. The number of vents, the petrologic and geochemical diversity, and the temporal distribution of mafic and silicic lavas are consistent with emplacement of many separate batches of mafic magma into the shallow crust beneath the Tatara–San Pedro complex over the past million years. Nearly two-thirds of the preserved volume of the Tatara–San Pedro complex comprises the two youngest volcanoes, which were active between ca. 188–83 ka and 90–19 ka. Repeated advances of mountain glaciers punctuated growth of the complex with major erosional episodes that removed much of the pre-200 ka volcanic record, particularly on the south flank of the complex. Dating the inception of a glaciation on the basis of preserved material is difficult, but the age of the oldest lava above a lacuna may be used to estimate the timing of deglaciation. On this basis, the argon ages of basal lavas of multiple sequences indicate minimum upper limits of lacunae at ca. 830, 790, 610, 400, 330, 230, 110, and 17 ka. These are broadly consistent with global ice-volume peaks predicted by the oxygen isotope-based astronomical time scale and with age brackets on North American glacial advances. Estimated growth rates for the two young volcanoes are 0.2 to 0.3 km3/k.y.; these are three to five times greater than a growth rate estimated from all preserved lavas in the complex (0.06 km3/k.y.). Removal of up to 50%–95% of the material erupted between 930 and 200 ka by repeated glacial advances largely explains this discrepancy, and it raises the possibility that episodic erosion of mid-latitude frontal arc complexes may be extensive and common.

Miocene to present magmatic evolution at the northern end of the Andean Southern Volcanic Zone, Central Chile

ABSTRACT. At the northern end of the Andean Southern Volcanic Zone (NSVZ; 33-34°S), subduction angle decreased and the magmatic arc migrated eastward to its present location along the Andean drainage divide during the Pliocene. Prior to arc migration, the isotopic compositions of the magmas emplaced along the locus of the Miocene volcanic front changed significantly, Miocene igneous rocks have relatively low initial 87Sr/86Sr=0.7038 to 0.7040 and high epsilon Nd=+1.9 to +3.3, while Pliocene rocks emplaced along the same belt have higher 87Sr/86Sr=0.7043 to 0.7049 and lower epsilon Nd=-1.1 to +0.8. These isotopic changes imply a temporal increase in the role of crustal components in magma-genesis. This may have resulted, in part, from increased assimilation of lower crust. However, the crust below the Pliocene arc never reached the same thickness as that below the active NSVZ, although the Pliocene igneous rocks are isotopically similar to NSVZ lavas. Altematively, these changes may be explained by an increase in the proportion of crustal components added to the sub-Andean mantle source region during the Pliocene. This resulted from an increase in the rate of subduction erosion and source region contamination below Central Chile due to a combination of both decreasing subduction angle and the subduction of the Juan Fernandez Ridge. Similar isotopic changes imply an increase in crustal components in Andean magmas beginning before 14 Ma at 30°S, by 9 Ma at 32°S, by 7 Ma at 33°S, and after 5 Ma at 34°S. This suggests a close relation with the southwards migration of the locus of subduction of the Juan Femandez Ridge. Differences in the rates of subduction erosion and mantle source region contamination also explain some of the chemical differences between the Andean Central (CVZ) and Southern (SVZ) Volcanic Zones. The authors conclude that lower plate parameters (subduction geometry, rates of subduction erosion and sediment subduction) may be as important as upper plate parameters (crustal age and thickness) for producing not only the spatial, but also the chemical segmentation of the active Andean arc. RESUMEN. Evolucion magmatica desde el Mioceno al presente en el extremo norte de la Zona Volcanica de los Andes del Sur, Chile central. En el extremo norte de la Zona Volcanica de los Andes del Sur (NZVS; 33-34°S), el angulo de subduccion disminuyo durante el Plioceno y el arco magmatico migro hacia el este, a su posicion actual a lo largo de la linea divisoria de aguas en la cumbre de los Andes. La composicion isotopica de los magmas emplazados en el Frente volcanico miocenico cambio significativamente antes de la migracion del arco. Las rocas igneas del Mioceno tienen razones iniciales de 87Sr/86Sr relativamente bajas, entre 0,7038 y 0,7040 y valores de epsilon Nd altos entre +1 ,9 y +3,3, mientras que las rocas del Plioceno, emplazadas en ese mismo cinturon, tienen razones iniciales de 87Sr/86Sr mas altas, entre 0,7043 y 0,7049, y valores de epsilon Nd mas bajos entre -1,1 y +0,8. Estos cambios isotopicos indican un aumento en el tiempo del rol de los componentes corticales en la genesis de los magmas, lo que podria ser el resultado, en parte, de una mayor asimilacion en la corteza inferior. Sin embargo, la corteza que infrayace al arco pliocenico nunca alcanzo el espesor de la corteza que existe en la actualidad bajo el extremo norte de la Zona Volcanica de los Andes del Sur, aun cuando las rocas igneas del Plioceno son isotopicamente similares a las lavas de la NZVS. En forma alternativa, estos cambios podrian ser explicados por un aumento en la proporcion de los componentes corticales en la fuente del manto subandino durante al Plioceno. Esto fue producto del mayor grado de erosion tectonica y de la contaminacion cortical de la fuente bajo Chile central, consecuencia de la disminucion del angulo de subduccion y de la subduccion de la Dorsal Juan Fernandez. Cambios isotopicos similares indican un aumento de los componentes corticales en los magmas andinos que comenzo antes de los 14 Ma en 30°S, a los 9 Ma en 32°S, a los 7 Ma en 33°S y despues de 5 Ma en 34°S. Esto sugiere una estrecha relacion con la migracion hacia el sur del lugar de subduccion de la Dorsal Juan Fernandez. Las diferencias en la velocidad de la erosion tectonica yen la contaminacion de las regiones fuente de magmas tambien explican algunas de las diferencias quimicas entre la Zona Volcanica de los Andes Centrales (ZVC) y la Zona Volcanica de los Andes del Sur (ZVS). Esto sugiere que los factores relacionados con la placa inferior (geometria de subduccion, velocidad de erosion tectonica y subduccion de sedimentos) pueden ser tan importantes como los factores de la placa superior (espesor y edad de la corteza) para producir no solo la segmentacion espacial, sino tambien la segmentacion quimica del arco andino activo.

Crustal xenoliths from Calbuco Volcano, Andean Southern Volcanic Zone: implications for crustal composition and magma-crust interaction

Crustal xenoliths from Calbuco Volcano, Andean Southern Volcanic Zone: implications for crustal composition and magma-crust interaction

Crustal xenoliths from Calbuco Volcano, Andean Southern Volcanic Zone: implications for crustal composition and magma-crust interaction

Crustal xenoliths in the 1961 andesite flow of Calbuco Volcano, in the southern Southern Volcanic Zone (SSVZ) of the Andes, consist predominantly of pyroxene granulites and hornblende gabbronorites. The granulites contain plagioclase+pyroxene+magnetite±amphibole, and have pronounced granoblastic textures. Small amounts of relict amphibole surrounded by pyroxene-plagioclase-magnetite-glass symplectites are found in some specimens. These and similar textures in the gabbronorites are interpreted as evidence of dehydration melting. Mineral and bulk rock geochemical data indicate that the granulites are derived from an incompatible trace element depleted basaltic protolith that underwent two stages of metamorphism: a moderate pressure, high temperature stage accompanied by melting and melt extraction from some samples, followed by thermal metamorphism after entrainment in the Calbuco andesite lavas. High ɛNd T values (+4.0 to +8.6), Nd-isotope model ages of 1.7–2.0 Ga, and trace element characteristics like chondrite normalized La/Yb< and La/Nb≤1 indicate that the protoliths were oceanic basalts. Similar oceanic metabasalts of greenschist to amphibolite facies are found in the Paleozoic metamorphic belt that underlies the Chilean coastal ranges. Mineral and bulk rock compositions of the gabbronorite xenoliths indicate that they are cognate, crystallizing from the basaltic andesite magma at Calbuco. Crystallization pressures for the gabbros based on total Al contents in amphibole are 6–8 kbar. These pressures point to middle to lower crustal storage of the Calbuco magma. Neither granulite nor gabbro xenoliths have the appropriate geochemical characteristics to be contaminants of Calbuco andesites, although an ancient sedimentary contaminant is indicated by the lava compositions. The presence of oceanic metabasaltic xenoliths, together with the sedimentary isotopic imprint, suggests that the lower crust beneath the volcano is analogous to the coastal metamorphic belt, which is an accretionary complex of intercalated basalts and sediments that formed along the Paleozoic Gondwanan margin. If this is the case, the geochemical composition of the lower and middle crust beneath the SSVZ is significantly different from that of most recent SSVZ volcanic rocks.

Environmental assessment of 1991 Hudson volcano eruption ashfall effects on southern Patagonia region, Argentina

The August 1991 eruption of the Hudson volcano in the southern Andean volcanic zone affected an area of 1000 km in radius to the east in the Argentine Patagonic meseta. The thickness of ash ranged from 20 cm in the Andean area to less than 1 mm in the Atlantic coast zone. Wind storms reactivated the ash deposits, together with terrigenous material, more than one year after the eruption. In order to assess the potential risk of the ash, analyses of concentration, size, mineralogical composition, toxic elements, and irritating effects of gases were performed. In all samples, the percentage of particles of the 2-to 5-μm range is below the toxic threshold level. Trace elements are below the toxic threshold concentration for humans and animals. The major impact of the ashfall was on sheep herds; about one third of them were lost in the areas close to the volcano. Soil incorporated the ash layers, and a fast recovery of orchards was reported two years later. Rivers were loaded with sediment in the immediate aftermath, but one year later returned to previous conditions. Along the shores of the Buenos Aires Lake, a fresh tephra layer can be distinguished.

Possible contribution of the asthenosphere, below the subducted oceanic lithosphere, to the genesis of arc magmas: Geochemical evidence from the andean southern volcanic zone (33–46°S)

Abstract The application of the Sr/Ca-Ba/Ca systematics to volcanic rocks of the Andean Southern Volcanic Zone (33°S–46°S) has revealed a good correlation between the estimated degree of partial melting required to generate primary magmas and the projected extensions of the oceanic Nazca plate fracture zones under the continental South American plate. Magmas erupted at volcanic centers situated above these projections are thought to have been derived from primary magmas generated by relatively high degrees of melting, whereas those erupted at other centers are thought to have evolved from magmas produced by comparatively low degree of fusion. We interpret this relationship to reflect the facilitation of heat and mass transfer from the asthenosphere below the subducted oceanic lithosphere to the subarc mantle by the fracture zones. This contribution enhances the degree of melting of the subarc mantle source as well as the fraction of material derived from the subducted oceanic crust. This model predicts the predominance of basalts depleted in incompatible trace elements in centers located above the Nazca plate fracture zone extensions and of basalts enriched in incompatible trace elements in centers situated between boundaries of fracture extensions.