Geochemistry and tectonics of the Chilean Southern Andes basaltic Quaternary volcanism (37-46°S)

Abstract

ABSTRACT. Between latitudes 37° and 46°S of the Andes, the Nazca-South America Plate convergence is currently slightly oblique, Postglacial volcanism has been continuous and intense, being expressed as numerous composite stratovolcanoes (SV) and hundreds of minor eruptive centres (MEC). The overall trend of the volcanic are is NNE (~N10°E), and its main structural feature is the 1,000 km long, also NNE-trending, Liquine-Ofqui fault zone (LOFZ). Some MEC are spatially associated with the main NNE-trending lineaments of the LOFZ; others, such as the Carran-los Venados volcanic group (40.3°S), form N50-70°E clusters, and oblique to the overall trend of the volcanic are. Between 37° and 41.5°S, Central Southern Volcanic Zone (CSVZ), the SV form either N50-600W or N50-700E alignments. Between 41.5° and 46°S, South Southern Volcanic Zone (SSVZ), the distribution of SV is similar. The alignment of parasitic vents, MEC and SV suggests that the direction of sigma H max in The CSVZ and SSVZ is roughly N50-70°E, which may reflect a transpressional tectonic regime, resulting from a combination of dextral strike-slip and shortening across the arc. Most young extensional NE-trending volcanic chains (late Pleistocene-Holocene), including SV±MEC contain mainly basaltic rocks. This is consistent with a short residence time of magmas in the crust. Older volcanic complexes (Early Pleistocene) and volcanic edifices controlled by NW-trending contractional Fractures and faults present not only basaltic rocks, but also medium andesites, dacites and even rhyolites. This is probably the result of a longer, intracrustal magma residence, yielding a more mature and differentiated composition. Basaltic rocks from either N50-60°W and N50-70°E transverse fractures tend to increase their incompatible elements abundances, Ba/La, and 87Sr/86Sr ratios and decrease their La/Yb and 143Nd/144Nd ratios in going from west lo east, suggesting that these geochemical features are mainly controlled by subcrustal features and processes; fractures and faults would only facilitate the ascent of magmas. Similar geochemical behaviours are observed between MEC basaltic rocks located al similar latitude, but different longitude. RESUMEN. Geoquimica y tectonica del volcanismo cuaternario basaltico de los Andes del Sur de Chile (37-46°S). Entre las latitudes 37° y 46°S de los Andes, la convergencia entre las placas de Nazca y Sudamericana es levemente oblicua. El volcanismo posglacial, que ha sido continuo e intenso, ha originado numerosos estratovolcanes compuestos (EV) y cientos de centros eruptivos menores (CEM). El arco volcanico tiene una orientacion general NNE (~N10°E), y su principal caracteristica estructural es la zona de Falla Liquine-Ofqui (ZFlO), de 1.000 km de longitud y orientacion NNE. Algunos CEM estan asociados espacialmente con los lineamientos NNE de la ZFLO; otros, como el grupo volcanico Carran-los Venados (40,3°S), estan orientados oblicuamente (N50-70°E) con respecto a la orientacion general del arco volcanico. Entre los 37° y 41,5°S, Zona Volcanica Sur Central (ZVSC), los EV presentan alineamientos N50-60°W o N50-70°E; algo similar acontece entre los 41,5° y 46°S, Zona Volcanica Sur Sur (ZVSS). El alineamiento de los conos parasitos, de los CEM y de los EV sugiere que la direccion de sigma H max en las regiones central y sur de los Andes del Sur es aproximadamente N50-70°E, lo cual reflejaria un regimen tectonico transpresional, que resultaria de la combinacion de una rotacion dextral, tipo 'strike-slip', y un acortamiento del arco. La mayoria de las cadenas volcanicas jovenes (Pleistoceno tardio-Holoceno), de orientacion noreste, son extensionales y contienen principalmente rocas basalticas, lo cual es consistente con un tiempo de residencia cortical corto. Complejos volcanicos antiguos (Pleistoceno temprano) y edificios controlados por fracturas y fallas compresionales, de orientacion noroeste, presentan basaltos y tambien rocas volcanicas mas acidas. Esto resultaria de un tiempo de residencia mas prolongado en la corteza, lo que permitiria la generacion de magmas mas maduros y diferenciados. Las rocas basalticas, tanto de alineamiento de orientacion N50-60°W como N50-70°E, tienden a aumentar su abundancia en elementos incompatibles y sus razones Ba/La y 87Sr/86Sr y a disminuir sus razones La/Yb y 143Nd/144Nd en direccion oeste-este, sugiriendo que estas caracteristicas dependen fundamentalmente de procesos subcorticales; las fracturas y fallas solo facilitarian el ascenso de los magmas.