Mafic monogenetic vents at the Descabezado Grande volcanic field (35.5A degrees S-70.8A degrees W): the northernmost evidence of regional primitive volcanism in the Southern Volcanic Zone of Chile

摘要

In the Andean Southern Volcanic Zone (SVZ), the broad distribution of mafic compositions along the recent volcanic arc occurs mainly south of 37A degrees S, above a comparatively thin continental crust (a35 km) and mostly associated with the dextral strike-slip regime of the Liquie-Ofqui Fault Zone (LOFZ). North of 36A degrees S, mafic compositions are scarce. This would be in part related to the effect resulting from protracted periods of trapping of less evolved ascending magmas beneath a thick Meso-Cenozoic volcano-sedimentary cover that lead to more evolved compositions in volcanic rocks erupted at the surface. Here, we present whole-rock and olivine mineral chemistry data for mafic rocks from four monogenetic vents developed above a SVZ segment of thick crust (similar to 45 km) in the Descabezado Grande volcanic field (similar to 35.5A degrees S). Whole-rock chemistry (MgO > 8 wt%) and compositional variations in olivine (92 a 88 and Ni up to similar to 3650 ppm) indicate that some of the basaltic products erupted through these vents (e.g., Los Hornitos monogenetic cones) represent primitive arc magmas reaching high crustal levels. The combined use of satellite images, regional data analysis and field observations allow to recognize at least 38 mafic monogenetic volcanoes dispersed over an area of about 5000 km(2) between 35.5A degrees and 36.5A degrees S. A link between ancient structures inherited from pre-Andean tectonics and the emplacement and distribution of this mafic volcanism is suggested as a first-order structural control that may explain the widespread occurrence of mafic volcanism in this Andean arc segment with thick crust.