Variaciones geoquimicas, locales y regionales, en el frente volcanico cuaternario de los Andes centrales (17°30'-22°00'S), norte de Chile

Abstract

ABSTRACT. Local and regional geochemical variations in the quaternary volcanic front of the central Andes (17°30'-22°00'S), northern Chile. Geochemical data, obtained in samples from the 17°30'-22°00'S sector of the Quaternary volcanic front of the Central Andes, suggest a significant involvement of the continental crust in the magmagenesis of this region. A regional segmentation in the composition of the volcanic centers and a gap in the Quaternary volcanism between 19°10' and 20°40'S (Altos de Pica zone) are important features in this Andean sector. Centers north of the Altos de Pica (16°00'-1909'S) differ from those south of it (2040'-28°00'S) in the abundance of incompatible trace elements and in the behaviour of the Sr- and Pb- isotopic ratios with degree of differentiation. Since the mean age and composition of the crust are the only important geophysical variables between 17°30' and 22°00'S, the regional segmentation seems to reflect an interaction of subcrustal magmas with crust of different age and composition. This interaction is interpreted as a MASH-type process occurring in the lower crust. Local variations in magma composition suggest a MASH zone that evolved in response to crustal thickening during the lower to middle Miocene. The isotopic composition of southern monogenetic con e lavas is similar to those of the northern region lavas and is interpreted to result from interaction of subcrustal magmas with northern type lower crust extending southward at depth.