Spatial and temporal variations in volcanic of the Andean Central Volcanic Zone (26 to 28°S)

Late Cenozoic volcanic rocks from the southernmost extremity of the Central Volcanic Zone (CVZ) of the Andes, near the first-order tectonic discontinuity at 27°S, display geochemical attributes typical of CVZ volcanic rocks: an exclusion of basic compositions, high concentrations of large ion lithophile (LIL) elements and elevated ⁸⁷Sr/⁸⁶Sr. Some of these geochemical attributes show intriguing and sometimes systematic spatial and temporal variation. Along the Miocene volcanic front, normalized concentrations of K₂O and Rb in volcanic rocks erupted from central vents systematically increase as the tectonic discontinuity is approached from the north. K/Rb systematically decreases in the same direction. These along-arc systematics are equivalent to the geochemical differences between the Miocene volcanic front, as a whole, and the Pliocene to Recent volcanic front, which lies some 50 km farther east. Pliocene to Recent volcanic rocks, however, generally also exhibit enrichments in light rare earth elements and La/Yb. In addition, local variations in LIL element contents rival those along and across the arc. All of these spatial-temporal geochemical variations are attributed to crustal differentiation processes, specifically crustal contamination. The spatial-temporal implications are that crustal contamination is increasingly effective: (1) toward the modern-day tectonic discontinuity at 27°S; and (2) underneath Pliocene to Recent, as opposed to Miocene, volcanoes. Garnet control and hornblende fractionation are important variables in crustal differentiation. Increasing crustal contamination along and across the arc correlates with flattening of the subducted Nazca plate and possibly with increasing crustal thicknesses. If so, the spatial-temporal geochemical variations in the southernmost CVZ strongly support the models of Kay and others (1987, 1988) for the volcanic and tectonic evolution of the present-day nonvolcanic region.