Geochemical Variations in Volcanic Rocks from Central-south Chile (33–42°S)

Abstract

Pliocene to Recent volcanic rocks from central-south Chile (33–42°S) have ¹⁴³Nd/¹⁴⁴Nd (0.51269–0.51291), ⁸⁷Sr/⁸⁶Sr (0.70381–0.70464) and ¹⁸O/¹⁶O (5.2–6.8‰) ratios which overlap with those in oceanic-island basalts. Although individual volcanic centres have distinct isotopic ratios, variations in these ratios do not vary systematically along or across the strike of the Andes. At two volcanic centres, Laguna del Maule (36°S) and Puyehue (40°S), rhyolites have Sr, Nd and O isotope ratios similar to associated basalts. As in other volcanic arcs related to subduction, basalts and basaltic andesites from central-south Chile have high K/La, Rb/La and Ba/La and low Rb/Cs_abundance ratios relative to ocean floor and island basalts. Although K, Rb, Ba and La abundances in basalts from 12 central-south Chilean volcanic centres vary by factors of 3 to 8, ratios of these elements such as Ba/La (15–28) and K/La (330–720) vary by only a factor of two and do not correlate with variations in elemental abundance.

We conclude that in central-south Chile there is no isotopic evidence that crustal contamination is an important process in creating calc-alkaline sequences ranging from basalt to rhyolite. Although isotopic characteristics of central-south Chilean lavas are consistent with partial melting of mantle sources similar to the sources of oceanic-island basalts, variations in incompatible trace element abundances and ratios are more consistent with a two-stage model involving addition of K, Rb, Ba and Cs-rich fluids derived from the subducted Nazca plate to the peridotite sources of the basalts.