Strontium isotope data for recent andesites in Ecuador and North Chile

New Sr isotope data are presented for andesite lavas from active volcanoes in Ecuador and North Chile. Twenty-three samples from five Ecuadorian volcanoes have 87Sr/86Sr ratios most of which are essentially within analytical error, and average 0.7044. In contrast, 16 samples from the San Pedro-San Pablo volcano complex in North Chile have 87Sr/86Sr ratios between 0.7058 and 0.7072. These samples show no correlation between 87Sr/86Sr and 87Rb/86Sr, nor between 87Sr/86Sr and other elements or ratios. The major difference in setting between the two volcanic provinces lies in the thickness of the continental crust; 40-50 km beneath Ecuador, 70 km beneath North Chile. Andesites from both areas are derived primarily from the mantle, but those from North Chile show evidence of a significant Sr isotopic contribution from the continental crust. (Auth.)