Oxygen isotopic compositions of Central Andean plutonic and volcanic rocks, latitudes 26°–29° south

Oxygen isotope data are reported for 27 igneous rocks of Mesozoic to Quaternary age from the Central Andes. 26–29°S. The plutonic rocks, and most of the volcanics, have δ18O values between 6.2 and 8.3‰.The whole-rock δ18O values show a weak correlation with initial87Sr/86Sr data. This O-Sr array differs from documented trends for calc-alkaline plutonic suites from California, Scotland and northern Italy, but overlaps with data for volcanic and plutonic rocks from Ecuador, northern Chile and southern Perú.The oxygen isotope results indicate that the magmas evolved without significant contamination from supracrustal rocks (e.g., rocks that experienced18O enrichment during surficial weathering). The available O, Sr and Pb isotopic data for these rocks are best explained by magma generation in the upper mantle or lower crust. From the Late Mesozoic on, the87Sr/86Sr values were modified at depth by isotopic exchange between the magma and a continually thickening crust of plutonic rocks of Late Precambrian to early Mesozoic age.