Isotope and Trace Element Data for Orogenic Andesites from the Austral Andes

Abstract

The Austral Volcanic Zone of the Andes (AVZ, 49–55°S) consists of six Pleistocene to Recent volcanic centres resulting from the subduction of the Antarctic plate beneath southernmost South America. Andesites from the southernmost volcano in the AVZ, the Cook Island volcanic complex, have very distinctive geochemical characteristics, including high MgO, CaO, Sr, K/Rb and LREE/HREE, low FeO/MgO, K, Rb, Ba and HREE, and Sr, Nd, O and Pb isotope compositions similar to mid-ocean ridge basalts. These high-MgO andesites are interpreted to have formed by small (<5%) degrees of partial melting of subducted oceanic lithosphere (MORB), followed by rapid upward migration of these partial melts so that near-surface fractional crystallization and crustal contamination were minimal. Andesites and dacites from Mt. Burney, the next volcanic centre to the north, and from Aguilera, Nunatak and Lautaro, the three most northerly volcanoes in the AVZ, have progressively higher SiO₂, K, Rb, Ba, ⁸⁷Sr/⁸⁶Sr and δ¹⁸O, and lower MgO, CaO, Sr, Ni, Cr, K/Rb and ¹⁴³Nd/¹⁴⁴Nd. These rocks are interpreted to have formed by increasing degrees of near-surface fractional crystallization, combined with small amounts of crustal contamination of a parental composition similar to the high-MgO andesites from Cook Island, the most primitive magma type identified in the AVZ. These south-to-north changes in the geochemistry and petrogenesis of the volcanic centres of the AVZ may result from variations in the stress pattern within the continental lithosphere due to the change from direct convergence to strike-slip plate motion which occurs along the southernmost margin of South America.