Evolution of Eocene to Oligocene arc-related volcanism in the North Patagonian Andes (39–41°S), prior to the break-up of the Farallon plate

Abstract

Voluminous Paleogene magmatic rocks (44 to 29Ma) are found in a retroarc position in the Northern Patagonian to Southern Central Andes (~39–42°S), whose origin remains controversial. Geochemical data in these Eocene to Oligocene volcanic associations are herein used to unravel their origin and understand changes in subduction parameters. Geochemical signatures indicate arc-related associations and reflect changing geodynamic boundary conditions of the Andean margin through time. In particular, Eocene magmatism (~44Ma; Pilcaniyeu Belt) shows an alkaline-like signature and limited slab influence. Reported contemporaneous within-plate magmatism (~47–43Ma) in an easternmost position reflects a more typical enriched source. Oligocene arc-like volcanism (~29Ma; El Maitén Belt), which developed in an extensional retroarc setting, shows a higher contribution from slab-derived fluids and a calc-alkaline source. A comparison with younger arc-related magmas from the region (~26–20Ma), emplaced in an intra- to retroarc position (Cura Mallín and Abanico basins), indicates a progressive increase in slab-signature, associated with a tholeiitic magma source. We propose that these compositional variations could be directly related to changes in plate configuration before and after the Farallon plate break-up and the initiation of a more orthogonal convergence typical of the present Andean-type subduction zone.