Eocene magmatic evolution of North Patagonian Andes

FERNÁNDEZ PAZ, LUCÍA; IANNELLI, SOFÍA; VALENCIA, VICTOR; LITVAK, VANESA D.; FOLGUERA, ANDRÉS; ECHAURREN, ANDRÉS; ENCINAS, ALFONSO

Santiago de Chile

Simposio; Primer Simposio de Tectónica Latinoamericana; 2016

ATECSUD

EOCENE MAGMATIC EVOLUTION OF NORTH PATAGONIANANDESLucía Fernández Paz[1], Vanesa D. Litvak[1], Andrés Echaurren[1], Sofía B Iannelli[1], Andrés Folguera[1], AlfonsoEncinas[2], Victor Valencia[3].1-Universidad de Buenos Aires. CONICET. Instituto de Estudios Andinos2-Universidad de Concepción3-Universidad de WashingtonAll through its evolution, the South American plate margin has undergone the collision of several seismicridges that conditioned its tectonic development. During the early Paleogene, the ridge between the Alukand Farallón plates was subducted beneath the North Patagonian Andes, almost parallel to the margin. Asa consequence of this, an asthenospheric window was opened and a bimodal volcanism was emplaced inthe retroarc zone. Under some hypothesis, it has been proposed that during this stage, arc activity wasinterrupted by Aluk plate detachment with the generation of a transform margin. Later, during lateOligocene-middle Miocene times, widespread volcanism was emplaced at North Patagonian latitudes. Itsvariable geochemical characteristics (arc-like to within plate signature), from the forearc toward theretroarc zone, were explained through either a hot spot event as well as an asthenospheric upwellingcaused by slab roll-back. Our study uses geochemistry and geochronology to unravel tectonicdevelopment of Andean volcanism included in El Maitén Belt (33-20 Ma), in order to fill the gap betweenprevious (within-plate) and posterior (arc-related) registered volcanic events. The studied volcanic rockscomprise basaltic and andesitic lavas of the Rivadavia range, near Esquel (43°S), with interbeddedintermediate pyroclastic flows. These strata display wedge-like geometries that were interpreted as thesynrift stage of an extensional regime that lasted until the early Miocene. This sequence was assigned byLA-ICP-MS U/Pb geochronology on a crystalolithic tuff to the late Eocene (37.0 Ma). Geochemically,these rocks are associated with subalkaline?tholeiitic series, characterized by trace elements signaturesthat show slab fluid contributions with limited sediment recycling. This magmatic association representsthe earliest evidence of arc volcanism under an extensional setting, which can be correlated with thedevelopment of extensional basins along the central and southern Andean margin (33°-46°S) during lateEocene-early Miocene times. In a regional context, this arc-related volcanism implies the resumption ofplate subduction during late Eocene times, after a protracted gap in arc volcanism coeval to the earlyPaleogene within plate stage during Aluk/Farallón ridge subduction.