EOCENE MAGMATIC EVOLUTION OF NORTH PATAGONIAN ANDES

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

Santiago

Simposio; Primer Simposio de Tectónica Sudamericana; 2016

Asociación tectónica Sudamericana (ATECSUD)

All through its evolution, the South American plate margin has undergone the collision of several seismic ridges that conditioned its tectonic development. During the early Paleogene, the ridge between the Aluk and Farallón plates was subducted beneath the North Patagonian Andes, almost parallel to the margin. As a consequence of this, an asthenospheric window was opened and a bimodal volcanism was emplaced in the retroarc zone. Under some hypothesis, it has been proposed that during this stage, arc activity was interrupted by Aluk plate detachment with the generation of a transform margin. Later, during late Oligocene-middle Miocene times, widespread volcanism was emplaced at North Patagonian latitudes. Its variable geochemical characteristics (arc-like to within plate signature), from the forearc toward the retroarc zone, were explained through either a hot spot event as well as an asthenospheric upwelling caused by slab roll-back. Our study uses geochemistry and geochronology to unravel tectonic development of Andean volcanism included in El Maitén Belt (33-20 Ma), in order to fill the gap between previous (within-plate) and posterior (arc-related) registered volcanic events. The studied volcanic rocks comprise basaltic and andesitic lavas of the Rivadavia range, near Esquel (43°S), with interbedded intermediate pyroclastic flows. These strata display wedge-like geometries that were interpreted as the synrift stage of an extensional regime that lasted until the early Miocene. This sequence was assigned by LA-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 signatures that show slab fluid contributions with limited sediment recycling. This magmatic association represents the earliest evidence of arc volcanism under an extensional setting, which can be correlated with the development of extensional basins along the central and southern Andean margin (33°-46°S) during late Eocene-early Miocene times. In a regional context, this arc-related volcanism implies the resumption of plate subduction during late Eocene times, after a protracted gap in arc volcanism coeval to the early Paleogene within plate stage during Aluk/Farallón ridge subduction.