Middle to late Cenozoic arc magmatism along the Patagonian Andean margin

ALFONSO ENCINAS; LUCIA FERNÁNDEZ PAZ; VICTOR VALENCIA; ANDRES FOLGUERA; SOFÍA IANNELLI; VICTOR RAMOS; VANESA LITVAK; FLORENCIA BECHIS; ANDRES ECHAURREN

Simposio; Primer Simposio de Tectónica Sudamericana; 2016

Variable composition and development of Paleogene to Neogene arc-related magmatism in North Patagonian Andes seem to have been controlled by contrasting contractional and extensional stages associated with changes in certain parameters of the subduction system (e.g. subduction angle, roll back and upper plate velocities). Our study focuses on the Eocene to Lower Miocene volcanism that evolved in intra- and retroarc positions along the transitional zone from the Southern Central Andes to the North Patagonian Andes. Bimodal middle Eocene (45-35 Ma) magmatism registered in southern Neuquén province (39-40°S) shows low influence of slab-derived products and a tholeiitic signature, although with slightly alkaline tendency. Relatively coetaneous volcanism (~40 Ma) to the south in northern Chubut province (43°SL) comprises basaltic lavas with tholeiitic signature that have low influence from arc-components and are associated with syn-extensional geometries. By early Oligocene times (33-29 Ma) arc magmatism evolved to a more calc-alkaline type as registered in the andesitic lavas from Auca Pan depocenter (~40°L) and the initial stage of the El Maitén volcanic belt (~41°S). A pronounced change in the evolution of arc magmatism is associated with plate tectonic reorganization after the break-up of Farallones plate that resulted in a widespread arc-related volcanism developed in an extensional regime from the forearc throughout the retroarc zone. In this context, Early Miocene (23-20 Ma) retroarc magmatism developed in the Nirihuau basin (~41°S) comprises subalkaline basaltic to andesitic lavas, with arc-like signatures and an NMORB source with no crustal contamination. Morover, these Early Miocene arc-related products are interbedded with marine deposits, which is coherent with the widespread extension achieved during this period.