Early Andean tectonomagmatic stages in North Patagonia: Insights from field and geochemical data

ECHAURREN, A; OLIVEROS VERONICA; ANDRES FOLGUERA; IBARRA F; CREIXELL C; LUCASSEN F

GEOLOGICAL SOCIETY OF LONDON SPECIAL PUBLICATION

geological society of london

Año: 2016

0305-8719

TheAndean cordillera in northern Patagonia is mainly formed by Mesozoic magmaticunits; the mostly Jurassic-Cretaceous North Patagonian Batholith and volcanismfrom the Jurassic Lago La Plata (Ibáñez) Fm. and the mid Cretaceous DivisaderoGroup. These rocks represent the development of a trench-parallel magmatic beltthrough Jurassic-mid Cretaceous time, during a tectonic switching fromextension to compression. In order to study the arc evolution during thistransition, we carried field work and geochemical sampling in the easternAndean slope at ~ 43º S, clearing structural relationships and characterizingmagmatic sources between Jurassic and Cretaceous units.Multi-element diagrams for both volcanic units evidence a slab-derivedsignature, while isotopic ratios (Sr, Nd and Pb) indicate parental melts fromthe subduction-modified asthenospheric mantle that interacted with crustalsources during their emplacement. An angular unconformity is identified betweenthe synextensional Jurassic volcanic rocks and Neocomian sedimentary rocksbeneath the mid Cretaceous sequences. Even though this deformational event wassimultaneous with generalized over-riding plate compression elsewhere, geochemicalratios evidence an  immature Aptian-Albianarc with no crustal thickening associated. Late Jurassic to Early Cretaceousarc installment after a retraction of magmatism from the foreland toward thetrench between ~41º-45º S would have provoked crustal softening triggering the initialfold-thrust belt, evidenced by the angular unconformity between Jurassic andCretaceous strata, that rapidly expanded to the foreland.