Early Andean tectonomagmatic stages in north Patagonia: insights from field and geochemical data

The Andes in northern Patagonia are mainly formed by Mesozoic magmatic units: the mostly Jurassic-Cretaceous North Patagonian Batholith and volcanism of the Jurassic Lago La Plata (Ibanez) Formation as well as the mid-Cretaceous Divisadero Group. These rocks represent the development of a magmatic belt through Jurassic-mid-Cretaceous time, during a switch of the tectonic regime from extension to compression. To study arc evolution during this transition, we carried out fieldwork and geochemical sampling at c. 43 degrees S, clarifying structural relationships and characterizing the magmatic sources. Multi-element diagrams for both volcanic units suggest a slab-derived signature, whereas isotopic ratios (Sr-Nd-Pb) indicate parental melts sourced from the subduction-modified asthenospheric mantle interacting with crustal sources during their emplacement. An angular unconformity is identified between the synextensional Jurassic volcanic rocks and Lower Cretaceous sedimentary rocks beneath the mid-Cretaceous sequences. Although this deformational event was simultaneous with generalized overriding plate compression, geochemical ratios indicate an immature Aptian-Albian arc with no associated crustal thickening. Late Jurassic to mid-Cretaceous arc settlement after a trenchward retraction of magmatism from the foreland between c. 41 and 45 degrees S, with an associated increase in slab dip angle, may have provoked crustal softening facilitating the subsequent initial fold-thrust belt growth.