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 (Ibáñez) 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°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°S, with an associated increase in slab dip angle, may have provoked crustal softening facilitating the subsequent initial fold–thrust belt growth.
Supplementary material: Petrographic descriptions and geochemical–isotopic data are available at https://doi.org/10.6084/m9.figshare.c.3677974