The origin of Patagonia: insights from Permian to Middle Triassic magmatism of the North Patagonian Massif

We conducted a U-Pb-Hf isotope study on zircon crystals from ignimbrites of the Changhsingian to Olenekian (253-248 Ma) Los Menucos Basin in the North Patagonian Massif (NPM), Argentina, in order to evaluate the age and petrogenesis of the magmas. Additionally, a compilation of whole-rock geochemistry and U-Pb-Hf in zircon isotope data for the Permian to Middle Triassic rocks of the NPM, for comparison with our data, was made to assess whether Patagonia would have been an exotic terrane accreted to SW Gondwana during the late Palaeozoic. We interpret the available U-Pb-Hf data to suggest that northern Patagonia experienced eastward arc expansion from the early Permian, about 273 Ma ago. This similar to 820 km arc expansion event involved crustal shortening and magmatism with high-silica adakitic affinity, resulting in Hf-isotopic pull-down. At 253 Ma, slab steepening became associated with the coeval emplacement of ignimbrites of the Los Menucos Basin, which involved post-orogenic to intraplate magmatism. During the Middle Triassic, a slab break-off triggered uplift and basaltic underplating, promoting the emplacement of dike swarms with C-type adakitic signature at 246-244 Ma. The Hf isotope data for SW Gondwana for the same period indicate distinct trends that are explained here by differential slab roll-back since the Guadalupian, in a slab-tearing setting. Therefore, Permian to Middle Triassic magmatism is interpreted as having been associated with an eastward-directed proto-Pacific subduction system, which ultimately supports an autochthonous origin for Patagonia.