Upper Triassic to Middle Jurassic magmatic evolution of northern Patagonia: Insights from the tectonic and crustal evolution of the Los Menucos area, North Patagonian Massif, Argentina

Abstract

The Upper Triassic magmatic and tectonic evolution of the central North Patagonian Massif is evaluated on the basis of the zircon U–Pb-Hf record and whole-rock geochemistry for volcanic rocks of the Los Menucos area. The sample analyzed in this study yields a 210 ± 3 Ma U–Pb zircon age, and the geochemical data for the Upper Triassic rocks are indicative of a high-K calk-alkaline arc-type magmatism, which also has adakitic signature related to melting of the subducted oceanic slab. The Hf isotope data for zircon from this sample, with ƐHf(T) values between – 6.1 and – 8.7, support a mixed source that may have involved melted slab and crustal reworking. A comprehensive evaluation of the Upper Triassic – Lower Jurassic magmatic and tectonic evolution of northern Patagonia is in agreement with the hypothesis of an Upper Triassic South Gondwanian flat-slab, which shifted the magmatic arc towards the continental interior at a time of crustal shortening and with arctype adakitic magmatism. The Lower Jurassic magmatic record is consistent with slab detachment and later roll-back, which would have promoted crustal extension and intraplate magmatism, and also shifted the arc magmatism to the paleo-trench. As a consequence of this stage, the Subcordilleran and North Patagonian batholiths were emplaced. The progressively – with time - less negative Hf isotope signatures for magmatic zircon are consistent with less evolved magmas generated throughout the Jurassic. The Upper Triassic-Middle Jurassic magmatism in northern Patagonia was part of a complex tectonic evolution, which involved crustal shortening and extension during plate reorganization during the critical period of Gondwana supercontinent break-up.