Evolution of the patagonian-west antarctica margin of gondwana in the palaeozoic–early mesozoic: new models constrained by zircon u-pb ages, and o and hf isotopic compositions

Abstract

A number of conflicting tectonic models have been proposed to explain the geological relationships between southern South America (Patagonia) and West Antarctica within the palaeo-Pacific margin of Gondwana. Extensive fragmentation and isolation of the various tectonic blocks during Gondwana break-up have complicated interpretations and palaeogeographic reconstructions. In order to explore and test the different tectonic models, I combine zircon U-Pb, Lu-Hf and O isotopic data for samples from key locations throughout the north and south of Patagonia, the Antarctic Peninsula and the Ellsworth Mountains in West Antarctica. Zircon is a robust refractory mineral that occurs in igneous and metamorphic rocks and survives multiple sedimentary cycles with little change to its isotopic composition. It therefore preserves a perfect archive for testing tectonic correlations. Igneous rocks from the Ellsworth Mountains were dated at ca. 680 Ma, older than previously reported. These zircons indicate that rifting, which affected Mesoproterozoic crust, likely occurred in the Cryogenian and supports a connection between the Ellsworth-Whitmore Mountain block and East Antarctica before the amalgamation of Gondwana. This agrees with the break-up of Rodinia in the context of the southwest United States and East Antarctica configuration. U-Pb zircon dating and O-Hf isotopic compositions of detrital zircons from the Ellsworth Mountains also support this connection, indicating a likely East Antarctic provenance. A Cambrian magmatic event is recorded by zircon at ca. 520 Ma, also related to an extensional setting – but in this case with crustal recycling. I interpret this Cambrian magmatism as a result of a tectonic escape after a collision between the Australo-Antarctic and West Gondwana/Indo-Antarctic plates. In Tierra del Fuego, samples from drill cores indicate that Cambrian magmatism occurred between ca. 540 and 520 Ma with strong similarities to the Pampean Orogen of Argentina. Metamorphism occurred at ca. 265 Ma, when zircon crystallised from high temperature hydrous fluids that previously interacted with Grenvillian rocks. Importantly, igneous rocks from Tierra del Fuego record the first evidence of Permian magmatism at ca. 255 Ma, arising from melting of Cambrian rocks. This suggests prolongation of Permian magmatism from the North Patagonian Massif in northern Patagonia and also connections to the Eastern Domain of the Antarctic Peninsula. Granitic rocks in northern Patagonia record mantle-like O magmatic inputs at ca. 280 Ma and 255 Ma, but with reworking of upper crustal materials between these two events. In northwestern Patagonia, early Permian granites indicate continuity of the Permian magmatic belt along the western margin of South America farther north. Further, detrital Permian zircons in late Palaeozoic–early Mesozoic accretionary complexes suggest a continuation of a slightly older Permian subduction-related magmatic arc, partly located in Patagonia and extending to the Antarctic Peninsula. All this data, together with other geological considerations, are in line with an autochthonous or parautochthonous origin for northern Patagonia. It also confirms connections between southern Patagonia and the Antarctic Peninsula from late Palaeozoic to Jurassic times. I suggest that Patagonia rifted from the South African-Ellsworth sector of the paleao-Pacific margin of Gondwana to then collide with the same sector during the Guadalupian.