Patagonia revisited: new detrital zircon and offshore geological evidences.

RAMOS, V.A.; NAIPAUER, M.

Viña del Mar

Congreso; International Geological Congress on the Southern Hemisphere; 2013

Universidad Andrés Bello

The recent finding of archeocyathids in molassic Middle Cambrian to Early Ordovician age-sequences of northern Patagonia makes necessary to reexamine the relationships between this southern part of South America and East Antarctica. The Early Cambrian age of the archeocyathids, and their derivation from the Shackleton Limestones, open several alternatives that are evaluated based on the lithology and the U-Pb zircon ages of the different metamorphic sequences of Patagonia and the Transantarctic Mountains. The U-Pb and Lu-Hf isotopic analyses of the different sedimentary sequences exposed in the Ventania System, an old Paleozoic orogenic belt exposed in the southern region of the Río de la Plata Craton in the province of Buenos Aires, show new evidence for the understanding of the tectonic evolution of the western sector of the Gondwanides mountain belt (Fig. 1). These ranges have been formed as result of the late Paleozoic collision of the Patagonia terrane against the continental margin of Gondwana. The provenance analysis together with the sedimentary paleocurrents confirm a dominant source from the Tandilia System, a Paleoproterozoic mountain belt formed during the amalgamation of the Rio de la Plata Craton at about 1,800-2,200 Ma, and incorporated to Western Gondwana during the Brasiliano Orogeny at 550-530 Ma. The local dominant source at the base of the early Paleozoic changed to more distant supplies toward the top of the sequences, when is recorded an increasing participation of detritus from first, Cambrian (560-520 Ma) zircons from the Pampean Orogen, and later on Ordovician (480-460 Ma) zircons from the Famatinian Orogen. The detrital zircon patterns and the maximum age of the units shed light to some previous discrepancies in the early Paleozoic stratigraphy. The new data indicate a southern provenance with the first evidence of Carboniferous and Permian magmatic zircons. The oldest Archean zircons together with the finding of clasts with archeocyathids in the Sauce Grande Tillites confirm the provenance from Patagonia, which was derived from Eastern Gondwana. The U-Pb ages of the ash-fall tuffs in the Tunas Formation confirm the Early Permian age of the Eurydesma Fauna in the Ventania System. The U-Pb data together with the Lu-Hf isotopic data enhance the comprehension of the tectonic evolution of the Ventania System as part of the larger Gondwanides Belt that amalgamated during Late Permian times to Western Gondwana with some independent pieces derived from Eastern Gondwana. Based on these data, it is proposed that the Somuncurá Massif of northern Patagonia is the conjugate margin of the Pensacola Mountains in East Antarctica. The main episodes of deformation within the Cambrian- Ordovician Ross Orogeny are correlated, as well as the passive margin setting during the Silurian-Devonian, which indicate that the lower section of the Beacon Supergroup of Antarctica corresponds to the Sierra Grande Formation. These facts show that the Patagonian terrane may have been situated as the conjugate margin of the Transantarctic Mountains from Southern Victoria Land to the Pensacola Mountains. The rifting of Patagonia from Antarctica and the beginning of subduction along western Patagonia, are correlated among different terranes, showing a robust coherent evolution through early Paleozoic times among these blocks. The new information on the offshore geology of the adjacent continental platform at the latitudes of the province of Buenos Aires, show the inner part of the orogen where lower and middle crustal rocks are overriding the sedimentary cover. This new finding explains the apparent lack of metamorphic rocks associated with the collision. The suture between the Cortijo and Tandilia terranes in figure 1 is indicated based on geophysical data recently published. The final amalgamation of Patagonia with Western Gondwana occurred in late Paleozoic times.