ORDOVICIAN ODYSSEY OF CUYANIA, THE GREATER PRECORDILLERA TERRANE OF WESTERN ARGENTINA

Cuyania, the greater Precordillera terrane of western Argentina, attracted great interest at the 7^th International Symmposium on the Ordovician System held in Las Vegas in 1995. It had just been interpreted as having rifted from the Ouachita embayment of Laurentia in the early Cambrian and traveled as a microcontinent across the Iapetus Ocean until it docked in the Middle Ordovician against the proto-Andean margin of Gondwana. Impressive successions of Ordovician strata in the Precordillera were considered to provide evidence of the drifting and accretion of Cuyania. These successions include abundant, widespread K-bentonites discovered by Stig Bergstrom, Warren Huff and colleagues and interpreted as evidence that linked Cuyania to the Famatinian orogenic belt on the proto-Andean margin. This link is used by proponents to support the Laurentian origin of Cuyania, but a Gondwana origin is also consistent with the K-bentonite evidence. Recently, Hf isotope values in Mesoproterozoic detrital zircons from Cambrian and Ordovician sandstones in the Precordillera were interpreted as further evidence for the Laurentian model. However, the same Hf isotope values are here reported for Mesoproterozoic detrital zircons from Ediacaran sandstones deposited on the Río de la Plata craton (RPC). They are evidence of a Mesoproterozoic orogenic belt on the proto-Andean margin of Gondwana that served as the basement for the Cuyania terrane and the detrital zircons within its Cambrian and Ordovician strata. The Hf isotope values are further evidence of a para-autochthonous origin of Cuyania. The terrane is interpreted as a continental fragment rifted from the southern margin of Gondwana. With subsidence, it was inundated and largely carbonate sediment aggraded and prograded with the platform expanding in size during the global Cambrian to Middle Ordovician eustatic rise in sea level. In the Middle Ordovician extensional basins opened on the carbonate platform with the development of a transcurrent fault system parallel to the proto-Andean margin. That change in plate motion terminated subduction at the Famatinian trench-arc, and Cuyania migrated along the fault system until it reached its present position outboard of the Famatinian Orogen in the Devonian or Carboniferous.