ACCRETION OF THE ARGENTINE PRECORDILLERA TO WESTERN GONDWANA

Alternatives for time of accretion of the Argentine Precordillera to western Gondwana variously incorporate Middle-Late Ordovician subduction, Late Ordovician extension/rifting, and Silurian-Devonian accretion.

Upward transition from platform carbonates to black shale, beginning in Arenig and progressing diachronously southwestward, indicates foreland flexural subsidence of the Precordillera in response to tectonic loading during initial subduction. Middle Ordovician synorogenic turbidite conglomerates contain clasts of igneous rocks, suggesting a volcanic and/or basement provenance; lack of dating of clasts leaves uncertainty. Early-Middle Ordovician volcanic-ash (bentonite) beds, 490-455-Ma granitoids, Gondwanan fauna in the Precordillera by late Middle Ordovician, and metamorphic cooling ages as old as 464 Ma are all compatible with Ordovician subduction of an eastern Iapetus(oceanic)/Precordillera(continental) plate and collision with a Gondwanan continental-margin arc (Famatina).

The stratigraphic range and sizes of boulders and olistoliths of intrabasinal platform carbonate rocks and black shale in Ordovician synorogenic deposits require high relief to expose the stratigraphic succession and to gravitationally drive block falls/slides. A high-relief horst-and-graben system suggests extension following Middle-Late Ordovician collision. Alternatives in compressional settings include block falls from westward-directed carbonate thrust sheets or from intraplatform horsts framed by reactivation/inversion of basement faults. Late Ordovician pillow basalts suggest rifting west of the Precordillera platform.

A clastic wedge in the central Precordillera, a melange/olistostrome in the eastern Precordillera, and metamorphic ages indicate Silurian-Devonian collision. Lack of Ordovician subduction of Precordillera crust inferred from age and geochemistry of the Famatina arc, combined with relating Ordovician extension to rifting from Laurentia, is consistent with Silurian-Devonian subduction/accretion of the Precordillera. Alternatively, subduction of Precordillera crust may have stopped after initial collision in Ordovician, and Silurian-Devonian events mark accretion of the Chilenia terrane west of Precordillera.