A REVIEW OF CHAITENIA: A DEVONIAN OCEANIC ISLAND ARC ACCRETED TO PATAGONIA SOUTH OF CHILENIA, SOUTH AMERICA

The Paleozoic growth of the West Gondwana margin is considered to have included successive terrane accretion events. In South America, a microcontinent called Chilenia, underlain by late Proterozoic basement rocks, is interpreted to have collided with Gondwana in the Devonian. The Huincul lineament of the Neuquén basin in Argentina has been proposed as the southern limit of Chilenia, which could extend further since Devonian collisional metamorphic rocks occur south of Bariloche. Following the Devonian collision, a mainly Carboniferous and Permian subduction complex formed in the trailing edge of the Chilenia terrane in Central Chile. SHRIMP U-Pb ages of detrital zircons have revealed that north of Chilenia, the Devonian was a period of magmatic quiescence. There are very few detrital zircons of that age in the sedimentary successions of the accretionary prism of Central Chile. South of the Neuquén basin, however, Devonian zircons are abundant in the accretionary complex, and there are some outcrops of Devonian plutonic rocks. The latter form two magmatic belts: an eastern one built on the continental crust of the North Patagonian Massif, and a western one formed in an oceanic environment as indicated by geochemical data. The western belt is interpreted to have constituted an oceanic island arc that has been called Chaitenia. Thus, in southern Chile, the microcontinent of Chilenia is replaced by an active Devonian oceanic arc which was also accreted to the continental margin of Gondwana. This scenario reflects a transition on the southwestern Gondwana margin from Mariana-type subduction to Andean-type subduction, commencing in the Carboniferous, with accretionary complexes built mainly of oceanic material replacing elongated margin-parallel continental terranes.