Early Tertiary volcanic rocks chronology from central Chubut, Patagonia, Argentina

HALLER MJ; ALRIC VI; FERAUD G; BERTRAND H

Friburgo

Congreso; 18. Geowissenschaftliches Lateinamerika-Kolloquium; 2003

Combining geological information with new radiometric and geochemical data allow to compare two basalt generation geodynamic models for Central Patagonia at Early Tertiary times. Fifteen 40Ar/39Ar ages obtained for Central Patagonia show that the alkaline basaltic volcanism in this region started at Paleocene times. After a gap in the volcanism, the basaltic activity reinitiated during the Eocene with the emplacement of gabbroic dikes, basaltic and basanitic dikes and lava flows. All the Paleogene rocks show an intraplate OIB magmatism signature. Paleocene volcanics are ne normative basaltic lava flows. The Eocene rocks are basanites with ultrabasic inclusions. An intrusive body of nephelinic syenite composition was also recognized. Tectonic reconstruction for the Paleocene of Patagonia suggests a similar model to those which presently exists at latitude 42° S, with relatively high convergence velocities and slight obliquity, where extensional efforts prevail. This geodynamic frame may have caused attenuation of the crust with astenospheric ascent, originating the basalt effusions during the Paleocene. On the other hand, the tectonic configuration for the Lower Tertiary of South America shows the passage of a triple point from the North of Chile to Tierra del Fuego, between 70 and 40 Ma, having reached the latitude of North Patagonia during the Eocene. In this way, the related astenospheric window could be related with the Eocene basaltic eruptions.