The southern Gondwana margin in Patagonia during lower Permian

FE NULLO; MJ HALLER

Mendoza

Congreso; Backbone of the Americas. Patagonia to Alaska; 2006

New radiometric dating and geophysical data allowed us to determine the activity and ongoing development of the Gondwana margin during the upper Carboniferous – lower Permian period and the correlation of the geological processes that occurred in the southern region of the Neuquén Province and the northern region of the Chubut Province. The oldest country rocks have a low to middle metamorphism, with ages of 368 ± 9 Ma in the North, and ages ranging between 372 ± 9 and 405 k8 Ma in the South, thus indicating a Devonian age event. Granite bodies produce metamorphism which extends from the northern region of San Martín de los Andes to Paso de Indios, and show magmatic arc activity with ages ranging between 299 ± 7 Ma to the North and 261 ± 6 Ma to the South. Intrusive bodies in the northern area are predominantly granite bodies featuring subordinated dioritic varieties. According to their geochemical composition they are mildly peraluminous, suggesting cortical contribution to the magma. Magmatic bodies in the southern region are primarily characterized by biotite granodiorites to tonalites, with subordinated granites. The described calcalkaline magmatism represents a Permian volcanic arc. The Gondwana magmatic arc has a NW-SE direction. It developed on older rocks located in the Northeast. The final stage of the magmatic arc activity featured an extensional process that slowed down the subduction stage and made room for surface magmas with epizonal granites. This process occurred during late upper Permian times. During the early Mesozoic and as a consequence of an intensification of the transpressional-transtensional faulting process, the fractured and moved Gondwana margin, adapted to the stress fields occurring in that sector of the active margin, although it maintain its belt-shaped continuity. The Neuquén basin began to develop to the northeast of this structure as the result of the strong extensional process that occurred from the upper Permian to the lower Triassic period. By that time, the volcanic arc moved towards west, while the extensional magmatism process that had started during the Upper Permian to Triassic in the continental crust continued until the Jurassic period.