The final assembly of southwestern Gondwana: a proto-Pacific perspective

RAPELA, C.W.; PANKHURST, R.J.

Montevideo

Simposio; XI Congreso Latinoamericano de Geología, Montevideo, Simposio IGCP 436,; 2001

IGCP 436

The stages and timing of the assembly of south-westem Gondwana have been mostly inferred from studies in orogenic belts along the Atlantic margin of SE Brazil, Uruguay and their SW Africa counterparts. These sectors are dominated by diachronous orogenies of the Briasiliano-Pan African Cycle that are associated with the closure of the Adamastor Ocean from 630 to 520 Ma. New evidence of the characteristics of the supercontinent assembly along the proto Pacific margin comes from recent U-Pb and Sm-Nd isotopic and petrological studies in different basement sectors of Argentina. Early Cambrian ages of 527 ± 6 and 530 ± 3 Ma for granites of the Sierra de la Ventana, are roughly coeval with the granites of the Saldania belt at the basement of the Cape Fold BeIt in South Africa. These events are probably related to the Cabo Frio orogeny that marked the closure of the Adamastor Ocean between the Rio de la Plata-San Francisco and the Kalahari Congo plates. The collisional Pampean orogeny (520-530 Ma) that coevally occurred at the opposite (proto-Pacific) margin of the Río de la Plata craton, confirms that the Early Cambrian was a major accretional period in the supercontinent. Gondwana assembly continued weII into Ordovician and Silurian times due (o oceanic convergence and micro continent accretions. The Famatinian orogeny (490-460? Ma) ended with the collision of [he Precordillera terrane and, perhaps, other associated Grenvillian fragments underlying Central Chile and central-west Argentina. The final tectono-magmatic event is the Achalian orogeny ¡11 the Sierras Pampeanas, which produced westward thrusting, strike-slip shearing and crust-derived granitic magmatism in the range 420-370 Ma. The tectonic setting of this later event is rather obscure, and possible connections with contemporaneous granitic magmatism in the basement massifs of Patagonia will require further detailed studies. The proto-Pacific cycle of orogenies is not restricted to south-western Gondwana, as Early to Middle Palaeozoic diachronous events occurred alt along the proto-Pacific rim in the Ross orogen of Antarctica, the Tasman orogen of eastern Australia and the Tuhua orogen of New Zealand: No further major continental accretion took place in southwestern Gondwana as indicated by Carboniferous Cordilleran type batholiths in the coast range of central Chile. The start of this subduction-related magmatism heralded the period of maximum extent and relative stability of the supercontinent stage, from Carboniferous until the rifting of Africa and the South American plates in Early Jurassic time. Magma sources changed with time during the Palaeozoic as the supercontinent grew westwards by lateral accretion. While the Early Paleozoic events reworked Paleo- to Mesoproterozoic lithosphere, the new Late Palaeozoic Pacific subduction region established on the accreted crust involved reworking of Meso- to Neoproterozoic lithosphere. -Mesozoic-Tertiary Andean subduction is characterized instead by melting of young lithophile-depleted sources as a consequence of the different plate dynamics prevailing after the break up of Gondwana.