Two Paleoproterozoic Orogenies in the Evolution of the Tandilia Belt, Buenos Aires, as Evidenced by Zircon U-Pb SHRIMP Geochronology

摘要

The superposition of two orogenies during the evolution of the Paleoproterozoic Tandilia Belt, Argentina, produced complex geological structures in the granitic-gneissic-migmatitic Buenos Aires Complex. Ion microprobe (SHRIMP II) dating of 61 spots in 56 zircon crystals from 10 rock samples shows tectonic activity related to the accretionary Encantadas orogeny of 2.25-2.12 Ga, although a major overprint of the collisional Camboriu orogeny of 2.10-2.08 Ga is also recognized. Only one zircon age is Neoarchean. Two age peaks for the Tandilia Belt are approx 2.16 Ga and approx 2.08 Ga, which occur along most of the Brazilian shield, both in granite-greenstone and in granulite-facies terrains. All zircon crystals show complex internal structures, but magmatic and metamorphic ages are restricted to the Trans-Amazonian Cycle between 2234 Ga and 2065 Ma. The Trans-Amazonian Cycle is a dominantly juvenile event followed shortly thereafter by crustal recycling. The Tandilia Belt exhibits a comparable petrotectonic evolution to the Piedra Alta Terrane of Uruguay and to the basement of the Dam Feliciano Belt in southern Brazil. These results require a re-evaluation and integration of the models proposed for the evolution of the southern Brazilian shield in relation to western Gondwana and other Precambrian supercontinents. Although only one Archean age is registered in the dated zircon crystals, Nd isotopic data on 13 samples show that the Tandilia Belt has Neoarchean crust formation ages, similar to other rocks from the southern Brazilian shield.