Early Evolution of the Proto-Andean Margin of South America

RAPELA, C.W.; PANKHURST, R.J; CASQUET; BALDO, E.G.; SAAVEDRA, J.; GALINDO, C.

Townsville

Congreso; 14th Australian Geological Convention; 1998

Geological Society of Australia

The crustal framework of the Southern Andes was formed during Early Paleozoic continental collisions. From detailed study of a 500 km transect in the Sierras Pampeanas, central-west Argentina, two pre-Silurian tectonomagmatic episodes are recognized, each culminating in separate micro-continental collisions against the proto-Andean margin of southwestern Gondwana (Pankhurst et al., in press; Rapela et al., in press). The earliest recognized event in the Eastern Cordillera and northwestern Sierras Pampeanas is deposition of a turbiditic passive margin sequence with trace fossils indicating a Vendian to Tommotian age (Puncoviscana Formation and metamorphic equivalents, Durand, 1996). The Pampean orogeny started in early Cambrian times with a short subduction phase, indicated by ca. 530 ± 3 Ma calc-alkaline granitoids (conventional U-Pb oil abraded zircons), partially emplaced along the recently developed supercontinent passive margin. After the Pampean terrane collision, the margin was buried to granulite facies conditions (M2 event, 8.6 ± 0.8 kbar, 810 ± 50 °C), closely followed by relaxation to pressures ca. 4 kbar (M3 event) and formation of widespread migmatites and highly peraluminous granites in the Eastern Sierras Pampeanas. According to concordant results from SHRJMP U-Pb data on migmatite monazite, Rb-Sr isochron and conventional U-Pb zircon dating of the anatectic granites and associated cordieritites, these events were essentially synchronous at ca. 525 Ma. After brief quiescence during the late Cambrian, a second major episode, the Famatinian orogeny, started with subduction at ca. 490 Ma, forming a wide continental arc and ensialic back-arc basin to the west of the accreted Pampean terrane. Conventional and SHRIMP U-Pb dating of the main ho + bi granodiorite phase of the batholith yielded a combined age of 490 ± 5 Ma. Younger monzogranites gave Rb-Sr whole-rock ages of 470- 450 Ma, suggesting that intrusive activity lasted until the late Ordovician. Both the main Early Ordovician metaluminous sequence and the monzogranites have high initial 87Sr/86Sr (0.7075 - 0.7 105) and low åNdt (-4.6 to -6.3) inherited from lower crust. Sm-Nd model ages of 1600-1700 Ma indicate that the underlying crust is identical to that beneath the Pampean mobile belt to the east, and therefore strongly suggesting the Famatinian arc was a continental one. The Famatinian subduction heralded the approach of eastern Laurentia to Gondwana, during which the Precordillera terrane separated from the southern Appalachian region, finally colliding with Gondwana in Silurian - Devonian times.