Early Palaeozoic evolution of the Gondwana margin of South America

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

Cape Town

Congreso; Gondwana 10: Event Stratigraphy of Gondwana; 1998

University of Johannesburg

Two main pre-Silurian tectonomagmatic episodes (Pampean and Famatinian cycles or orogenies) wore involved in the construction of the western margin of Gondwana (Fig. 1). From a multi-disciplinary study of a 500 km transect in the Sierras Pampeanas, central-west Argentina, these are identified and re-defined more precisely than in previous literature and interpreted in a more tightly constrained context. Each of these cycles culminated in a microcontinental collision against the proto-Andean margin of Gondwana, but the first entirely pre-dated the rifting of the Argentine Precordillera Terrane from Laurentia (Astini et al, 1995; Keller el al., in press), and the second, we believe, pre-dated the final accretion of that terrane the earliest recognized event is deposition of an Early Cambrian passive margin sequence (Puncoviscana Formation and equivalents over an unexposed Mesoproterozoic cratonic basement (Nd model ages of 1 500 ¡À 200 Ma, inherited zircons 800-1400 Ma; Pankhurst et al., in press). The Pampean Orogeny (Rapela et al., in press), previously thought to be Neoproterozoic in age, started in Early Cambrian times with a short subduction phase, indicated by 530¡À3 Ma calc-alkaline granitoids in the Sierras de C¨®rdoba (dated by conventional U-Pb on abraded zircons). During the Pampean Terrane collision, the mar gin was buried to granulite4acies conditions (M2: 8.6 ¡À0.8 kbar, 810 ¡À 50¡ãC, determined using the TWEEQU thermo-barometric programme of Berman, 1991), generating widespread migmatites. Relaxation of P to ca 4 kbar following the climax of metamorphism led to anatexis and formation of highly peraluminous granites in the Eastern Sierras Pampeanas. These events were essentially synchronous at ca 525 Ma according (o concordant results from SHRIMP U-Pb data on orthogneiss monazite, Rb-Sr whole rock on the mw-grade passive margin rocks, and Rb-Sr isochron and conventional U-Pb zircon dating on the anatectic granites. After a brief period of quiescence, subduction of the southern lapetus Ocean floor beneath Gondwana recommenced. The second major episode, the Famatinian cycle is predominatly represented by intense subduction- relted magmatism of early-Middle Ordovician age (Pankhurst et al., in press). A wid volcanic and plutonic arc and ensialic back-arc basin developed to the west of the Pampean accreted terrane. The main granodiorite phase of the batholith of the southern Famatinian arc is associated with an S2 fabric and shear zone formation, and was emplaced late in the deformational history of the passive margin metasediment. Conventional SHRIMP U-Pb zircon dating yielded a combined age of 490 ¡À 5 Ma. Younger monzogranites gave Rb-Sr whole rock age of 470-450 Ma., typical of granites in the Sierra de Famatina, but geochemical continuity with the main granodiorite suite raises the possibility than these are partially reset age. All the calc-alkaline rocks of the southern Famatinian arc have high initial 87Sr/86Sr (0.7075 ¨C 0.7105) and low ¦ÅNdt (-4.6 to -6.3), inherited from lower crust. Sm-Nd model ages of 1600-1700 Ma., indicated than the underlining crust is identical to that beneath the foreland to the east. This part of Famatinian arc at least was thus a continental one, developed on the margin of Gondwana during the rifting and drifting of Precordillera Terrane. The Precordillera Terrane itself consists of a lower Paleozoic Carbonate platform, formed over subsided Grenvillian crust while still part of Laurentia (Keller et al., in press). It finally collided with Gondwana in Silurian-Devonian times.