The A-type event in the eastern Sierras Pampeanas, La Rioja and Catamarca provinces (Argentina): geochronological and isotopic considerations

DAHLQUIST, J., ALASINO, P., GALINDO, C., CASQUET, C., GRANT, C. AND MEDINA, C.

Santiago, CHILE

Congreso; International Congress on the Geology and Geophysics of the Southern Hemisphere; 2007

Initial 87Sr/86Sr ratios of ca. 0.7088 (for a U-Pb SHRIMP zircon crystallization age of 340 ± 3 Ma) were  reported by Dahlquist et al. (2006) for the San Blas A-type granitic pluton (Sierra de Velasco; La Rioja). Remarkably, this isotopic ratio is indistinguishable from the initial 87Sr/86Sr ratio (0.7086) of the Capilla del Monte granite in the Sierra de Córdoba, which is also Early Carboniferous (ca. 337 Ma) and A-type too. This coincidence points toward an isotopically similar magma source with a significant continental crust component for the A-type event of Eastern Sierras Pampenas (ATEESP). The term ATEESP is used here to embrace A-type  magmatism and related tectonics of Early Carboniferous age in the Eastern Sierras Pampeanas (Dahlquist et al., this volume). Consistently, multielement diagrams for different ATEESP plutons such as the Zapata and Huaco granitic complexes (Sierras de Zapata in Catamarca, and Sierra de Velasco in La Rioja, respectively) also attest to a significant continental crust component (unpublished data). The main zircon crystallization phase in the Huaco granitic complex was dated by Söllner et al. (2007) at 354 ± 3.9 Ma (LA-ICP-MS,  U-Pb  zircon dating). This age is close to that of the San Blas pluton. New whole rock Rb-Sr isotope data  for  the Zapata and Huaco granitic complexes however, yielded  errochrons (284.7 ± 9.9 Ma; 87Sr/86Srinitial = 0.7217, MSWD = 11.6,  and 308 ± 17 Ma; 87Sr/86Srinitial = 0.7166; MSWD = 3.9, respectively), which suggests that some post-crystallization alteration took probably place sometime between the Late Carboniferous and the Early Permian, that incompletely reset  the Rb/Sr system. Söllner et al. (2007) also recognized in the Huaco granite an alteration event at 315 ± 42 Ma, which is within error of the errochrone age shown above. In conclusion, U-Pb zircon ages along with Rb-Sr isotope data, reveal an history of Early Carboniferous  A-type magmatism in the Eastern Sierras Pampeanas (ca. 340-350 Ma) that was overprinted by hydrothermal alteration (probably much time later), suggesting that the crust remained hot during a long time. The hydrothermal event although of uncertain age could be correlated with foreland fracturing and accompanying pervasive hot water flow during collision of the Deseado terrane (Patagonia) to SW Gondwana in the Middle Carboniferous (Pankhurst et al. 2006).