Deformed carbonatite-syenite complex in the Western Sierras Pampeanas of Argentina: U-Pb SHRIMP zircon age and isotope (Nd,Sr) constraints

CASQUET, CÉSAR; PANKHURST, ROBERT; GALINDO, CARMEN; RAPELA, CARLOS W.; FANNING, C. MARK; BALDO, EDGARDO; DAHLQUIST, JUAN A.; GONZÁLEZ CASADO, JUAN M.; COLOMBO, FERNANDO

San Carlos de Bariloche

Congreso; VI South American Symposium on Isotope Geology.; 2008

We describe here the case of a deformed syenite-carbonatite dyke from a Grenville-age terrane in the Sierra de Maz, one of the Western Sierras Pampeanas of Argentina. (Ne)-albite syenite and enclave-rich foliated biotite carbonatite are the two main lithologies. 87Sr/86Sr values in calcite are very unradiogenic (0,70275-0,70305). A Rb-Sr isochron age of 582 Ma ± 60 Ma (MSWD = 1,8; Sri = 0,7029) was obtained. Sm-Nd data yield epsilon values (ca. 570 Ma) of between +3,3 and +4,8. SHRIMP U-Pb syenite zircons 206Pb-238U ages are between 433 and 612 Ma with four possible growth-crystallization events. The most definitive statement that can be made concerning the age of this carbonatite complex is that it is at least 570 Ma old. Combined geochronological, textural and geologic evidence however favour an emplacement age of ca. 570 Ma, i.e., Ediacaran. Rifting at ca. 570 Ma leading to the opening of the Clymene Ocean is the most probable scenario for the intrusion of the carbonatite-syenite complex. Magma source was a depleted mantle.