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

CASQUET, C.; PANKHURST, R.J; GALINDO, C.; RAPELA, C.W.; FANNING, C.M; BALDO, E.G; DAHLQUIST, J.A.; GONZÁLEZ-CASADO, J.M.; COLOMBO, F.

Bariloche

Simposio; VI South American Symposium on Isotope Geology; 2008

INGEIS-CIG

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 calcites are very unradiogenic (0.70275-0.7035). A Rb-Sr isochron of 582 ± 60 Ma (MSWD = 1.8; Sri = 0.7029) was obtained. Sm-Nd data yield epsilon values (ca. 570 Ma of between  +3.3 and +3.8) . SHRIMP U-Pb syenite zircons 206Pb/238 U 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 is at least 570 Ma old. Combined geochronological , textural and geological evidence however favour an emplacement age of ca. 570 Ma, i.e. Ediacaran. Rifting at ca. 570 Ma leading to opening of the Clymene Ocean is the most probabe scenario for the intrusion of the carbonatite-syenite complex. Mantle source was  depleted.