A DEFORMED ALKALINE IGNEOUS ROCK – CARBONATITE COMPLEX FROM THE WESTERN SIERRAS PAMPEANAS, ARGENTINA: EVIDENCE FOR LATE NEOPROTEROZOIC OPENING OF THE CLYMENE OCEAN?

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

PRECAMBRIAN RESEARCH

Elsevier Ltd.

Lugar: Oxford. UK; Año: 2008 vol. 165 p. 205 - 220

0301-9268

A deformed syenite–carbonatite dyke is reported from a Grenville-age terrane in the Sierra de Maz, one of the Western Sierras Pampeanas of Argentina. This is the first recognition of such a rock assemblage in the basement of the Central Andes, and could prove to be of economic importance. The two main lithologies are coarse-grained (Ne)-syenite and enclave-rich fine-grained foliated biotite-calcite carbonatite. Samples of carbonatite and syenite yield an Rb–Sr isochron age of 582 ± 60 Ma (MSWD = 1.8; Sri = 0.7029); SHRIMP U–Pb spot analysis of syenite zircons shows a total range of 206Pb–238U ages between 433 and 612 Ma, with a peak at 560–580 Ma. Textural interpretation of the zircon data, combined with the constraint of the Rb-Sr data suggest that the carbonatite complex formed at ca. 570 Ma. Further disturbance of the U-Pb system took place at 520±4 Ma (Pampean orogeny) and 454±4Ma (Famatinian orogeny). Very unradiogenic 87Sr/86Sr values in calcites (0.70275 – 0.70305) provide a close estimate for the initial Sr isotope composition of the carbonatite magma. Sm–Nd data yield eNdt570 of +3.3 to +4.8. The complex was probably formed during early opening of the Clymene Ocean.