A deformed alkaline igneous rock-carbonatite complex from the Western Sierras Pampeanas, Argentina: Evidence for late Neoproterozoic opening of the Clymene Ocean?

Abstract

A deformed ca. 570 Ma syenite-carbonatite body is reported from a Grenville-age (1.0-1.2 Ga) 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. The two main lithologies are coarse-grained syenite (often nepheline-bearing) and enclave-rich fine-grained foliated biotite-calcite carbonatite. Samples of carbonatite and syenite yield an imprecise whole rock 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 prominent peak at 560-580 Ma defined by homogeneous zircon areas. 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 525 ± 7 Ma (Pampean orogeny) and at ca. 430-440 Ma (Famatinian orogeny) and it is concluded that the Western Sierras Pampeanas basement was joined to Gondwana during both events. Highly 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 εNd570 values of +3.3 to +4.8. The complex was probably formed during early opening of the Clymene Ocean from depleted mantle with a component from Meso/Neo-proterozoic lower continental crust.