Sr, C and O isotope composition of marbles from the Sierra de de Ancasti, Eastern Sierras Pampeanas, Argentina: age and constraints for the Neoproterozoic–Lower Paleozoic evolution of the proto-Gondwana margin

MURRA, J; BALDO, E.; GALINDO, C.; CASQUET, C.; PANKHURST, R.J; RAPELA, C.W.; DAHLQUIST, J.A.

GEOLOGICA ACTA

UNIV BARCELONA

Lugar: Barcelona; Año: 2011 vol. 9 p. 1 - 23

1695-6133

The Sierra Brava Complex on the eastern flank of the Sierra de Ancasti consists of marbles, metabasites, calc-silicaterocks, psammo-pelitic schists and gneisses. In the central part of this sierra a thick succession of banded schists (AncastiFormation) crops out. Regional metamorphism of these rocks is attributed to the Famatinian orogeny (Ordovician),metamorphic grade increasing westwards and southwards and culminating in a migmatite complex on the westernside of the Sierra. The meta-carbonate rocks are subdivided into a northeastern group (low-grade calcite marbles), anda southeastern group (high-grade calcite and calcite-dolomite marbles). Twenty-three marble samples were analysedfor Sr isotope composition and Rb, Mn, Mg and Ca contents, and six for C and O isotope composition. An Ediacarandepositional age of 570 –590Ma is inferred by reference to the trend of 87Sr/86Sr in Neoproterozoic seawater. Thus themetacarbonates are probably slightly older than the Ancasti Formation (equivalent to the Puncoviscana Formation ofnorthern Argentina), which has a maximum sedimentation age of ca. 570Ma. Ediacaran depositional ages have alsobeen reported for metacarbonates elsewhere in Argentina, Uruguay and Brazil. We propose that the Sierra de Ancasticarbonates on one hand, and those in the Western Sierras Pampeanas (Difunta Correa Sequence) and -tentativelytheCorumbá Group of Brazil on the other, represent platforms on opposite margins of the extinct Clymene Ocean,whereas Neoproterozoic carbonate successions such as the Loma Negra Formation (Tandilia, southern Argentina)and the Arroyo del Soldado Group (Uruguay) were deposited on the eastern side (present coordinates) of the Rio dela Plata craton, which at the time occupied a position farther to the north.