Publication: Sr, C and O isotope composition of marbles from the Sierra
de Ancasti, Eastern Sierras Pampeanas, Argentina: age and
constraints for the Neoproterozoic–Lower Paleozoic evolution of
the proto-Gondwana margin
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Publication Date
2011
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Instituto de Ciencias de la Tierra "Jaume Almera" (Barcelona)
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Abstract
The Sierra Brava Complex on the eastern flank of the Sierra de Ancasti consists of marbles, metabasites, calc-silicate
rocks, psammo-pelitic schists and gneisses. In the central part of this sierra a thick succession of banded schists (Ancasti
Formation) 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 western
side of the Sierra. The meta-carbonate rocks are subdivided into a northeastern group (low-grade calcite marbles), and
a southeastern group (high-grade calcite and calcite-dolomite marbles). Twenty-three marble samples were analysed
for Sr isotope composition and Rb, Mn, Mg and Ca contents, and six for C and O isotope composition. An Ediacaran
depositional age of 570 –590Ma is inferred by reference to the trend of 87Sr/86Sr in Neoproterozoic seawater. Thus the
metacarbonates are probably slightly older than the Ancasti Formation (equivalent to the Puncoviscana Formation of
northern Argentina), which has a maximum sedimentation age of ca. 570Ma. Ediacaran depositional ages have also
been reported for metacarbonates elsewhere in Argentina, Uruguay and Brazil. We propose that the Sierra de Ancasti
carbonates on one hand, and those in the Western Sierras Pampeanas (Difunta Correa Sequence) and -tentativelythe
Corumbá 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 Río de
la Plata craton, which at the time occupied a position farther to the north.