Abstract

The origin of Late Paleozoic limestone and marble interleaved within accretionary complexes in Southern Patagonia is not comprehensively understood. We focus on the marbles of the Eastern Andes Metamorphic Complex near Lago General Carrera zone (ca. 46.5°S), here referred as the Chelenko marble. This unit crop out as tectonic lenses interleaved with metasedimentary rocks and minor metabasaltic rocks (Augustsson and Bahlburg, 2008). The sedimentary features of limestone in the Chelenko marble are obliterated due to intense deformation and recrystallization associated with metamorphism under green schists facies, resulting in the absence of preserved microfossils. A late Cisularian maximum depositional age of precursor limestones is interpreted through U-Pb dating of detrital zircons separated from a meta-sandstone interleaved with layers of deformed marbles. Based on this geochronological constraint, the C-O-Sr isotope data reported by Lacassie and Hervé (2005) on a decameter-scale section composed of horizontal marble layers, presumably representing the original sedimentary bedding, is discussed. The oxygen isotope composition of the marbles is negative (δ18O <-8 ‰) pointing to post-depositional modifications associated with tectono-thermal events, but the δ13C values are positive and comparable with the signature of marine carbonates. In the case of the Sr isotopes, the 87Sr/86Sr compositions of the marbles are higher than the global seawater composition proposed for Permian times, but similar to the Sr isotopic composition of Pennsylvanian seawater (McArthur et al., 2020). A possible explanation is that the metamorphic fluids during the regional metamorphism and deformation of the precursor limestone were derived from the prograde dehydration of the neighboring metapelites, which have very low carbon contents and more radiogenic 87Sr/86Sr signatures (Augustsson and Bahlburg, 2008). This process would have modified the 87Sr/86Sr isotopic composition of the marbles towards more radiogenic values without affecting the bulk the δ13C, which remain like the δ13C composition of the calcareous protolith. An alternative possibility is that the interleaved metasandstones are younger than the marbles, and that the protolith of the latter was deposited during the Pennsylvanian. In such a case the marbles would preserve the 87Sr/86Sr and δ13C compositions of the precursor limestone. The preliminary results suggest that the deposition of limestones that constitutes the protolith of the Chelenko marbles occurred in a marine environment during the Late Paleozoic. Although it is unknown if that occurred in an open ocean or in an epicontinental sea in Gondwana. Further analyzes are needed to test these hypotheses and elucidate their derived tectonic implications. This study was financed through EXPLORACION grant 13220141 (ANID), and FONDECyT grants 2010010, 130227 and 180457 (ANID). Augustsson, C., and Bahlburg, H. 2008. Provenance of late Palaeozoic metasediments of the Patagonian proto-Pacific margin (southernmost Chile and Argentina). International Journal of Earth Sciences, 97, 71-88. Lacassie, J. P., Hervé, F. 2005. Isotopic study of the carbonate rocks of the Patagonian Andes Metamorphic Complexes. In Abstracts of the Gondwana 12: geological and biological heritage of Gondwana: 6-11 November, 2005: Mendoza, Argentine. McArthur, J.M., Howarth, R.J., Shields, G.A., Zhou, Y., 2020. Strontium Isotope Stratigraphy, Geologic Time Scale 2020. BV. https://doi.org/10.1016/B978-0-12-824360-2.00007-3