Abstract

LAJOINIE, María Florencia; LANFRANCHINI, Mabel. E; ETCHEVERRY, Ricardo O  and  RECIO, Clemente. Primeros registros del “evento lomagundijatuli” en mármoles paleoproterozoicos del basamento de las Sierras septentrionales de la Provincia de Buenos Aires, Cratón del Río de la Plata. Rev. Asoc. Geol. Argent. [online]. 2014, vol.71, n.4, pp.585-597. ISSN 0004-4822.

The marble from the San Miguel area represents one of the two marbles identified in the basement of the Sierras Septentrionales. It is placed in a garnet-biotite gneiss which is intruded by igneous bodies of ∼2100 Ma. These rocks are whitish with a coarsegrain size; the calcite + diopside + quartz mineral association indicates a high-amphibolite metamorphic facies grade. Chemical analyses indicate CaO values higher than 50 % and lower than 1 % of MgO. The intrusion of thin interbedded granitic bodies composed of quartz + plagioclase + potassic feldspar generated the formation of a wollastonite-vesuvianitegrossular- diopside skarn. Determinations of δ13C and δ18O in calcite crystals from the marble revealed important positive anomalies with a decrease pattern of + 5.89 to + 4.26 ‰ in δ13C and of + 17.85 to + 13.84 ‰ in δ18O, towards the contact with the granitic bodies. These fluctuations are caused by the crystallization of calc-silicates due to desvolatilization reactions during metamorphic and metasomatic processes. δ18O values obtained from two diopside populations, proximal and distal to the contact marble-granitic bodies, were used together with δ18O contents of calcite (in paragenetic association) in order to determine two geothermometric pairs (calcite-diopside): distal and proximal, which allowed to establish temperatures of 716 °C (metamorphic process) and 451 °C (metasomatic process). The concordance of the geological and isotopic characteristics of the marble of San Miguel with those of the worldwide records of Paleoproterozoic carbonates, together with the estimated age of the protolith indicate that this protolith was a marine carbonate deposited during the “Lomagundi-Jatuli Event”.

Keywords : Skarn; Stable isotope; Metasomatism; Metamorphism; Tandilia.

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