First geochronological constraint for the Palaeoproterozoic Lomagundi-Jatuli δ13C anomaly in the Tandilia Belt basement (Argentina), Rio de la Plata Craton

Geochronological data of Palaeoproterozoic carbonates with delta C-13 values between + 5 parts per thousand and +16 parts per thousand suggest that the positive carbon isotope excursion of the global Lomagundi-Jatuli event (LJE) occurred mostly between 2220 and 2060 Ma. In the southern portion of the Rio de la Plata Craton, the LJE was recorded in calcite-diopside marble of the Tandilia Belt basement. The marble was found at the San Miguel area as part of a metamorphic rock assemblage, mainly composed of gneisses and migmatites, formed during the Transamazonian Orogeny. The latter represents the maximum magmatic and metamorphic activity in the Tandilia Belt basement. The marble, derived from a calcareous-siliciclastic sedimentary marine sequence, was intruded by leucogranites that provoked the San Miguel skarn formation. First U-Pb LA-ICP-MS age of 2206 +/- 14 Ma, obtained from titanites of the skarn, also placed the metasomatic processes into the Transamazonian Orogeny. In addition, this age corresponds to the minimum age for the marble's protolith deposition which matches with the beginning of the LJE worldwide. These new data would favor the correlation between the San Miguel marble and the Montevideo Formation marble of the Tandilia Terrane (Uruguay), within the Rio de la Plata Craton, and not with the Paso Severino Formation of the Piedra Alta Terrane, which is at least 60 Ma younger. New delta C-13 determinations in calcite crystals from the marble allowed the lowest and the highest carbon values (+3.19 parts per thousand and +6.90 parts per thousand, respectively) to be obtained. The lowest values are within the range of normal marine carbonates (between - 5 parts per thousand and +5 parts per thousand), despite having been altered by contact metamorphism, while the highest values are typical of the LJE carbonates. In this sense, the early deposition of the carbonate protolith within the LJE, together with the effect of post-depositional metamorphic processes would justify the wide range of values shown by the isotopic ratios of C.