U-Pb age and C isotopes in Neoproterozoic rocks from the Tucavaca Belt, Murciélago Group, Bolivia: records of a glacial event followed by a carbonate platform

Sedimentary records of Neoproterozoic glaciations are observed on several continents, and in South America they were recorded in Brazil and Argentina. In Bolivia, Neoproterozoic rocks are found in the Tucavaca Belt, the southern portion of the country, southeast of the city of Santa Cruz de La Sierra, is here interpreted as glacial origin due U-Pb age (Gaskiers event) and C and O isotopes signatures.

The Tucavaca belt is limited to the southwest by Quartenary sedimentary rocks along the railway between Santa Cruz (Bolivia) and Corumbá (Brazil). Its outcrops are divided into two groups: (a) the first is observed near the Bolivia - Brazil border in continuity with the Paraguay Belt, on the Brazilian side. These outcrops comprise a thick package of rocks from the Boquí Group overlain by carbonates (Jacadigo and Corumbá Groups, respectively, of the Paraguay belt). A second group of outcrops corresponds to the Tucavaca belt and extends in an elongated manner for more than 500 km. These rocks are aligned in a SW-NE direction, unlike the rocks in the Paraguay belt, which are in a N-S direction.

  The rocks of the Tucavaca Group are about 2500m thick and overlap the Cahama Formation and comprise a sequence of conglomerates, finely laminated pink to gray limestones, coarse sandstones with granocrescence, sometimes micaceous and with rare dripping pebbles. Finally, shales, mudstones and siltstones are observed with sometimes thick intercalations of clastic material. In the Porto Soarez region, sandstones and limestones were identified close to the town of Murciélago, which motivated the definition of the homonymous group. The sediments of the Murciélago group overlie the Boquí group, which are more than 500m thick and are essentially horizontal. These rocks were later correlated with the Pororó Formation of the Tucavaca Group.

After performing the CL images, the samples were taken to the LA-ICP-MS equipment at the Multilab UERJ laboratory, where were laser ablasion analyzes were carried out with a spot size of 150µm with analysis of carbonate samples interspersed with reference materials (NIST 612 and WC-1). The concordia diagram yiended an age of 580 ± 30 Ma interpreted as the carbonate sedimentation age. This age is consistent with a sample of rhyolite with a K-Ar age of 572 Ma, suggesting volcanic activity probably correlated with the initial period of the rift responsible for the opening of the deposition basin of the Boquí and Tucavaca groups.

Seventeen samples were pulverized and analyzed for C and O isotopes and present α 13CPDB values between -0.92‰ to -1.78‰ indicating intermediate signatures between sediments of biological origin and carbonates altered by hydrothermal solutions of metamorphic origin. The α18OPDB values range from -4.09‰ to 6.32 and, if primary, indicate a source of biological origin without interference from solutions of metamorphic origin. The δ13C values of the best preserved samples indicate fluctuations in α 13CPDB compatible with those of carbonates deposited at the beginning of the Neoproterozoic.

The outcrops visited indicate a transition from glaciogenic sediments (characterized by poorly sorted sandstones, conglomerates and shales) to carbonate sediments and suggest a sudden variation in temperature in the deposition environment, indicating rapid deglaciation and deposition of sediments of biological origin. The age of ca. 580 Ma indicates that the rocks studied are correlated to the Gaskiers event.