The occurrence of Mesoproterozoic successions in the southern part of the Nico Pérez Terrane has remained controversial, but is now confirmed by different lines of evidence. We present here chemostratigraphic data suggesting Mesoproterozoic depositional ages for three different units.
The Mina Verdún Group is a volcanosedimentary succession including thick limestone and dolostone deposits in its middle part. Carbonates are mainly stromatolitic and dominated by Conophyton. The unit is pre-Ediacaran in age, but no maximum age constraints are available. A δ13C curve obtained at its stratotype is characterized by a basal negative excursion to -3.3‰ PDB, followed by moderately positive values for the rest of the section, representing a plateau around +2‰ PDB (Gaucher et al., 2006).
The Parque UTE Group (former Lavalleja or Fuente del Puma Group) represents a volcanosedimentary succession dated at 1429±21Ma (U-Pb SHRIMP zircon age of rhyolites: Oyhantçabal et al., 2005). Carbonates are mainly clastic dolostones or dolomitic limestones, interbedded with shales, basalts and acid tuffs. The obtained δ13C curve shows a basal negative excursion of -1‰ PDB, followed by a positive plateau at +1/+1.5‰ PDB, returning to negative values of -3‰ PDB at the top.
The Mataojo Formation of the Carapé Group is constrained to be younger than 1,900 Ma by detrital zircon ages (Oyhantçabal et al., 2005). It is characterized by white dolomitic marbles associated with quartz-arenites and metapelites. In all studied sections δ13C varies within a narrow range around 0‰ PDB (-0.6 to +0.4).
A common characteristic of all studied units is the relatively low amplitude of δ13C oscillations and occurrence of isotopic plateaus, which comprise most of the sections. Although the global Mesoproterozoic carbon isotope curve is poorly constrained, the following conclusions can be drawn:
(1) The Mina Verdún Group was probably deposited in the late Mesoproterozoic or Early Neoproterozoic (Tonian) according to global δ13C curves.
(2) The Mataojo Formation of the Carapé Group shows a δ13C curve consistent with an age between 1,300 and 1,900 Ma, the latter date being provided by the youngest detrital zircon ages. The mentioned time interval is characterized by carbonates with δ13C of 0 ± 1‰ PDB.
(3) The δ13C curve obtained for the Parque UTE Group suggests that the transition from isotopic invariance around 0 ‰ PDB to moderate-amplitude δ13C excursions probably took place by ca. 1,450 Ma, thus earlier than currently accepted (Bartley et al., 2007)