Chemostratigraphy of the Mina Verdún Group and other cement-grade Proterozoic limestone deposits in Uruguay

GAUCHER, CLAUDIO; SIAL, ALCIDES; POIRÉ, DANIEL; CERNUSCHI, F.; FERREIRA, V.; CHIGLINO, L.; GONZÁLEZ, PABLO D.; MARTÍNEZ, G.; PIMENTEL, M.

Punta del Este (Uruguay)

Simposio; 5th South American Symposium on Isotope Geology; 2006

Facultad de Ciencias (Universidad de la República)

The Neoarchean metasedimentary succession informally named “Las Tetas Complex” (LTC) crops out extensively in the Nico Pérez Terrane. It is composed, from base to top, by: (1) metapelites and quartz-arenites; (2) up to 700 m dolostones, partially stromatolitic, here assigned to the Cerros de Villalba Formation (CVF) of Gaucher et al (1996); (3) metapelite-metamarl-dolostone intercalations; and (4) quartz-arenites overlain by quartz-pebble conglomerates. It represents a shallowing-upward sequence up to 1500 m in measured thickness. Hemispheroidal, laterally-linked stromatolites predominate in the CVF, but planar and columnar stromatolites also occur. Lamination is microbial in origin, as demonstrated by its irregular, wavy geometry in thin section. The age of the CVF is radiometrically constrained by: (1) Rb-Sr datings (2001 ± 117 Ma; Ro=0.699628) of the intrusive Arroyo Perdido Granite; (2) U-Pb SHRIMP datings of cores and metamorphic overgrowths of detrital zircons, placing deposition between 2762 ± 8 and 2721 ± 7 Ma (Hartmann et al., 2001).d13C values in the CVF range between –2.55 and +0.64 ‰ V-PDB. The d13C curve obtained near Pirarajá is the closest approximation to the primary isotopic values, as indicated by Mn/Sr, d18O values and absence of contact metamorphism. This section yielded consistent d13C values between +0.64 and –0.49 ‰ V-PDB. Low amplitude of d13C excusions seems to characterize the CVF, suggesting an assignment either to the Archean-lowermost Palaeoproterozoic (3500-2450 Ma) or to the upper Palaeoproterozoic-lower Mesoproterozoic (1800-1400 Ma). Radiometrical ages mentioned above allow to discard an upper Palaeoproterozoic-lower Mesoproterozoic age, supporting a Neoarchean age for the CVF. Thus, the stromatolitic assemblage preserved there represents the oldest fossils so far reported from South America.