Sr Isotopic Composition and Pb/Pb Age of Neoproterozoic-Lower Paleozoic Carbonate Sequences at Salinas Hill and Pie de Palo Range, Western Argentina

CINGOLANI CARLOS ALBERTO; KAWASHITA KOJI; NAIPAUER MAXIMILIANO; VARELA RICARDO; CHEMALE JR. FARID

Salvador, Brasil.

Simposio; IV South American Symposium on Isotope Geology; 2003

Many authors have suggested a sedimentation age for the Angaco Fm. as a range in between the Neoproterozoic to the Lower Paleozoic time. The lack of accurate fossils from the Caucete Group, with the exception of trace fossils, was an important stratigraphic limitation. Our 87Sr/86Sr isotopic ratios from 0.7089 to 0.7099 are in coincidence with data obtained by Sial et at. (2002) in Pie de Palo Range. Those ratios are rather high for the best-known Neoproterozoic seawater curve but are close to Middle and Late Cambrian 87Sr/86Sr values that rose to a maximum 0.7091. Another explanation for these high values is that samples underwent secondary alteration (diagenetic or metamorphic events) as we shown in the fluid-rock interaction models, or influence of continent sedimentation in the seawater during the carbonate deposition. A good lineament on Pb/Pb isochron of c. 546 Ma determined from carbonates is interpreted as a last diagenetic/metamorphic event affecting the Angaco Fm. at Salinas Hill area. This suggests that the depositional age of this unit could be older, and range the Neoproterozoic time. If we consider the c .670 Ma U-Pb detrital zircon age obtained by van Staal et al. (2002) in the El Quemado siliciclastic rocks, the deposition age of the Angaco Fm. could be constrained in between 670 and 546 Ma.