CONSTRAINING DIAGENETIC ALTERATION OF MIDDLE ORDOVICIAN (DARRIWILIAN) LIMESTONE FROM THE ARGENTINE PRECORDILLERA: IMPLICATIONS FOR PRIMARY SEAWATER SIGNATURES

The elemental and isotopic compositions of marine limestone are often utilized to investigate geochemical cycling and environmental changes in the rock record; however, the primary geochemical signal may be obscured or even completely overprinted by post-depositional alteration. The middle Darriwilian Las Chacritas and Las Aguaditas Formations of the Argentine Precordillera have been the target of recent biostratigraphic and geochemical studies. These units record the drowning of the Precordillera terrain associated with the formation of the Guandacol foreland basin prior to docking with Gondwana. Here we constrain the history of post-depositional alteration for a discrete fossiliferous interval from the Las Chacritas and Las Aguaditas Formations with detailed petrographic, elemental, and isotopic data. These formations contain numerous fractures and veins filled with multiple generations of carbonate cements making them ideal for investigating their alteration history.

Carbonate hand samples were collected within a 1x1 m grid at 0.2 m intervals. Petrographic analyses were used to characterize the preserved carbonate fabrics and aid in determination of primary and later carbonate microfacies. Fine-grained micrite, which shows little evidence of recrystallization, was targeted as a primary phase since it has been repeatedly shown to reliably record primary seawater values for Paleozoic carbonate strata. Coarsely crystallized phases such as voids and fracture vein fill, which likely formed during (Cenozoic) uplift of the Precordilleran succession, were then sampled to determine the extent to which secondary diagenesis may have influenced the composition of primary phases. Individual phases were discretely sampled with a bench-mounted drill press. Sample powders were analyzed for elemental (Ca, Mg, Fe, Sr, Mn) and isotopic (C- and O-) values. Variability of elements and isotope values of primary and diagenetic features utilized to constrain the effect of each on the preservation of primary sweater signals.