Antiquity of the Rı́o de la Plata craton in Tandilia, southern Buenos Aires province, Argentina

Rb–Sr and Sm–Nd whole-rock data for granitoids and orthogneisses from the western part of the Sierras Septentrionales of the southern Buenos Aires province yield an errorchron of 2009±71 Ma (initial 87Sr/86Sr=0.7041, MSWD=69) and an isochron of 2140±88 Ma (initial 143Nd/144Nd=0.50977), respectively. As in previous investigations, the Rb–Sr data are clearly disturbed, but the Sm–Nd isochron may record the age of emplacement of igneous precursors. These results reaffirm that this region is the southern extension of the crystalline basement of the Rı́o de la Plata craton. The Sm–Nd age, though not very precise, is slightly older than previously demonstrated but consistent with most recent U–Pb studies of the craton exposed in Uruguay and Brazil. Crust-derived Sm–Nd model ages averaging 2620±80 Ma indicate that, though the principal rock-forming events were Paleoproterozoic, a Late Archaean prehistory is possible. However, the data place strict constraints on the nature and intensity of post-2000 Ma activity in this area, which seems to be confined to tholeiitic dyke emplacement and hydrothermal reactivation.