Geochronology of metamorphic and eruptive rocks of southeastern Neuquén and northwestern Río Negro Provinces, Argentine Republic

A synthesis of geologic and geochronologic information on metamorphic and eruptive rocks cropping out in the region located between 70°W and 71°W and 40°S and 41°S in Neuquén and Río Negro Provinces, Argentine Republic, is presented. In particular the metamorphites, migmatites, and granitic rocks of the so-called “Crystalline Basement” have been investigated and some data are presented on the Cenozoic basalts. The studies carried out so far permit the following preliminary conclusions. The metamorphic rocks belong to two different events. The oldest, with an age of 1190 ± 16 Ma and an initial 87Sr/86Sr ratio of 0.7043 ± 0.0002, corresponds to a regional metamorphic event. The youngest, with an age of 615 ± 26 Ma and an initial ratio of 0.7061 ± 0.0005, belongs to a second metamorphic event that has been recognized only locally. Some of these rocks have undergone a calco-sodic migmatization process. The Rb/Sr whole-rock isochrons for these rocks give ages between 490 and 420 Ma and initial 87Sr/86Sr ratios from 0.7045 to 0.7100. The best isochron has an age of 480 ± 10 Ma and an initial ratio of 0.7049 ± 0.0002. The KAr dates obtained for samples from the three metamorphic events are, in general, non-concordant with the results given by the Rb/Sr method, indicating that the KAr systems were modified by thermal and deformational processes that occurred after crystallization of the metamorphites. However, a KAr date on biotite-schist gives an age of 643 ± 20 Ma in accord with the second metamorphic event. Furthermore, samples collected from some of the localities studies have given KAr ages between 450 and 420 Ma, in agreement with the Rb/Sr whole-rock ages obtained for the migmatization process. A KAr age of 440 ± 20 Ma was obtained for a tonalite body, the emplacement of which would seem to be related to the migmatitic event. The initial 87Sr/86Sr ratios of the metamorphic and migmatitic rocks are low in comparison with those of similar rocks from other areas of Argentina. The Rb/Sr ages obtained for the granites show the existence of two cycles (or subcycles). The oldest took place in the Late Devonian, with an age of 370 ± 6 Ma. The other cycle occurred in the Early-Middle Permian and has an age of 280 ± 5 Ma. The KAr ages obtained on the same granite samples are concordant with these Rb/Sr ages. Preliminary study of the Cenozoic basalts reveals three different cycles: in the Eocene (39–44 Ma), Oligocene (28–32 Ma), and Miocene (16–18 Ma). Previous studies of these rocks had assigned them to the Oligocene.