The magmatic rocks from Alto Paraguay (High Paraguay River extensional lineament), western Apa craton, mainly consist of several major circular alkaline complexes and some rhyolitic domes and ignimbrites. The former are characterized by intrusive Naealkaline rock-types (nepheline syenites and syenites and effusive equivalents) topped by lava flows and ignimbrites. Two main evolved suites were defined using petrochemical and Sre isotope data: an agpaitic suite in the north and a miaskitic suite in the south. The domes of subalkaline rhyolitic lavas and ignimbrites occur to the north of the alkaline complexes, along the Paraguay River, near the town of Fuerte Olimpo. The emplacement ages of the alkaline complexes were constrained using the KeAr, AreAr, RbeSr and SmeNd dating methods on whole rocks and/or mineral separates (amphibole, alkali feldspar and biotite).
Ages are quite variable (Upper Permian to Middle Triassic), with average KeAr and AreAr ages of 248.8 ± 4.8 and 241.8 ± 1.1 Ma, respectively, and RbeSr and SmeNd age data giving best values from 248 ± 4 to 244 ± 27 Ma and from 256 ± 3 to 257 ± 3 Ma, respectively. In contrast, the Fuerte Olimpo volcanics show a Mesoproterozoic age (1.3 Ga, KeAr and AreAr radiometric methods; and 1.42 ± 0.24 to 1.30 ± 0.03 Ga, RbeSr and SmeNd methods, respectively). RbeSr systematics (87Sr/86Sr initial ratios 0.7038) highlight a relatively “primitive” character of the Naealkaline magmatic source(s), in contrast with the “crustal” values (87Sr/86Sr initial ratio ~ 0.7105) of the Fuerte Olimpo rhyolites. Thus, magmatism in the Alto Paraguay area is related to two extensional events: a younger event corresponding to the PermianeTriassic alkaline rocks, and an older event connected to the Precambrian volcanic acidic rocks.
