Seven circular igneous intrusions of the Velasco alkaline province define a northeast-southwest lineament north of San Ignacio de Velasco in the headwaters of the Rio Paragua. Nepheline-bearing and quartz-bearing syenites make up ring complexes that are contained within a homogeneous and structureless biotite granite that is also part of the alkaline province. All intrude Precambrian gneisses. Rb-Sr whole-rock isochron ages (140 ± 6 m.y., 143 ± 4 m.y.) and K-Ar mineral ages (134 to 142 m.y.) indicate that the alkaline province was emplaced in Late Jurassic or very early Cretaceous time. K-Ar ages of 1,336 and 1,323 m.y obtained from the gneisses date the last period of metamorphism and deformation in the area. Similar alkaline provinces occur in Namibia, Angola, Brazil, and Uruguay, and this period of alkali magmatism may be related to the initial rifting of the Africa–South America plate and the formation of the proto-Atlantic.