This volcanic province extending for 70 km north-south and 20 km east-west comprises flows and plugs of basanite and olivine basalt, with some pyroclastics, and plugs and necks of nephelinite, tephrite and phonolite.
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Among the basaltic rocks of the Rio Chubut Medio are rocks identified as pantellerites.
In the Sarmiento area occur numerous sills, ring dykes and radial dykes of alkaline basic rocks, principally teschenites, with differentiates of nepheline, analcime and zeolite syenites. They intrude tuffs of the Bajo Barreal Formation. Analcime basanite lavas are widespread in the area.
Remnants of the Patagonian basalt plateau between 45°and 47°S extend from the border with Chile eastwards to about 68° 30'W (Baker et al., 1981). The basalts form conspicuous mesetas and in the northeast of the area intrusive alkaline complexes occur around Sarmiento (No. 14).