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). The lavas range from quartz tholeiites through olivine tholeiites to alkali basalts, basanites and leucite basanites. The basanites and leucite basanites occur as late, post-plateau cones and flows. The basanites contain only normative nepheline but the leucite basanites include modal leucite, which is interstitial and does not form euhedra, with sanidine and nepheline. Rock and some mineral analyses and Sr isotope ratios are available in Baker et al. (1981).