Chemical data obtained from Olivine samples from the Tertiary basalts of Rio Grande do Norte and Paraíba, as well as from Mesozoic basalts of the Maranhão Basin in northeastern Brazil, allowed to estimate the crystallization temperatures of the plagioclási contained in these rocks according to the geothermometer of KUDO & WEILL (5). In the calculations made, assuming equilibrium conditions between the crystals and the liquid, the chemical composition of the border of the plagioclase and the interstitial matrix was considered. In the olivine basalts, the temperatures recorded for 1 Kbar water pressures were between 1078 ° -11680 ° C, showing perfect agreement with the HAKLI & WRIGHT (3) geothermometer used for these rocks by SIAL (8). With respect to the tololithic basalts, the values were between 1140 ° - 1230 ° C,
