The pre-Tertiary oceanic crust exposed on the west coast of Costa Rica has been broadly referred to as the Nicoya Complex. This study was designed to determine the age of the Nicoya Complex in the Nicoya Peninsula, Playa Jac¿, and the Quepos Peninsula using 40Ar-39Ar radiometric dating and to assess the petrologic relationships between the different localities using major element, trace element, and Sr, Nd, Pb isotopic data. Radiometric ages of basalts and diabases from the Nicoya Peninsula are 88--90 Ma (with a weighted mean of 88.5 Ma), and those of two intrusive rocks (a gabbro and plagiogranite) are both 83--84 Ma. The combined geochemical data indicate that the sampled Nicoya Peninsula rocks belong to a single suite related by fractional crystallization of similar parental magmas. Nd and Pb isotopic ratios indicate a common mantle source distinct from that of mid-ocean ridge basalts. Both the age and composition of the Nicoya rocks are consistent with the idea that they are a part of the Caribbean Cretaceous oceanic plateau <Donnelly, 1994>. The Jac¿ lavas are geochemically similar to the Nicoya Peninsula suite, and a single age of 84 Ma is identical to the age of the Nicoya Peninsula intrusives. The one analyzed Quepos basalt has a radiometric age of ~64 Ma, and it is enriched in incompatible elements relative to the Nicoya rocks. Similarities in Nd and Pb isotopic ratios indicate that the Quepos and Nicoya/Jac¿ lavas were derived from a similar mantle source to that which produced the Nicoya rocks, possibly the Galapagos plume.¿ 1997 American Geophysical Union