This study presents new petrologic and sedimentologic data from northwestern Costa Rica concerning the provenance of Cretaceous forearc sandstones that contain plutonic detritus. Plutonic rock fragments are important accessory particles in pyroxene-bearing sandstones overlying the ophiolite named the Nicoya Complex. Through the use of modal analysis of the framework grains, I studied three sandstone suites of the El Viejo and Rivas Formations that include both shallow- and deep-water deposits, ranging from late Campanian to Maastrichtian in age. In terms of primary framework components, the sandstones resemble those derived from magmatic arcs. Two modal parameters are introduced to evaluate detrital plutonic contributions and affinity of source rocks: the ratio of plutonic to total lithic fragments [(Lp + iQF)/Lt], and the ratio of uralitized pyroxene to total pyroxene grains (uralPx/Px). Modal values for (Lp + iQF)/Lt indicate that plutonic fragments comprise up to 9% of total lithic fragments. A strong correlation between these two parameters suggests that uralitized pyroxene grains were also derived from intrusive rocks of probably basic and intermediate compositions. In particular, significant concentrations of lithic fragments exhibiting micrographic textures and uralitized pyroxene grains are interpreted to have been derived predominantly from eroded plagiogranites. Sandstone suites containing plutonic detritus signal an unroofing of deeper levels of the Mesozoic ophiolitic sequence as a consequence of strong uplift of the Costa Rican arc in late Senonian time. This tectonic event began in the Campanian at ca. 75 Ma, ∼9 m.y. after the intrusive magmatic activity on Nicoya Peninsula, and is consistent with the onset of the Laramide orogeny.