The investigation of intermediate rocks in two granite-gabbro contact zones in the Wichita province of southwestern Oklahoma has established petrologic and geochemical constraints which suggest intrusive relationships exist between the major silicic and basic plutonic rocks. Field studies of the Poko Mountain area in the eastern portion of the province indicate the contact of the younger granite with the underlying gabbro is discordant and is characterized by the presence of gabbro xenoliths within the granite. Whole-rock and trace element chemistry disclose the intermediate rock found highest in the gabbro section at this exposure is more fractionated than the contact phase of the granite. Negative Eu anomalies demonstrate the granite has experienced strong plagioclase fractionation. The gabbro, however, shows no complimenatry positive Eu patterns. In addition, the plagioclases of the gabbro exhibit reverse cryptic variation with the most calcic plagioclase (An67) being found in the intermediate rock immediately below the granite contact. Petrogenesis of the intermediate rock at Poko Mountain is therefore attributed primarily to crystal fractionation of the gabbro and secondarily to alkali metasomatism from the overlying granite. Examination of the Twin Mountains outcrop in the western part of the province reveals the contact between granite and intermediate rock is knife-sharp. The intermediate rock, originally an anorthositic cumulate, is volumetrically composed of approximately 8% plagioclase but is now identified by a negative Eu anomaly. Contact thermal alteration and resorption of plagioclase within the intermediate rock are evident and indicate disequilibrium. The data suggest the development of the intermediate rock at this locale is due solely to hydrothermal contamination of the basic rock by alkali metasomatism from the adjacent granite. The evidence thus suggests that at neither outcrop are the granites a product of differentiation of the basic rocks. ' Most probably the granites originated by partial melting of the Precambrian granitic basement by upwelling basic magmas during the rifting associated with the evolution of the southern Oklahoma aulacogen.