Apollo 14 VHK (Very High Potassium) basalt is a newly discovered variety of low-Ti, high-Al mare basalt characterized by unusually high concentrations of K, Rb, and Ba, and high ratios of these elements to other incompatible elements that exceed normal lunar values. Potassium-feldspar and K, Si-rich glass are ubiquitous and widespread components of these basalts. Other phases include magnesian olivine (Fo 60--73), Fe-rich olivine Fo 31--50), pigeonite, plagioclase (An 79--95), ilmenite, Cr-ulv8-spinel, Fe-metal, and troilite. VHK basalts have low initial 87Sr/86Sr ratios, with Rh/Sr ratios estimated to be 15¿ higher than their sources. This Rb/Sr fractionation occurred at their time of formation, 3.75 aeons. Hypotheses involving volatile/nonvolatile fractionations or long-term enrichment of the source regions in K are inconsistent with the high Ba/La ratios and low initial 87Sr/86Sr ratios with VHK basalt. The most plausible origin for these rocks is partial assimilation of lunar granite by a normal low-Ti, high-Al mare basalt. Assimilation occurs by diffusion-controlled exchange of alkalis and Ba between two melts. Trace element compositions of the granitic melt are controlled in part by refractory accessory phases in the granite. Significant geochemical interaction between the lunar crust and mare basalts is implied.