High-alumina olivine tholeiites (HAOT) and lesser basaltic andesites dominate late Miocene to Quaternary mafic volcanism of southeastern Oregon and adjacent California/Nevada. In the western Harney Basin, within a ~30x40 km² area, numerous mafic vents erupted phenocryst-poor basaltic rocks ranging from primitive HAOT (MgO > 8 wt.%, K₂O ±0.3 wt.%, Rb ±3 ppm, La 2-5 ppm) to strongly incompatible trace-element enriched trachyandesites or to LREE- but otherwise less-enriched calc-alkaline andesites. This bifurcation of enrichment trends, towards trachyandesite and calcalkaline andesite, occurs after evolution of enriched HAOT (K₂O ±0.6 wt.%, Rb ±7 ppm, La ±10 ppm) from primitive HAOT. Incompatible trace-element enrichment towards trachyandesite is associated with Fe-enrichment and ~constant La/Yb culminating in compositions containing ~60 ppm La and ~700 ppm Zr. In contrast, evolution towards calc-alkaline andesites is characterized by decreasing Fe and progressively higher La/Yb but with lesser enrichment of La and Zr (~25 ppm La and ~200 ppm Zr) than in the trachyandesite suite. The mafic trachytic trend formed mainly by multiple cycles of fractionation and recharge of HAOT as suggested by the basaltic major element composition combined with the great enrichment of incompatible elements. In contrast, calc-alkaline andesites are likely derived by fractionation from HAOT with Fe enrichment suppressed by mixing with felsic magmas and (or) crustal assimilation. HAOT lavas have up to ~four-fold variations in incompatible trace-element concentrations, even among those with MgO near or above 8 wt.%. Isotopic variation is insufficient to argue for variable mantle sources and strongly declining K/La ratios with increasing K₂O preclude their derivation only by variable degrees of partial melting of the mantle. Contamination of primitive HAOT with an enriched magma is more consistent and a suitable contaminant exists in strongly enriched Fe-tholeiite that lies on the liquid-line to trachyandesite. The Harney Basin basaltic suite is exemplary to illustrate calc-alkaline vs tholeitic magma trends, how alkalic mafic magmas can originate from olivine tholeiites, and the effect of "self-contamination", where parameters for "primitive", such as MgO, do not suggest much fractionation nor involvement of crustal material.