In this paper we present geologic mapping, K-Ar chronology, major and trace elment data, mineral chemistry, and Nd, Sr, and O isotopic data for volcanic rocks of the Mount Taylor volcanic field (MTVF). The MTVF lies on the tectonic boundary between the Basin and Range province and the southeastern Colorado Plateau and is dominated by Mount Taylor, a composite volcano active from ≈3 to 1.5 m.y. ago. Growth of the volcano began with eruption of rhyolite, followed by quartz latite and finally latite. Basalts erupted throughout the lifetime of the volcano. The compositional variations of MTVF rocks resulted primarily from fractional crystallization of midly alkaline hy-hawaiite magmas. Differentiation of magmas occurred in multiple, short-lived magma chambers, a result of a low magma flux from the mantle that could not support a single, long-lived chamber. Rare mixing of evolved hy-hawaiite and rhyolite produced a few intermediate magmas, primarily in the early history of the field. Mixing may have occurred when rhyolite magmas in the lower crust ascended to upper crustal levels and were injected into the bases of mafic magma chambers. Small amounts of crustal assimilation accompanied fractional crystallization and affected all the evolved MTVF rocks. Assimilation/fractional crystallization occurred primarily in the lower crust as hy-hawaiite differentiated to mugearite or latite. Early in the history of field, evolved lower crustal magmas ascended into the upper curst, where density filtering and a reduced tensional stress field inhibited further ascent until magmas evolved to rhyolite or quartz latite. Later in the history of the field, latite magmas ascended directly from the lower crust and erupted without further significant differention because of increased crustal extension. ¿ American Geophysical Union 1990