Continuous core from research drill hole Inyo-4 through a thick 300 m thick sequence of mafic lava flows in the west moat of the Long Valley Caldera has provided an unusual opportunity to investigate the chemical evolution of this exceptionally complete record of postcaldera mafic magmatism. Lavas are mainly basalts and trachyandesites ranging from 48 to 58% SiO2 having a nearly fourfold range in MgO contents. The lavas fall into five distinct chemical groups with little or no compositional overlap. These groups correlate remarkably well with stratigraphic position, and they define a trend toward more evolved compositions with time. The groups appear to represent periodic eruptions from a continuously evolving magma body. Preliminary 40Ar/39Ar dates indicate that these lavas erupted over a span of at least 0.264 m.y. between 0.415 Ma and 0.151 Ma. Except for the most evolved group, the chemical variation within a group was dominated by crystal fractionation. Except from the least evolved groups, the chemical contrasts between adjacent groups were dominated by assimilation. Warming of the crustal environment facilitated increasing assimilation. A zoned lava flow (more mafic upward) followed by a mafic flow in an otherwise progressively evolved sequence of flows provides evidence for eruption from a zoned magma reservoir. Deeper, more mafic portions of this zoned magma body were drawn up to shallower levels in the chamber during a period of high eruption rates. The heterogeneity of mafic clasts in the vent breccia dike beneath the 600-year-old South Inyo phreatic explosion crater indicates that these breccia clasts dropped >300 m down the vent from the overlying lava sequence during waning stages of the phreatic activity, rather than forming by brecciation of an older feeder dike, as previously proposed (Eichelberger et al., 1988). ¿ American Geophysical Union 1994