The 1350 years B.P. Big Obsidian Flow (BOF) at Newberry Volcano in central Oregon contains a wide variety of mafic magmatic inclusions. Although little grain size variation is observed within single inclusions, within the suite textures vary continuously from fine-grained (''quench'') to coarse-grained (''cumulate''). The BOF inclusion suite, therefore, defies conventional genetic classifications. Detailed petrographic and geochemical observations require a new model for inclusion formation. Only a few of the finer-grained inclusions have margins suggestive of liquid-liquid interaction. Interstitial glass content (≈20 vol%) and composition (high-silica rhyolite) are approximately constant through the suite. Mineral compositional zoning is greater in the coarse-grained than fine-grained inclusions. Whole inclusion compositions vary significantly but do not correlate well with textural diversity. We interpret the BOF inclusion suite to be a heterogeneous sample set of whole inclusions and parts of disaggregated inclusions. The compositional variability of the suite can be explained by a combination of preentrapment (hybridization) and postentrapment (interstitial liquid loss/grain) processes. Preentrapment hydridization involved mixing of basaltic andesite magma similar to Holocene flank flows and up to 30% rhyolitic liquid. The BOF inclusions may demonstrate the recent existence of mafic magma in the Newberry system. Inclusion features characteristic of small ΔT are consistent with the crystal-free nature of the BOF, suggesting that the rhyolite may have been superheated. If true, a crystal fractionation origin for the BOF rhyolite is unlikely. The disaggregation of grain size and compositionally zoned inclusions, which produced the observed textural diversity in the BOF suite, compounds problems of interpreting mafic inclusions in silicic volcanic rocks and, more so, in plutonic equivalents. ¿American Geophysical Union 1990