<p>Assimilation of crust by hot basaltic liquids changes physical magma characteristics. Advances have been made in understanding assimilation through isotope and trace element geochemistry, thermodynamic models and theory. Physical examples of assimilation in basaltic magma chambers, however, are less common due to limited exposures capturing the processes. Here, field observations, petrology and bulk rock chemistry, including ⁸⁷Rb-⁸⁷Sr data, are reported for lithologies, including sedimentary blocks and hornfels, within the hypersthene gabbro of the Paleogene-aged Ardnamurchan Igneous Complex, Scotland. Mesozoic sediments and Neoproterozoic Moine schist, into which the hypersthene gabbro intruded, have radiogenic ⁸⁷Sr/⁸⁶Sr (&gt; 0.71). Significant initial Sr isotope variability in the hypersthene gabbro and autoliths (0.70402–0.70921) demonstrates inhomogenous and extensive (&gt; 10%) assimilation in some instances. Mineralogy and radiogenic initial ⁸⁷Sr/⁸⁶Sr (0.70753–0.71030) of hornfels masses are consistent with their derivation by replacement after sedimentary protoliths. Partial assimilation of sediment during cumulate formation accounts for variable quantities of crustal assimilation in the hypersthene gabbro and implies concomitant hydration of the magma. Incomplete assimilation of stoped and foundered sediments enabled greater extents of assimilation, while also enhancing accommodation space for intrusion emplacement. Crustal assimilation may also have played a role in the layering and structure in the intrusion due to cooling and modifying melt composition within a crystal mush. Larger or hotter basaltic magma chambers, and lavas erupted in continental flood basalts or in oceanic settings show variable chemical evidence for assimilation. Consequently, crustal contamination during formation of crystal mushes is likely to be a ubiquitous process in basaltic feeder intrusions.</p>