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The Tonsina ultramafic and mafic assemblage, located in the northern Chugach Mountains along the Trans‐Alaska Crustal Transect (TACT), is interpreted to be the remains of a magma chamber that crystallized at the base of a mature interoceanic island arc. It is one of a number of isolated masses of peridotite and gabbro that comprise the basal part of the Peninsular terrane along the northern fringe of the Chugach Mountains in southern Alaska. These masses, the roots of a mid‐Jurassic island arc, have been uplifted along the north dipping Border Ranges fault system and crop out sporadically for 700 km from Kodiak Island to eastern Alaska. The Tonsina ultramafic‐mafic assemblage is a layered sequence whose basal rocks are dunite and harzburgite (1 km thick) overlain by a narrow zone of websterite and clinopyroxenite. Above these ultramafic rocks is a 5‐ to 6‐km thick layered gabbro that is garnet bearing at its base. Textural relationships and chemical trends throughout the series of dunite, pyroxenite, garnet bearing gabbro and spinel gabbronorite point to an origin by igneous accumulation from a hydrous, tholeiitic parent magma at deep levels, i.e., at the base of a mature island arc before being emplaced along the Border Ranges fault. The rocks were crystallized from a melt at pressures between 9.5 and 11 kbar (28‐ to 33‐km depth) and equilibrated at temperatures of 800° to 90O°C based on phase relationships in the cumulus assemblages, pyroxene thermobarometry, and olivine‐spinel thermometry. Transitional sequences from ultramafic to mafic rocks are characterized by increasingly AL‐rich pyroxenes and spinels (or garnet) and the absence of olivine + plagioclase assemblages. Deep‐level fractionation of Al‐poor phases such as olivine and pyroxene at moderately high pressures and temperatures resulted in concentration of Al in the evolving magma. As crystallization continued, the phases recorded this alumina concentration and the resultant gabbros are Al‐rich. This observed trend supports the theory of high‐alumina basalt generation by fractional crystallization of olivine and pyroxene. Comparison of the Tonsina rocks with Aleutian lower crustal xenoliths as well as other exposures of arc‐related lower crustal magma chambers in Japan, Pakistan, and western North America indicates that this process is a common mechanism for island arc magma evolution at depth.

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