The Hall Cove ultramafic area, Duke Island, Southeastern Alaska: Supplement 2 from "The ultramafic complex and related rocks of Duke Island, southeastern Alaska" (Thesis)

Duke Island, 59 square miles in area, is at the southern end of southeastern Alaska. Sedimentary and volcanic rocks, possibly Mesozoic in age, are metamorphosed and intruded by gabbroic, ultramafic and granitic plutons, in that order. The granitic rocks may be of Cretaceous age. Primary gabbroic rocks are dominantly two-pyroxene gabbro and norite. Their plagioclase is An50-An70. Ultramafic rocks crop out as two main areas and more than a dozen minor ones. The rocks in the main areas probably are continuous at depth forming the Duke Island ultramafic complex. Constituent minerals are olivine, clinopyroxene, and hornblende; orthopyroxene and plagioclase characteristically are absent. Rock units are classified as dunite, peridotite, olivine pyroxenite, and hornblende pyroxenite. Hornblende pyroxenite contains 10-20 per cent magnetite and typically occurs as a border zone. The olivine-bearing units have remarkable layering which developed by gravitational settling of crystals from a body of circulating magma. Most of the olivine pyroxenite is cut by an intrusion represented at the present surface by dunite and peridotite. Hornblende-anorthite (An95) pegmatite, an ultramafic derivative, occurs in an aureole around the complex. In the aureole, pyroxene gabbro is altered to hornblende gabbro with plagioclase intermediate between those of pegmatite and primary gabbro. The relationship of ultramafic and primary gabbroic rocks indicates that they formed from ultramafic magma and normal gabbroic magma respectively. Mechanisms of crystallization differentiation, multiple intrusions, solid intrusion, and vapor transfer are examined as possible explanations of the distribution of rock types within the ultramafic complex. No one is sufficient, but all have applicability. The border zone is accounted for by transfer of water, silica, lime, and iron from the ultramafic magma to olivine-bearing rocks initially solidified from the magma body onto its walls. The required reactions are demonstrable in other parts of the complex, and the process is related to the development of the surrounding aureole. Evidence is given of late magmatic recrystallization in the complex and of local replacement of olivine pyroxenite by dunite. Disequilibrium, largely arising from multiple intrusion, and transfer of materials by an aqueous-rich vapor phase are probable causes. A sequence of events is summarized