Ultramafic Intrusives and Associated Magnetite Deposits at Union Bay, Southeast Alaska

Citation

Ruckmick, John Christian (1957) Ultramafic Intrusives and Associated Magnetite Deposits at Union Bay, Southeast Alaska. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/7B6M-0K73. https://resolver.caltech.edu/CaltechETD:etd-10072004-161749

Abstract

An intrusive body of ultramafic rocks, which crops out over an area of 7 square miles at Union Bay, southeast Alaska, has been mapped and studied petrographically. The ultramafic rocks are probably Jurassic or Cretaceous in age, and constitute a spoon-shaped lopolith with a dunite feeder plug at the east end. The rocks of the lopolith are composed of various proportions of diopside and forsteritic olivine, with some magnetite and hornblende, and no feldspar. The most basic rocks, dunite and peridotite, occur in the central portion of the lopolith, and are enveloped by olivine-pyroxenite, pyroxenite, and hornblende-pyroxenite in that order. The peridotite rocks in the central portion of the complex are composed of olivine-pyroxenite intruded by sills and irregular bodies of dunite. Attitudes of the dunite sills conform to the spoon-shaped structure of the logolith. Large areas of pyroxenite containing 10% to 25% of primary magnetite crop out around the periphery of the lopolith. An intrusive body of gabbro adjoins the south side of the ultramafic lopolith. The gabbro is earlier than, and has been intruded by, the ultramafic rocks. The structure and petrology of the ultramafic lopolith are explained by an hypothesis of successive injections of increasingly basic magmas composed essentially of: (1) a cotectic blend of diopside and magnetite; (2) a cotectic blend of diopside and forsteritic olivine; and (3) forsteritic olivine. The intrusion of the magmas in that order, the reverse order of crystallization of the component minerals, is a reflection of the differential melting of the ultramafic source in the mantle of the earth. Each arriving magma must have found the hot, still crystallizing, central portion of the preceding magma the most accessible locus of intrusion. The lack of a very high grade of contact metamorphism in the pelitic sediments adjacent to the ultramafic rocks is explained by the assumption that the ultramafic magmas contained very little or no water, and, therefore, expelled no significant amounts of volatile phases during crystallization.

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