Abstract

Calcic pelite from the contact aureoles around plutons in the Grand Island diorite complex, southeastern Alaska, records episodic flow of metamorphic fluid and Ca-metasomatism. Grains of garnet adjacent to quartz veins are comprised of almandine, spessartine, grossular, and pyrope components and show oscillatory zoning in mole fraction of grossular (ranging from 0.10 to 0.35) and antipathetic zoning in mole fraction of almandine. With some exceptions, the mole fraction of spessartine decreases and Fe/(Fe+Mg) increases toward the rim. Na, Y, Yb, and Zr zoning in these garnet grains have opposite trends to grossular, but Ti in garnet is positively correlated with grossular content. Garnet grains ca. 2 m from the veins have fewer cycles of zoning, indicating that infiltration started near veins and that some infiltration events penetrated only a short distance into the pelite. In contrast, andradite-rich garnet and epidote in the veins contain negligible concentrations of Y, Yb, and Zr, consistent with growth of andradite-rich garnet from infiltrating fluids containing low concentrations of these elements. The data on zoning can be explained by episodic Ca-metasomatism, with or without mobility of other components, producing episodic growth of garnet, plagioclase, and epidote. The reason for pulses of fluid flow or changes in fluid compositions is unclear, but may be related to episodic plutonism in the complex. Staurolite and other minerals in the amphibolite-facies rocks were partially replaced in a series of fluid-infiltration pulses. The first produced a second generation of garnet, the second produced retrograde clinozoisite+albite+muscovite+sulfide in the contact-metamorphic rocks and the plutonic rocks, and a later event produced chlorite + sericite. Retrograde metamorphism is inferred to be associated with emplacement of plutons in the Coast plutonic-metamorphic complex 10 km to the east, and possibly with mineralization in the Juneau gold belt.

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