CA-METASOMATISM AND CHEMICAL ZONATION OF GARNET IN CONTACT-METAMORPHIC AUREOLES, JUNEAU GOLD BELT, SOUTHEASTERN ALASKA

Calcic pelite from the contact aureoles around plutons in the Grand Is land diorite complex, southeastern Alaska, records episodic flow of me tamorphic fluid and Ca-metasomatism. Grains of garnet adjacent to quar tz veins are comprised of almandine, spessartine, grossular, and pyrop e 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 d ecreases and Fe/(Fe + Mg) increases toward the rim. Na, Y, Yb, and Zr zoning in these garnet grains have opposite trends to grossular, but T i in garnet is positively correlated with grossular content. Garnet gr ains ca. 2 m from the veins have fewer cycles of zoning, indicating th at infiltration started near veins and that some infiltration events p enetrated only a shea distance into the pelite. In contrast, andradite -rich garnet and epidote in the veins contain negligible concentration s of Y, Yb, and Zr, consistent with growth of andradite-rich garnet fr om infiltrating fluids containing low concentrations of these elements . The data on zoning can be explained by episodic Ca-metasomatism, wit h or without mobility of other components, producing episodic growth o f garnet, plagioclase, and epidote. The reason for pulses of fluid flo w or changes in fluid compositions is unclear, but may be related to e pisodic 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 + s ulfide in the contact-metamorphic rocks and the plutonic rocks, and a later event produced chlorite + sericite. Retrograde metamorphism is i nferred to be associated with emplacement of plutons in the Coast plut onic-metamorphic complex 10 km to the east, and possibly with minerali zation in the Juneau gold belt.