Introduction

Calcic pelitic schists from the contact aureole of the Grand Island pluton, in the Juneau Gold Belt, southeast Alaska, contain garnets that display compositional zoning that is the result of episodic metasomatism (Stowell et al., 1996). Zoning of the major elements suggests that periodic influx of Ca-rich fluid produced zones with high XGrs. In addition, zoning of trace elements suggests that many other elements were also involved.

Compositional zoning of garnet

Backscattered electron image of garnet with strong cylcic zoning. Field of view is approximately 450 microns wide.

Compositional traverse across the garnet shown above. The extreme cyclic zoning would require cyclic changes in P or T of several kbar or several 100 degrees if the rock was metamorphosed in a closed system. This seems unlikely. Instead the zoning can be easily explained by episodic addition of Ca.

Metasomatism is consistent with andradite garnet found in veins in the same outcrop. Repeated metasomatism may have been the consequence of repeated intrusion by phases of the Grand Island diorite.

Trace element compositions in the garnet

. Na is antipathetic to Ca, suggesting that Xanorthite varies with Xgrossular. If so, this requires a source of Ca in addition to the garnet and plagioclase. Y is also antipathetic to Ca, suggesting that epidote (a common sink of Y) grew during at the same time as the high Xgrossular portions of garnet. Together with cyclic zoning, this suggests that Y, Ca, and other elements were added to the rock during metamorphism. Ti zoning may reflect episodic growth of sphene during these events.