THE POTENTIAL ROLE OF EPIDOSITE IN WATER-SATURATED PARTIAL MELTING OF MAFIC AMPHIBOLITES: AN EXAMPLE FROM POLLOCK MOUNTAIN, WESTERN IDAHO
Layers and boudins of epidosite (epidote + quartz ± garnet ± magnetite ± plagioclase) occur throughout both garnet-bearing and garnet-free amphibolite varieties, and may provide a small-scale analog to partial melting in the Pollock Mountain Amphibolite as a whole. Pods of epidosite are of three end member types: 1) epidote + quartz, 2) epidote + quartz + magnetite, and 3) epidote + quartz + garnet. Quartz and plagioclase are intimately associated with garnet. Accumulations of almost pure hornblende typically surround epidosite pods, and leucosome is commonly observed in low-pressure interboudin partitions. These observations suggest that the breakdown of epidote (either during prograde metamorphism or adiabatic decompression) resulted in the formation of garnet and the release of H2O. This H2O escaped into the surrounding amphibolite, resulting in localized melting of plagioclase and the formation of a hornblende-rich restite. The calculated melt compositions are unlike any of the tonalitic/trondhjemitic plutons observed in the Blue Mountains province, and probably reflects the inability of H2O-saturated melts to rise through the crust.