THE POTENTIAL ROLE OF EPIDOSITE IN WATER-SATURATED PARTIAL MELTING OF MAFIC AMPHIBOLITES: AN EXAMPLE FROM POLLOCK MOUNTAIN, WESTERN IDAHO

Migmatized metabasalts of the Pollock Mountain Amphibolite are exposed on the northern ridge of Pollock Mountain near the Western Idaho Suture Zone, which marks the tectonic boundary between island arc and oceanic terranes of the Blue Mountains province and the North American continental margin. Tonalitic and trondhjemitic leucosomes formed in both garnet-free and garnet-bearing assemblages of hornblende + plagioclase ± biotite ± rutile. Leucosome is typically associated with coarse-grained hornblende, which occurs as large idiomorphic grains within the leucosome or as thick (up to 1 cm) accumulations adjacent to leucosomal veins. The coarse-grained hornblende is interpreted to be peritectic. Rare earth element patterns of calculated melt compositions in equilibrium with the hornblende have pronounced positive europium anomalies and concave-upward shapes. Together, these features are suggestive of water-saturated melting of the amphibolite, during which plagioclase melted and hornblende was stabilized. This begs the question, “What was the source of water?”

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.