MIGMATIZATION OF AMPHIBOLITES ON POLLOCK MOUNTAIN, BLUE MOUNTAINS PROVINCE, WESTERN IDAHO

Pollock Mountain is located adjacent to the Salmon River Suture Zone, where oceanic and island arc terranes of the Blue Mountains province were accreted to the North American continent. Amphibolites on Pollock Mountain exhibit progressive stages of migmatization, from: 1) unmigmatized amphibolite, 2) amphibolite with leucocratic (plagioclase + quartz) haloes enclosing magnetite porphyroblasts (akin to “flecked gneiss”), 3) formation and coalescence of melt veins along foliation planes, 4) concentration of leucosome into discrete pods, with coarse-grained hornblende-rich melanosome, and 5) separation of melt and its migration through disrupted amphibolite (diatexite). Calculated melt compositions in equilibrium with the coarse-grained hornblende melanosome exhibit LREE-enriched chondrite-normalized patterns, with pronounced positive Eu-anomalies and slightly concave-upward MREE and HREE. These REE patterns are consistent with water-saturated partial melting at low to moderate pressures (<10 kbar; garnet-absent) in which plagioclase was destabilized. These calculated melt compositions are unlike any observed in the Blue Mountains province, and likely reflect the inability of water-saturated magma to separate from its source. If the migmatites on Pollock Mountain are related to collision of the Blue Mountains province with the North American continent (128 ± 3 Ma; Selverstone et al., 1992, Tectonics, v.11, pp.124-144), then they demonstrate that the collision event was accompanied by relatively shallow melting in the crust.

The leucocratic haloes described in 2 (above) have a granoblastic texture, suggesting a metamorphic segregation origin. However, leucocratic material is observed to extend from the magnetite porphyroblasts in veins several centimeters in length, which may even merge with other veins. This suggests that the leucocratic material represents melt that was mobilized. The origin of this flecked gneiss is currently the focus of study.