TONALITIC-TRONDHJEMITIC MAGMATISM FROM GARNET-BEARING AND GARNET-FREE MIGMATIZED AMPHIBOLITES ON POLLOCK MOUNTAIN, WESTERN IDAHO: ESTIMATING MELT COMPOSITIONS FROM HORNBLENDE TRACE ELEMENT DATA

Pollock Mountain is located near the Salmon River Suture Zone (SRSZ) along which accreted terranes of the Blue Mountains province collided with the North American continent during Cretaceous time (~128 Ma). Pollock Mountain sits atop the Pollock Mountain Plate (PMP), which represents arc crust that experienced high grade metamorphism during the collision event. Two occurrences of migmatized amphibolite (approximately 2.5 km apart) were examined, one resulting in peritectic garnet and the other garnet-free. Leucosomes in both migmatites comprise plagioclase and quartz. Their whole rock compositions are tonalitic to trondhjemitic, but with a strong cumulate signature. Hornblende crystals in equilibrium with leucosome in six samples were analyzed for trace element abundances by LA-ICPMS. Those from the garnet-free migmatite exhibit chondrite-normalized REE patterns characterized by slight LREE depletion, flat MREE and HREE, and no Eu-anomaly. In contrast, hornblende crystals from the garnet-bearing migmatite have concave-downward REE patterns, with steep positive slopes from the LREE to the MREE, steep negative slopes from the MREE to the HREE, and slight negative to positive Eu-anomalies. Compositions of melt in equilibrium with the hornblende were calculated using the hornblende-melt partition coefficients of Klein et al. (1997). Calculated melt compositions from the garnet-free migmatite have concave-upward REE patterns, with pronounced positive Eu-anomalies and enrichment in the HREE, relative to the MREE. These patterns are unlike any known plutonic rocks in the Blue Mountains province. Melts produced in this way were probably H₂O-saturated and not able to separate from their source. Melt compositions calculated for the garnet-bearing migmatite, however, have relatively straight LREE-enriched patterns, low HREE abundances, and positive Eu-anomalies. These REE patterns are very similar to those of the nearby epidote-bearing Round Valley pluton, ~13 km to the southeast. If these migmatites experienced the same P-T conditions of partial melting, then the differences in the melt compositions must be a function of mineralogical heterogeneities in the amphibolite.