PETROGENESIS OF TONALITIC-TRONDHJEMITIC MAGMAS AT MID- TO LOWER-CRUSTAL DEPTHS IN AN ARC-CONTINENT SUTURE: A COMPARISON OF THE GEOCHRONOLOGY, GEOBAROMETRY, AND GEOCHEMISTRY OF THE DEEP CREEK AND ROUND VALLEY PLUTONS, WESTERN IDAHO

The petrogenesis of tonalitic-trondhjemitic magmas in oceanic arc terranes are the result of either fractional crystallization of high-Al, hydrous basaltic magmas or partial melting of mafic amphibolites within the crust; however, distinction between the two processes remains a matter of debate. In western Idaho, the tonalitic Deep Creek Pluton (DCP) and Round Valley Pluton (RVP) are intermediate to felsic intrusions that were emplaced into arc crust only 20 km apart, but at very different depths. Both plutons are hornblende- and biotite-bearing, but the RVP also contains magmatic epidote. Aluminum-in-hornblende barometry for the DCP indicates emplacement pressures of ~2.4-2.8 kbar. Previous studies yielded pressure estimates of ~7.3-7.5 kbar for the RVP. The DCP and RVP magmas were emplaced during the Early Cretaceous shortly following arc-continent collision at ~128 Ma. Zircons yielded U-Pb (SHRIMP-RG) ages of 123.2 ± 2.4 Ma and 122.9 ± 1.0 Ma for the DCP main stage unit and a finer-grained inlier, respectively, and 117.9 ± 1.6 Ma for the RVP. Although the REE, Sr/Y and La/Yb behavior is markedly different for both plutons, the steady increase in Yb abundances and constant to slight decrease in La with increasing SiO₂ content for both RVP and DCP are consistent with an origin by partial melting of amphibolite (Brophy, 2008). Furthermore, the chondrite-normalized REE patterns of RVP rocks are essentially identical to those of calculated melt compositions determined from hornblende in migmatized garnet amphibolites from nearby Pollock Mountain (~13 km to the northeast; Richter et al., 2013). Compositional differences between DCP and RVP rocks probably reflect the nature of melt-generating reactions at different depths (e.g., variable residual garnet abundances, different degrees of melting).