Geochemical Evidence for a Dense Mafic Root of Units In the Wallowa Batholith, Wallowa Mountains, Northeast Oregon

A suite of Jurassic-Cretaceous intrusive bodies, including the Wallowa Batholith (WB) in the Wallowa Mountains, intruded into amalgamated Permian to Jurassic terranes of the Blue Mountains Province in northeastern Oregon. These rocks are exposed ~50 km west of the ⁸⁷Sr/⁸⁶Sr 0.706 line, denoting formation within lithosphere that is allocthonous with respect to cratonic North America. Recent modeling of the Miocene uplift patterns of the WB (Hales et al., 2006, Nature) suggests that a lower crustal delamination event occurred as a precursor to the eruption of the Columbia River Basalt Group, but lacks evidence for the identity of the delaminated body. The goal of this study is to test whether or not the formation of part of the WB created a dense lower crustal residuum that was involved in the delamination event. Wallowa Batholith rocks are mainly tonalitic to granodioritic I-type granitoids that are undersaturated in aluminum (ASI values < 1.1), have N₂O > K₂O, and have low δ¹⁸O. While these chemical values are similar to other Jurassic-Cretaceous intrusive rocks of the Blue Mountains province, the WB also contains units that are saturated in aluminum and have pronounced LREE enrichment (La/Gd ≈ 4) and HREE depletion (Gd/Yb ≈ 2), indicating fractionation of garnet and amphibole during partial melting. Trace element modeling suggests that the Blue Lake trondjhemite and Cornucopia stock (Johnson et al., 1997, J. Pet.) granitoid magmas of the WB formed by melting within the garnet stability field, and support a garnet-amphibolite residuum that formed by melting of metabasalts in the lower crust of the Wallowa terrane. Dehydration-melting experiments on amphibolite at 10 kbar (Wolf and Wyllie, 1994, Contrib. Mineral Petrol.) suggest that residuum with a density near that of underlying upper mantle can form in equilibrium with melts similar to granitoids of units within the WB.