Petrology, Geochemistry, and Genesis of High-Al Tonalite and Trondhjemites of the Cornucopia Stock, Blue Mountains, Northeastern Oregon

Abstract The Cornucopia stock, in the Blue Mountains of northeastern Oregon, is a small composite intrusion comprising five distinct intrusive units: a hornblende biotite tonalite, a biotite trondhjemite, and three cordierite-bearing two-mica trondhjemites. The stock was emplaced at shallow levels (<2 kbar) in island arc-related metasedimentary and metavolcanic rocks of the Wallowa terrane. The age of the intrusion is 116.8±1.2 Ma determined by 40Ar/39Ar incremental heating of biotite. These ages and volume constraints imply coeval emplacement of the tonalitic and trondjhemitic magmas. Tonalitic and trondhjemitic compositions span a narrow range of SiO2 content (65–74 wt %) and exhibit characteristics of a high-Al tonalite–trondhjemite–granitoid (TTG) suite, including light rare earth element (LREE) enrichment, low Y, Nb and Rb/Sr, and high Al2O3 and Sr. These compositions are consistent with an origin by 10–40% partial melting of a low-K tholeiitic source (with a relatively flat REE pattern), similar to metaigneous basement rocks of the Wallowa terrane, in equilibrium with a garnet pyroxene hornblendite residue. High Sr in the TTG rocks, lack of calculated residual plagioclase, and abundant calculated residual amphibole suggest that H2O in excess of that produced by amphibole dehydration was present at the site of melting. Conditions of partial melting are loosely constrained to pressures ≥10 kbar (residual garnet implied by REE abundances) and temperatures exceeding 900–950°C (magmatic temperatures based on apatite solubility). We suggest the Cornucopia tonalitic and trondhjemitic magmas formed by hydrous partial melting of lower island arc crust, probably as a result of underplating by, or intrusion of, mantle-derived basaltic magmas. Mantle melting may have been triggered by rapid uplift of Wallowa terrane crust, as the Blue Mountains terrane assemblage collided with the continental margin during Early Cretaceous time. Results of this study suggest that deformation and crustal melting associated with the accretion event were not strictly limited to the suture zone.