ZIRCON GEOCHEMISTRY AND PB-U (SHRIMP-RG) AGES OF THE CORNUCOPIA STOCK, NORTHEASTERN OREGON, AND A RE-EXAMINATION OF MAGMA SOURCES

The Cornucopia stock in the Blue Mountains Province (BMP) of NE Oregon and W Idaho is a composite intrusion comprising the hornblende-biotite-bearing Cornucopia tonalite, the biotite-bearing Tramway trondhjemite, and three cordierite-two mica trondhjemites (Pine Lakes, Big Kettle, and Crater Lake trondhjemites). These were emplaced in the Wallowa island arc terrane and represent the last episode of magmatism associated with accretion of the BMP to the North American continent in the Early Cretaceous. New Pb-U zircon ages were determined using SHRIMP-RG from all five intrusions. Zircon exhibits simple compositional zonation in CL images, which yielded relatively simple age distributions (MSWD: 0.53 to 1.44): Cornucopia tonalite, 121.3±1.6 Ma (2σ; n=11/12); Tramway trondhjemite, 122.0±1.5 Ma (n=12/12); Pine Lakes trondhjemite, 125.7±1.6 Ma (n=13/13); Big Kettle trondhjemite, 121.4±2.5 Ma (n=12/12); Crater Lake trondhjemite, 121.6±1.5 Ma (n=13/14). A minor ~132 Ma inherited component was observed in zircon populations from the Pine Lakes and Crater Lake intrusions. Average Ti_(zirc) for each sample ranges from 2.95 to 3.75 ppm, resulting in average Ti-in-zircon temperatures of 711-725 °C, similar to calculated zircon saturation temperatures from whole-rock compositions. Biotite ⁴⁰Ar/³⁹Ar ages of 116.8 Ma (Johnson et al., 1997, J. Petrol.) indicates the Cornucopia magmas cooled at a rate of ~72 °C/Ma (from zircon crystallization through the closure temperature of Ar in biotite), which suggests the magmas were emplaced into upper crust heated by previous magmatism. Chondrite-normalized REE patterns of melt compositions calculated from zircon geochemistry are similar in shape to corresponding whole rock patterns but differ in having slight negative Eu-anomalies and higher ∑REEs. REE patterns for cordierite-bearing melts have slightly elevated HREE contents relative to cordierite-free melts. The patterns are consistent with partial melting of metabasaltic rocks at P>10 kbar. However, an additional immature sedimentary component (greywacke?) may have played a role in the generation of the cordierite-bearing trondhjemites, which is consistent with a recent study by Peck (2016; Am. Mineral.) that showed that CO₂ in cordierite from the Crater Lake intrusion has an organic carbon isotope signature.