TERTIARY GRANITOID STOCKS OF THE WESTERN OREGON CASCADES: PETROLOGY AND REGIONAL GEOCHEMICAL PATTERNS
Among Oregon Cascades stocks the Nimrod is unique as it is the only true granite. The rock is fine to medium grained, equigranular, and locally miarolitic. Samples collected along a ~4.1 km transect where the pluton is bisected by the McKenzie River reveal that the pluton is chemically zoned from pyroxene quartz diorite at the western margin to true granite in the east, with swarms of mafic enclaves near the center of the transect. Systematic west to east chemical variations, including decreasing Sr (551 - 157 ppm), Sc (27.4 - 6.0 ppm), and Eu/Eu* (1.10 - 0.49), and increasing Rb (24 - 74 ppm), Th (1.8 - 14.4 ppm), LREE (La=13.1 - 31.0 ppm), and La/YbN (5.6 7.1) are consistent with >60% crystallization involving plagioclase + pyroxene. REE data suggest amphibole did not play a major role in the evolution of the Nimrod stock. This is also indicated in thin sections of the granite, where amphibole and biotite (now altered) appear to have crystallized late, implying a low water content in the magma. These results indicate that the tendency of Oregon Cascades plutons to be less hydrous than those in the Washington Cascades is apparent even in the most felsic intrusion.