TERTIARY GRANITOID STOCKS OF THE WESTERN OREGON CASCADES: PETROLOGY AND REGIONAL GEOCHEMICAL PATTERNS

Distributed along the length of the Oregon Cascades are at least eight small granitoid stocks associated with the Oligocene-Miocene Western Cascades. Many of these plutons occur to the west of, but at about the same latitude as, the major Quaternary volcanic centers of the High Cascades. This study focuses on three such plutons: (1) the 16.3 Ma Nimrod pluton (~10 km²) located ~50 km to the west of the Three Sisters, (2) the 13.4 Ma Detroit-Bull of the Woods pluton (~3 km²) located ~40 km west of Mt. Jefferson, and (3) the 11.6 Ma Laurel Hill / Still Creek pluton (~15 km²) located ~20 km west of Mt. Hood. Goals of the study are to compare the geochemical characteristics of these plutons with those of their “paired” Quaternary volcanic centers and with the characteristics of granitoid batholiths in the Washington Cascades. Plutons in the Oregon Cascades differ from the latter in that they were emplaced into thinner crust, tend to be less hydrous, and were generated during a time interval (~22 - 11 Ma) that was magmatically quiescent farther north.

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.