GEOCHEMISTRY OF CENOZOIC PLUTONIC ROCKS IN THE WESTERN CASCADES: TRACERS OF ARC EVOLUTION & ORE GENESIS

The western Cascades arc extends from northernmost California to southern British Columbia and contains volcanic rocks (~45-10 Ma) and plutonic rocks (~25-10 Ma) that are ancestral to the modern Cascades arc.

Miocene epizonal plutons are a low-K suite ranging from pyroxene diorite to hornblende-biotite tonalite-granodiorite (52 to ~74 wt.% silica). With increased silica, Sr, V/Sc, and Sr/Y decrease (~500 to 200 ppm, ~9 to 7, and ~50 to <10) and Ba, Nb, Th, and Ta increase (ca. 100-700, 3-10, 3-15, 0.2-1.0 ppm). Ba/Nb (~20-100) increases from north to south. REE patterns generally lack negative Eu anomalies, which are only present in evolved compositions that have U-shaped patterns reflecting amphibole crystallization. These geochemical characteristics suggest modest oxidation states (ΔNNO=0 to +1), fractionation dominated by pyroxene and plagioclase, contributions of subducted slab-derived components (Ba), and evolution to more hydrous compositions.

New SHRIMP-RG and LA-ICP-MS U-Pb analyses of zircon from twelve samples collected along strike of the ancestral arc in Washington and Oregon indicate plutons associated with porphyry Cu-Au and epithermal Au deposits were emplaced between 23 and 13 Ma. Zircons from eleven intrusions have large negative Eu/Eu* anomalies and higher Gd/Yb ratios that differ from the small negative Eu/Eu* anomalies and generally lower Gd/Yb ratios of the 19.3 Ma Mount Margaret porphyry (MM) and other porphyry Cu mineralizing plutons elsewhere in the Cordillera. Likely, the large negative Eu/Eu* anomalies of most Cascades plutons reflect thin crust and relatively low water contents, consistent with observations of smaller hydrothermal mineral deposits. The MM porphyry is associated with a large porphyry Cu-Au deposit and was produced by hydrous melt with low Gd/Yb likely due to amphibole fractionation.

We also identified 23 (inherited) zircon grains from the intrusions that have ages between 63 and 30 Ma consistent with derivation from the Eocene Tyee turbidite sandstones and older Western Cascades volcanic rocks. These inherited grains suggest the Western Cascades in northern Oregon and southern Washington is built on thin Cenozoic crust consisting of primarily Siletzia oceanic basalts covered by the Tyee.