FELSIC EOCENE VOLCANIC ROCKS ON THE OLYMPIC PENINSULA AND THEIR IMPLICATIONS FOR RIDGE SUBDUCTION

Recent work (Haeussler et al., 2003) attributes adakites and felsic near trench magmatism from Oregon to Alaska to the subduction of two spreading ridges: the Kula-Resurrection and the Kula-Farallon. This model implies that both ridges should have been subducted in the vicinity of the Olympic Peninsula / southern Vancouver Island at ~50 Ma. We present data on four felsic volcanic deposits of Eocene age on the Olympic Peninsula that may help to constrain the timing and geometry of one or both of these ridge subduction events. Adakites have been identified at three of these volcanic centers: Bremerton Hills (BH; Tepper et al., 2002), Chimacum (CR; Hahn et al., 2004), and most recently at Mt. Zion (MZ). MZ adakites occur as one or more porphyritic lava flows interbedded within Lyre Formation conglomerates and also as clasts within this unit. MZ lavas share many attributes of BH and CR adakites (e.g., Sr/Y > 20, Yb < 1 ppm) but differ in both phenocryst assemblage and chemistry. CR and BH adakites contain plag + hb ± qtz while MZ lavas also contain cpx ± ol. The three adakite sites also differ in mean Sr/Y (MZ=28, CR=61, BH=58), mean Y content (MZ=10.4 ppm, CR=7.3 ppm, MZ=7.6 ppm) and mean La/Yb(n) (MZ=12, CR=14, BH=17). These chemical and mineralogical differences are consistent with field evidence that the rocks at each locality were deposited in close proximity (< ~5 km) to a separate and now-eroded eruptive center, and imply there were multiple sites of adakite volcanism within a >400 km² area of the NE Olympic Peninsula.

Felsic volcanic rocks at Striped Peak (SP) are ~50 km west of the adakite localities and include water-lain crystal-vitric tuff with pumice clasts up to 3 cm in size and tuffaceous sandstones. These rocks are interbedded within Crescent Formation basalts (>48 Ma), implying that they are older than the adakites (46-48 Ma at BH). Based on their inferred age and proximity to southern Vancouver Island, we suggest the SP volcanics may be associated with ~50 Ma magmatism in the Leech River Complex or the ~51 Ma Flores volcanics. Ongoing geochemical and geochronological work will further examine this possibility.