INTRUSIVE ROCKS OF THE GAME LAKE AREA AND IMPLICATIONS FOR THE ORIGIN OF THE SNOWCAMP OPHIOLITE

Relatively undeformed stocks of hornblende gabbro and muscovite tonalite intrude serpentinized peridotite and deformed amphibolite near Game Lake, Oregon. There, peridotite alternately correlated with the Coast Range or Josephine Ophiolite is in east-dipping fault contact with underlying amphibolite, likely that of the Rogue-Chetco arc complex. The undated gabbro primarily consists of medium-sized grains of brown hornblende and twinned plagioclase with trace olivine; both display igneous textures. The rock is unfoliated but slightly metamorphosed to greenschist grade with minor epidote, chlorite, actinolite, and pumpellyite. The 149.1 ± .4 ma tonalite is primarily plagioclase feldspar and quartz with minor amounts of muscovite, chlorite and biotite and trace zoisite. Neither rock is foliated in hand sample, but the tonalite shows minor deformation in thin section as bent micas, slightly recrystallized muscovite, and a mortar texture in quartz and feldspar.

Geochemically, the hornblende gabbro is classified as basalt while the tonalite plots as trachyte and rhyolite on geochemical classification diagrams. Chondrite-normalized spider diagrams show that the gabbro has a relatively flat LREE trend and are only moderately enriched relative to chondrite, while the tonalite is enriched in LREEs. Both the gabbro and the tonalite show a slight Ta-Nb negative anomaly on MORB-normalized diagrams.

The hornblende gabbro and the muscovite tonalite were likely intruded during the late stages of the back-arc basin collapse when thrusting of the Rogue-Chetco arc complex eastward beneath the ophiolitic rocks occurred; both are found intruding both the upper and lower units of the faulted sequence. The gabbro may be related to other reported hornblende gabbro intruding the Josephine Ophiolite and its roof fault, the Orleans Thrust. The tonalite has previously been correlated to the 145 Ma Pearce Peak Diorite that dominantly occurs to the northwest in areas, such as the Elk outlier, that have alternately been interpreted to be part of the Coast Range Ophiolite and the Josephine Ophiolite. The central location of the Game Lake rocks between the main body of the Josephine Ophiolite to the southeast and the Elk outlier to the northwest suggests a link between these areas.