AMPHIBOLITE OF THE GAME LAKE REGION AND ITS RELATION TO THE JOSEPHINE AND COAST RANGE OPHIOLITE

Geochemical analysis of amphibolite (Briggs Creek Amphibolite) from the Game Lake area of the Snowcamp terrane, southwest Oregon shows that it is arc-related, rather than MORB-related. Well-foliated 154 Ma amphibolite is thrust beneath serpentinized peridotite along the Game Lake Thrust; the timing and T-P conditions of thrusting are similar to that of the Madstone Cabin Thrust that separates the Josephine Ophiolite from an underlying amphibolite sole of the Rogue-Chetco arc complex. This similarity suggests that the Snowcamp Ophiolite, previously correlated with the Coast Range Ophiolite may be related to the Josephine Ophiolite, or alternately, the Snowcamp Terrane is part of the Josephine Ophiolite and not part of the Coast Range Ophiolite.

The Briggs Creek Amphibolite at Game Lake was metamorphosed to amphibolite grade and is mainly composed of medium-grained metamorphic plagioclase and blue-green hornblende, with smaller amounts of calcite, biotite, and opaques. The hornblende and plagioclase are strongly foliated and likely formed during thrusting beneath the peridotite at Game Lake. Few brown hornblende grains are interpreted to be relict igneous phases. The amphibolite was exposed to greenschist retrograde metamorphism with actinolite, chlorite, and epidote replacing hornblende. Plagioclase is variably sausseritized.

The amphibolite has previously been interpreted to be ocean crust based on its geochemistry. In contrast, our geochemical data indicate an arc origin; the MORB-normalized spider diagram shows a negative Ta-Nb anomaly that resembles arc basalts. On the chondrite-normalized diagram, most amphibolite samples are depleted in LREE although a couple are enriched in LREE. Element mobility is a concern due to amphibolite facies metamorphism, however our data overall are relatively consistent; Harker diagrams show some mobility in a few elements, but there is reasonable confidence in an arc interpretation. The Ti-V diagram also supports an arc interpretation for the amphibolite.

The interpretation that the amphibolite at Game Lake is arc-related strengthens the argument that the ophiolitic rocks there are part of the Josephine Ophiolite and not the Coast Range Ophiolite. Alternately, the Josephine and Coast Range ophiolites have a common origin.