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The Co~ebrooke ScMst occupies a triangular area of about 10(} square miles within the Coast Range province of southwestern Oregon. Late c~etaceous thrusting on ,serpentinite "tectonic sheets" has placed much of the Oolebroo•ke Schist on top of Meso~oic eugeosynelinal and miogeosynclinal sedimenta·ry rocks such as the Galice, Otter Point, and Dothan Formations, and the Myrtle Group. Eocene eonglomer,ates of the Umpqua Formation containing clasts from both the Colebrooke Schist a1nd serpentinite rest unconformably on the Oolehrooke Schist and its thrust contacts with the underlying Mesozoic sedimentary rocks. Graywackes of the Otter Point and Dothan Formations are chemically and lithologically similar. Structural and stratigr:aphic evidence suggests that these graywackes may have been deposited at the same time. The large amount of volcanic debris and associated submarine volcanic centers within the westernmost exposed margin of the Otter Point suggests that it may represent the eastern flank of a Late Jt~:ra.ssic island arc. The serpentinite "tectonic sheets" are derived ~rOilll har~burgites and d:unioos. The highly sheared se.rpentinites contain tectonic inclusions of glaucophane schist, diabase, gabbro, and eolebrooke Schist. The serpentinites may repreS<ent parts of the oceanic mantle that were transported to •their present position by repeated tectonic movements. The Colebrooke Schist is predominantly metamorphosed thin-bedded shale and sandstone and associated minor pillow lavas, tuff, and chert. The mineral assemblages of the pelitic and bas;altic rocks reflect metamorphism intermecUate between blueschist and g.reenschist facie·s. The metaba.salts typieaHy contain actinolite, epidote, albite, chlorite, (pumpellyite). The mine.ral assembla.ge fm the pelitic rocks consists of quartz, chlorite, phengitic mica, albite, lawsonite, ·rund epidote. Bulk chemical compositions of Oolebrooke pelitic schists and Galice sedimentary rocks are similar to but distinct from Dothan and Otter Point graywackes. The big'h silica and aluminum, together with low calcium and sodium of the Colelbrooke and Galice Formations, make them compositionally different from typical graywackes.

The Colebrooke Schist of southwestern Oregon and its relation to the tectonic evolution of the region