STRATIGRAPHIC, PETROGRAPHIC, GEOCHEMICAL, AND STRUCTURAL ANALYSIS OF PILLOW LAVAS AND OVERLYING METASEDIMENTS: COLEBROOKE SCHIST, SOUTHWESTERN OREGON

The Colebrooke Schist (CS) is part of the Late Mesozoic-Cenozoic Franciscan Complex. It consists of metamorphosed shale and sandstone with minor pillow lava (PL), tuff and chert of blueschist-greenschist grade. We examined a quarry in the CS, 7.3 km NW of Agness, Oregon, 1 km south of Upper Jurassic Galice metasediments, to obtain a detailed history of the Colebrooke sea floor. The quarry exposes a shallowly NW-plunging anticline, cored by flattened PL and pillow breccias overlain by foliated metasediments. The rocks are cut by steep quartz-epidote veins and faults. Locally, a distinct, dark-colored, metalliferous metasediment (MM) overlies the PLs.

PLs contain relict augite, altered plagioclase, and secondary pumpellyite, chlorite, quartz, and sphene, calcite, chlorite, and stilpnomelane or biotite. Pillow breccias have altered pillow fragments in a fine matrix of augite grains, basalt fragments, and secondary calcite, quartz, biotite, pumpellyite, epidote, and blue amphibole. The overlying metasediments are foliated cherts and argillites with varying amounts of opaque minerals and randomly oriented biotite or stilpnomelane.

The MM and a PL , located 2 cm beneath it, were analyzed geochemically. The PL is LREE-depleted, has 16.55 wt. % FeO* 3.62 wt.% TiO₂making it a Fe-Ti basalt, has a MORB-like Ti/V of 41, plots in the MORB field within the mantle array on a plot of Th/Yb vs Ta/Yb. The MM contains 28.4 wt% FeO*, 8.9 wt.% MnO, 1567 ppm Cu, 342 ppm Zn, 264 ppm Ni and 1531 ppm V; however, the extremely high concentrations of Cu, Mn, and V exceed the lab’s highest calibration standards. In addition, sulfur-loss on heating led to low totals. Although the results are semi-quantitative, the high metal concentrations are typical of modern metalliferous sea-floor sediments.

The MM’s location immediately above the PLs is evidence for on-axis hydrothermal activity suggesting a ridge-transform intersection rather than a propagating rift-tip where metal-rich layers would be higher in the section although such sediments occur in both environments. The chert and argillite evidence a pelagic environment with pulses of terrigenous mud. The folded foliation records two episodes of deformation. Pumpellyite, greenschist- and blueschist-facies metamorphic minerals suggests progressive metamorphism and indicates subduction.