SOURCE OF META-IGNEOUS BLOCKS IN THE COLEBROOKE SCHIST, PICKETT PEAK TERRANE, SW OREGON

The Colebrooke (CB) Schist of the Pickett Peak Terrane, SW Oregon, is the easternmost, structurally highest unit of the Late Mesozoic-Cenozoic Franciscan Accretionary Complex and is in fault contact with the Upper Jurassic-Cretaceous Great Valley Group fore-arc basin deposits and its Jurassic Coast Range Ophiolite (CRO) basement. The CB consists of mostly transitional greenschist-blueschist-facies meta-sedimentary rocks with common greenstone blocks ranging from >7 m to >1 km across. A zircon U-Pb age from a meta-plutonic block similar to one analyzed in this study was determined by Plake (1989) to be 162 +/- 1 Ma, which could provide the upper limit to the age of the CB protolith. U-Pb zircon ages in the Snowcamp Mountain area are 169 +/- 1 Ma (Saleeby, 1984) from the CRO and 162 +/- 1 Ma (Harper, 1994) from the nearby Josephine Ophiolite (JO). Fifteen meta-volcanic samples and one meta-plutonic sample were analyzed by XRF and ICP-MS. The CB meta-volcanic rocks plot in Mid-Ocean Ridge basalt (MORB), island arc tholeiite (IAT) and transitional MORB/IAT fields on V-Ti, Th/Yb-Ta/Yb and MORB-normalized spider diagrams. The range of magma types suggests that the CB meta-volcanic blocks are derived from a back-arc basin basalt source. Several of the CB meta-volcanic samples are unusual high iron and titanium (Fe-Ti) MORB, which are also present in the JO. When plotted with CRO and JO volcanic and plutonic samples, there is overlap between the CB samples and the CRO and JO samples. This overlap; the similar age of a block to the JO and CRO; the presence of unusual Fe-Ti basalts; and the back-arc basin affinity of the CB greenstones, JO, and CRO suggest that the CB meta-igneous blocks are sedimentary blocks from the JO, and possibly the CRO as well, deposited as olistostromes in the Early Cretaceous trench off western North America.