The Colebrooke Schist of the Pickett Peak terrane, southwestern Oregon, is the easternmost, structurally highest unit of the Late Mesozoic-Cenozoic Franciscan Accretionary Complex. The Colebrooke Schist consists of mostly transitional greenschist-blueschist-facies meta-sedimentary rocks with common blocks of meta-volcanics and serpentinites, rare talc-schists and meta-plutonic rocks. The Colebrooke Schist meta-volcanic blocks are greenstones, in many cases with visible relict pillow structures and relict igneous textures.
Fifteen meta-volcanic samples and one meta-plutonic sample were analyzed by XRF and ICP-MS and were plotted with analyses from Plake (1989) and Coleman (1972). The Colebrooke Schist 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 Cr-Y diagrams. The range of magma types suggests that the Colebrooke Schist meta-volcanic blocks are derived from a back-arc basin basalt source. The Colebrooke Schist contains unusually high iron and titanium (Fe-Ti) MORB. The Colebrooke Schist analyses were separated into five individual large blocks. The Quosatana Butte, Skookumhouse Butte and Copper Canyon blocks have MORB affinities, whereas blocks, in a serpentinite melange underlying the Colebrooke Schist, from Mineral Hill and Saddle Mountain are transitional MORB/IAT. The geochemical similarity of Colebrooke Schist samples with Coast Range ophiolite and Josephine ophiolite, the similar age of a block in the Colebrooke to the Josephine ophiolite, as well as geochemical similarities and probable pebbly mudstone matrix all suggest that some of the Colebrooke Schist meta-igneous blocks may be sedimentary blocks derived from the Josephine and possibly also Coast Range ophiolites, deposited as olistostromes in the Early Cretaceous trench off western North America. Very large, MORB affinity blocks allows the possibility that at least some of the Colebrooke Schist blocks are dismembered remains of the ocean floor basement.
The Colebrooke Schist has undergone three deformation events. D1 consists of foliation first observed by Coleman (1972) and later described by Plake (1989) as alignment of platy minerals and the flattening of relict pillow structures. D2 consists of crenulation cleavage, S2, and folds, F2 (Coleman, 1972 and Plake, 1989). Plake (1989) described D3 features as an S3 crenulation cleavages and F3 folds without axial planar cleavage. Foliations, equivalent to S1 from Plake (1989), measured throughout the field area generally vary in strike from northwest to northeast and dip to the east and define a broad girdle on an equal area projection. The orientations of stretching lineations (equivalent to L1 of Plake, 1989) are scattered with no clear average value, although most plunge gently to moderately southeast or northwest. These shallow plunges and variable trends are consistent with rotation about a vertical axis, which supports Plake's (1989) proposal that the Colebrooke Schist has undergone rigid block rotation. The variation in foliation and lineation measurements is likely the result of one or a combination of three possibilities: 1) post-metamorphic folding following D3, 2) drag folding along the late north- to northeast-striking strike-slip faults, or 3) extensive shearing associated with veining throughout the Colebrooke Schist.
Katrib, J., 2006. Source of meta-igneous blocks and structure of the Colebrooke Schist in the Snowcamp Peak area, Pickett Peak terrane, southwestern Oregon.
Unpublished MSc. thesis, State University of New York at Albany. 111 pp., + xiii
University at Albany Science Library call number: SCIENCE Oversize (*) QE 40 Z899 2005 K38
MS thesis scanned
text pdf file (9MB);
Fig. 30 - Geological map of the Snowcamp Mountain area, southwest Oregon
(coloured geological map; scale ~1:50,000; detailed local inset) - 0.5MB pdf file