Abstract
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