Petrology of the Crystalline Rocks of Vedder Mountain, British Columbia

Vedder Mountain lies in Washington state and British Columbia on the western flank of the North Cascades. Metamorphic rocks, which comprise most of the western half of the mountain, can be divided into two separate and distinct units. The southern unit is composed of foliated and non-foliated hornblende-plagioclase gabbro with minor serpentinite and pyroxenite. Most of the gabbroic rocks are sheared and altered and exhibit a lower greenschist facies metamorphism overprinted by a subsequent prehnite-pumpellyite facies metamorphism. Whole-rock chemical analyses of the gabbros plot in and near the fields defined by oceanic gabbro (Engel and Fisher, 1969; Miyashiro et al, 1970; Thompson, 1973), California ophiolite gabbro (Bailey and Blake,1974) and Turtleback gabbro from Fidalgo Island (Brown et al, 1977). The northern unit is composed of albite-epidote amphibolites, barroisite schists, actinolite schists and graphitic-pelitic phyllites and schists. These rocks lack a prehnite-pumpellyite facies overprinting, and metamorphic mineral assemblages and mineral compositions are similar to those found in high pressure rocks of the Sanbagawa crystalline schist terrane of Sikoku, Japan (Iwasaki, 1963; Banno, 1964, and personal communication, 1976; Kanehira, 1967). Although the rocks comprising the northern metamorphic unit have been thought to be part of the pre-Middle Devonian Yellow Aster-Turtleback Complex (Misch, 1966), they appear strikingly similar to and could be high-grade equivalents of the Shuksan Greenschist/Blueschist and Darrington Phyllite units which comprise the Shuksan Metamorphic Suite found to the south in the North Cascades of Washington. Radiometric ages obtained by Dr. Richard Armstrong (personal communication, 1976) on northern unit muscovites, hornblendes and whole-rock samples are the same as the Late Permian-Early Triassic radiometric age of Shuksan metamorphism (Misch, 1966). Whole-rock chemical analyses of northern unit metabasites and the Shuksan Greenschist/Blueschist indicate that both units were derived from basaltic parent material. Analyses of the former cluster tightly, and plots of the analyses demonstrate that the basalts were tholeiitic. Analyses of Shuksan Greenschist/Blueschist show that the rocks have a diverse chemistry and could also be tholeiitic. Graphitic-pelitic phyllites and schists of the northern unit are chemically similar to samples from the Darrington Phyllite. Critical mineral assemblages of the northern metamorphic unit are adjacent or in close proximity to one another. This fact indicates that the variation in the bulk-rock chemical composition, rather than variation in the P-T conditions of metamorphism, accounts for the observed variation in assemblages. The following reaction has been defined for Fe3+-poor epidote amphibolite: .233 quartz + 1.069 chlorite + 2.663 albite + 1.000 epidote = 1.118 hornblende + 1.908 paragonite + .207 garnet + 1.750 H2O + .143 Fe2O3 Conditions of metamorphism in the northern metamorphic unit are unknown but are believed to have been close to those in the Sanbagawa schists. Although the pressures of metamorphism in the northern unit and the Shuksan Metamorphic Suite are believed to have been similar, temperatures in the former were probably higher, as indicated by greater AlIV values of northern unit amphiboles. This means that if the rocks comprising the northern metamorphic unit are equivalents of the Shuksan Metamorphic Suite, then they are higher grade equivalents.