Wales Group, undivided

Unit symbol: CPxwg
Age range Cambrian to Proterozoic (1000 to 485.4 Ma)
Lithology: Metamorphic
Group name: Wales Group, undivided
Complex assemblage of intercalated metabasite, metafelsite, metaclastic rocks, marble, and siliceous and carbonaceous black phyllite derived predominantly from andesitic to basaltic marine fragmental volcanic rocks and flows, graywacke, mudstone, and shale that contains locally interlayered marble (unit _<wgm). All rocks of the unit have been regionally deformed and metamorphosed to greenschist and, locally, amphibolite facies. The most abundant and widely distributed lithology is greenish-gray, thinly foliated, commonly crenulated albite-epidote ± quartz ± actinolite schist that is compositionally layered parallel to schistosity and is probably derived from tuffaceous mudstone, siltstone, and graywacke (Eberlein and others, 1983; S.M. Karl, unpub. data). Pillows and centimeter-scale pyroclastic rock fragments are locally preserved. Metasedimentary rocks show relict rhythmic and graded bedding, but other protolith features are mostly obscured by metamorphic recrystallization, penetrative foliation, high degree of flattening, and moderate elongation. Unit includes subordinate black phyllite and schist, meter-thick layers of silicic metavolcanic rocks, and light-colored, coarsely recrystallized marble. Metakeratophyre layers up to 3 m thick are common and typically contain rounded blue quartz eyes and phenocrysts or glomeroporphyritic clots of twinned albite set in a chert-like microscopic groundmass of quartz and albite (Herreid and others, 1978, Eberlein and others, 1983). Unit is folded, crenulated, and lineated (showing preferred orientation of minerals such as actinolite), and has quartz and carbonate boudins up to 5 m thick and 20 m long. Quartz segregation layers parallel and crosscut the foliation, and are concentrated in the crest regions of folds. There is evidence for at least two, and as many as four deformation events in these rocks. Unit believed to be at least several thousand meters thick (Eberlein and others, 1983). K/Ar hornblende age of 483 Ma (Turner and others, 1977) suggests deformation and metamorphism happened prior to the end of the Early Ordovician. Available constraints indicate that rocks in the Wales Group were deposited prior to Late Cambrian time and regionally metamorphosed and deformed before or during Early Ordovician time (Gehrels, 1992)

Source map information

Source map Karl, S.M., 2001, Unpublished data.
Symbol PzZmv
Unit name Metabasite
Description On Prince of Wales Island, includes greenstone and greenschist- to amphibolite-facies schist and gneiss, with lenses of marble and stretched-pebble metaconglomerate. Greenish-gray to greenish-black tuffaceous schist forms great thicknesses in Hetta and Kassa Inlets, with occasional interbedded metakeratophyre and basic metavolcanic layers up to 3 m thick. Schist is fine-grained with alternating chloritic and quartzose layers from 1 mm to several cm thick. Contains up to 5 per cent albite grains up to 3 mm long. The foliation swirls around the grains, indicating they formed prior to the end of deformation. Protolith inferred to be tuffaceous mudstone. Metamorphic minerals include Twinned albite, chlorite, carbonate, iron ore, leucoxene, biotite, actinolite. Includes beds of metakeratophyre tuff(?), metakeratophyre flows, and metaspilite. Metakeratophyre tuff(?) is the dominant rock type, and occurs as nearly massive porphyritic metafelsite beds 8 cm to 3 m thick, comprising up to 10 per cent of green tuffaceous schist and gray phyllite sections. Contacts are sharp to gradational. Keratophyre tuff(?) is light to medium gray with chlorite, and contains approximately 3 per cent macroscopic albite phenocrysts, and a smaller percent of quartz anhedral. Chloritic, bluish- to dark greenish-gray phyllitic amygdaloidal porphyritic metafelsite keratophyre flows are up to 300 m thick at Eek Point. These flows contain twinned albite phenoblasts to 3 mm long, and almond-shaped amygdules to 1 cm in diameter composed of quartz, carbonate, and epidote. Metaspillite west of Eek Point and near Copper City mine are 100 m thick and consist of bluish-gray fine-grained rock with flow bands, 5- to 25-mm long amygdules, and contain spillitic bombs up to 1 m long. Metamorphic minerals include quartz, albite, chlorite, sericite, epidote, and magnetite
Lithology Metamorphic

Correlated geologic units

Label PzZmv
Description Wales Group mafic volcanic rocks
Geologic age Ediacarian to Neoproterozoic
Geologic setting Extrusive, mafic
Lithology Form Importance
Greenstone < Metaigneous < Metamorphic Major
Metabasalt < Metavolcanic < Metaigneous < Metamorphic Indeterminate, major
Amphibole-schist < Schist < Metamorphic Indeterminate, major