Metavolcanic rocks west of the Coast plutonic complex of Brew and Morrell (1979b)

Unit symbol: MzPzsv
Age range Triassic to Paleozoic (541 to 201.3 Ma)
Lithology: Metamorphic
Group name: Metamorphic rocks associated with the Coast plutonic complex of Brew and Morrell (1979b)
Interbedded or structurally intercalated with unit }|ss are andesitic to basaltic metavolcanic rocks. In the northern part of southeast Alaska green, greenish-gray, and greenish-black very fine to fine-grained, massive to pillowed and amygdaloidal mafic volcanic flows and minor tuff constitute some of the least metamorphosed rocks of this unit (Plafker and Hudson, 1980; Redman and others, 1984). “Characteristically, the metabasalt is vesicular and (or) amygdaloidal, and the amygdules are mainly chlorite or quartz. Most of the metavolcanic unit appears to consist of massive to inconspicuously layered flows, with local breccia and thin zones of recrystallized tuff. At the top of the unit on the Chilkat Peninsula an estimated 300–400 m of section contains well developed pillow structures. * * * An incipient cleavage is present in some of the metavolcanic rocks on the Chilkat Peninsula. Epidotized shears and clots, chlorite patches, and disseminated pyrite are common throughout the unit” (Plafker and Hudson, 1980). This part of unit is up to 3.4 km thick (Redman and others, 1984). Plafker and Hudson (1980) report fossiliferous gray to dark-gray coquina limestone that commonly has nodular to layered black chert and abundant light-green fragments of devitrified glass that occurs in discontinuous lenses as thick as 90 cm between pillowed flows within a few tens of meters of the top of the volcanic unit. Late Triassic (Carnian?) well preserved ammonites, belemnites, and flat clams were found in situ and as float on the adjacent beach. The rocks of the Chilkat Peninsula are generally metamorphosed to lower greenschist facies; the metamorphic grade increases to lower amphibolite facies and the rocks become more strongly foliated near the Canadian border (Gilbert and others, 1987; Gilbert and others, 1988b; Dodds and Campbell, 1992). Southeast of the Chilkat Peninsula, Gehrels and others (1992) mapped Triassic metabasalt flows in their Perseverance group where “basalt low in the section is commonly massive and dark green, which contrasts with the light green color, chloritic foliation, and fragmental or pillowed nature of the underlying Permian metabasalt. Some of the massive layers are microgabbro sills. Higher in the section the metabasalt remains dark green but is commonly pillowed and feldspar porphyritic. Fragmental basalt and pillow flows with interpillow carbonate occur locally. Interpillow carbonate and associated thin limestone layers in the Carnian [Upper Triassic] part of the section commonly contain moderately deformed ammonites.” Also included in this map unit are Permian metavolcanic rocks of the Gastineau group of Gehrels and others (1992) that consist of light green pillow flows, pillow breccia, flow breccia, tuff breccia, and tuff of basaltic composition; crinoid-rich marble layers that range from less than 1 meter to more than 50 m thick; interpillow carbonate and calcareous layers and matrix in tuffaceous rocks; and up to several-meter-thick layers of interbedded black phyllite and light gray chert. “In general, the proportion of marble, metapelite, and chert increases upsection, and the top of the sequence is in many areas a massive, semi-continuous marble layer. There is also an increase upsection in fragmental basalt relative to pillow flows. Most metabasalt is aphyric, but small pyroxene and/or feldspar phenocrysts are present in some areas” (Gehrels and others, 1992). The Gastineau group is up to 3 km thick and contains conodonts of probable Permian and Carboniferous(?) age (Gehrels and others, 1992). We include both the Triassic and Permian metavolcanic rocks in this unit because, in general, they are not separately mapped to the southeast where equivalent rocks are grouped together in the Alava sequence of Rubin and Saleeby (1991) or mapped as undivided and unnamed hornblende schist (Brew and others, 1984; Berg and others, 1988). The metavolcanic rocks of the Alava sequence are metamorphosed intermediate to mafic volcanic pillowed and flow rocks, breccia and tuff, agglomerate, volcaniclastic rocks, and subordinate marble and siliceous limestone, with preserved primary textures. The Triassic part of the Alava sequence is inferred to rest unconformably on the upper Paleozoic part of the unit and is associated with black phyllite and carbonaceous and argillaceous marble. The late Paleozoic volcanic rocks are associated with finely laminated light-brown to light-gray crinoidal marble and argillite. Generally the Triassic rocks are less deformed than the Paleozoic rocks; rocks closer to the Coast Plutonic Complex of Brew and Morrell (1979b) are higher metamorphic grade (Himmelberg and others, 1991; Stowell and Crawford, 2000). In the Juneau and Sumdum quadrangles the unit consists of mafic and quartzofeldspathic schist derived from basaltic, rhyolitic, and probably dacitic volcanic rocks, as well as biotite schist derived from pelitic strata, quartzite derived from quartz-rich clastic strata, marble. In general, the proportion of metavolcanic rocks increases upsection (southwestward). Quartz-porphyritic metarhyolite with large blue quartz phenocrysts in a centimeter-scale fragmental texture is a diagnostic rock type in the Sumdum quadrangle. Metarhyolite is commonly interlayered with metabasalt that displays fragmental and pillow structures. In the lower part of the section in these quadrangles, metaclastic quartzite and quartz-cobble conglomerate are common (Brew and Grybeck, 1984; Gehrels and others, 1992). Unit is mostly greenschist facies but is transitional to amphibolite facies to the southeast, where it consists of fine- to medium-grained, medium to dark green hornblende schist and gneiss, including significant amounts of biotite gneiss, quartzite, biotite-hornblende gneiss, and poorly foliated amphibolite

Source map information

Source map Brew, D.A., and Ford, A.B., 1985, Preliminary reconnaissance geologic map of the Juneau, Taku River, Atlin, and part of the Skagway 1:250,000 quadrangles, southeastern Alaska: U.S. Geological Survey Open-File Report 85-395, pamphlet 23 p., 2 sheets, scale 1:250,000.
Symbol TKs
Unit name Schist, undivided
Description MAINLAND BELT: METAMORPHIC ROCKS OF COAST PLUTONIC-METAMORPHIC COMPLEX: Includes chlorite schist, greenschist, biotite and garnet biotite schist, hornblende and garnet hornblende schist, calc-silicate schist, marble, and quartzite; exposed between the Coast Range megalineament and the intrusive Rocks of the Coast Plutonic metamorphic Complex Sill Belt, etc. unit.
Lithology Metamorphic

Correlated geologic units

Label PzZyv
Description Metabasite, massive mafic semischist and agmatite in the Coast Mountains
Geologic age Permian to Triassic
Geologic setting Metamorphic, undivided
Lithology Form Importance
Hornblende-gneiss < Gneiss < Metamorphic Major
Amphibolite < Metamorphic Indeterminate, major
Amphibole-schist < Schist < Metamorphic Minor
Biotite-schist < Mica-schist < Schist < Metamorphic Minor