Blueschist of southern Alaska

Unit symbol: JTrsch
Age range Early Jurassic to Triassic (235 to 174.1 Ma)
Lithology: Metamorphic
Group name: Blueschist of southern Alaska
Diverse, well-foliated, multiply-deformed, variably foliated, metasedimentary and metavolcanic rocks closely associated with the Border Ranges Fault System. Sedimentary protoliths were shale, chert, tuffaceous arenite, and limestone; volcanic protoliths were probably mostly basalt (Winkler, 1992). Consists of thinly layered and complexly folded quartz-mica schist, greenschist, crossite-epidote schist (blueschist), and epidote amphibolite (Connelly and Moore, 1979). Whereas metamorphism varies from greenschist to amphibolite facies (Winkler, 1992), locally glaucophane-bearing and crossite-bearing rocks suggest unit was metamorphosed under conditions transitional to and including blueschist facies (Forbes and Lanphere, 1973; Carden and Decker, 1977; Carden and others, 1977; Connelly and Moore, 1979; Winkler and others, 1981; Bradley and others, 1999). Outcrops along northern flank of Chugach Mountains in Anchorage, Seldovia, and Valdez quadrangles are closely associated with Border Ranges Fault System and the McHugh Complex. Unit is also exposed along the west side of Afognak, Shuyak and Kodiak Islands, associated with the Border Ranges Fault System and the Uyak Complex. Includes the schist of Liberty Creek of Winkler and others (1981; Richter and others, 2006), the Knik River schist of Winkler, 1992 (also called the Knik River terrane of Pavlis, 1983; Pavlis and others, 1988), the Seldovia Schist of Roeske (1986; see also Cowan and Boss, 1978; Bradley and others, 1999), and the Raspberry Schist of Roeske and others (1989). Raspberry Schist of Roeske and others (1989) was subdivided into two units that consist of (1) metabasite and metasedimentary rocks that have some relict textures and phases preserved, including pillow shapes and bedding and (2) a unit that is completely recrystallized and preserves no original textures or phases. Carden and others (1977) reported 10 K/Ar ages on mica, chlorite, and amphibole from unit that range from 196.6±5.8 to 157.8.3±4.8 Ma, and Bradley and Karl (2000) reported 40Ar/39Ar plateau ages on mica of 191.7±0.3 and 190.98±0.3 Ma and on hornblende of 191.92±0.6 Ma. Roeske and others (1989) reported Rb/Sr isochron ages ranging from 212 to 189 Ma. Dating of the blueschist facies metamorphism of this metamorphic complex supports a pattern of decreasing age from west to east, as originally suggested by Sisson and Onstott (1986); however, it is unlikely that the reported ages are a true reflection of the timing of metamorphism. Clark (1972, cited in Winkler, 1992) reported a single Permian fossil collection from limestone. Winkler (1992) suggested that the diverse protolith lithologies may indicate tectonic mixing prior to metamorphism

Source map information

Source map Winkler, G.R., compiler, 1992, Geologic map and summary geochronology of the Anchorage 1° x 3° quadrangle, southern Alaska: U.S. Geological Survey Miscellaneous Investigations Series Map I-2283, scale 1:250,000.
Symbol JPzm
Unit name Metamorphic rocks undivided
Description Metasedimentary and metavolcanic rocks. Strongly to weakly foliated and variably metamorphosed from greenschist to amphibolite facies. Intruded by Jmip and Jg rocks. Sedimentary protoliths include shale, chert, tuffaceous arenite, and limestone. Volcanic protoliths are most probably basalt. The diversity of protoliths may indicate tectonic mixing prior to metamorphism. Fabric of metasedimentary rocks is cataclastic or recrystallized.
Lithology Metamorphic

Correlated geologic units

Label Jsch
Description Schist of Kodiak Island, Raspberry Schist of Roeske and others (1989). Equivalent to Seldovia Schist and Schist of Iceberg Lake in Cook Inlet Region as well as Schist of Liberty Creek
Geologic age Late-Triassic to Early-Jurassic
Geologic setting Metamorphic, undivided
Lithology Form Importance
Mica-schist < Schist < Metamorphic Greenschist Major
Metabasalt < Metavolcanic < Metaigneous < Metamorphic Glaucophane-schist Indeterminate, major
Marble < Metacarbonate < Metasedimentary < Metamorphic Indeterminate, major
Phyllite < Metaclastic < Metasedimentary < Metamorphic Minor