Kanektok metamorphic complex and Idono Complex

Unit symbol: Xio
Age range Paleoproterozoic (2500 to 541 Ma)
Lithology: Metamorphic
Group name: Kanektok metamorphic complex and Idono Complex
The Kanektok metamorphic complex, informally named by Hoare and Coonrad (1959a, 1961a, 1978, 1979), crops out in the northwest Goodnews Bay quadrangle as a narrow belt that trends northeast extends northward 160 km into the southern part of the Bethel quadrangle. The Idono Complex of Gemuts and others (1983; Miller and others, 1991; Miller and Bundtzen, 1994; Wilson and others, 1998) is similar in many respects to the Kanektok metamorphic complex and crops out in a number of small isolated exposures in the Iditarod quadrangle. The Kanektok metamorphic complex, which was also called the Kilbuck terrane by Jones and others (1981), is composed of gneiss and schist derived from sedimentary, volcanic, and plutonic rocks that range from upper greenschist to granulite facies (Hoare and Coonrad, 1978; Turner and others, 2009). It is an antiformal crystalline complex cored by amphibolite facies orthogneiss that range in composition from granite to diorite intercalated with metavolcanic and metasedimentary rocks (Turner and others, 2009; Moll-Stalcup and others, 1996). Orthogneiss of the core is intercalated with pyroxene granulite, garnet amphibolite, locally kyanite-bearing garnet-mica schist, and rare quartzite and marble (Turner and others, 2009). The amphibolite-facies rocks grade to greenschist-facies rocks that dip away from the core on the northwest and southeast. These lower grade rocks are schist and quartzite, as well as calc-phyllite, marble, and metaconglomerate (Turner and others, 2009). The character of the main complex changes at the Kanektok River. South of the river, mineral foliation tends to parallel compositional layering, and both consistently strike northeast; “* * * dip of the foliation changes from northwest to southwest two to four times across the width of the complex” (Hoare and Coonrad, 1979). Compositional layering and foliation are disrupted by northwest-trending faults, including a significant left-lateral offset at the Kanektok River. North of the river, the rocks are more highly deformed, and the dip and trend of foliation are more variable (Hoare and Coonrad, 1979). The Kanektok metamorphic complex is fault-bounded on the southeast; the complex is thrust over calcareous schist (unit ^Ps), the Kuskokwim Group (unit Kk), calcareous graywacke and conglomerate (unit Kcgc), and the undivided Togiak-Tikchik Complex (unit KDt) in the Goodnews Bay quadrangle, and green amphibole-bearing schist (Jgs) in the Bethel quadrangle. It is depositionally overlain by the Kuskokwim Group (unit Kk) in the Goodnews Bay and Bethel quadrangles. Turner and others (2009) report extensive radiometric dating on the Kanektok metamorphic complex using a variety of techniques; later studies by Box and others (1990), Moll-Stalcup and others (1996), and Miller and others (1991) in both the Kanektok and Idono complexes confirm Turner and others’ results. Most K/Ar and 40Ar/39Ar samples yielded discordant Mesozoic ages when multiple minerals from the same sample were dated. The relative within-sample discordance increases west to east across the complex as the apparent ages also increase. If these ages are considered simply as cooling ages, they suggest that a latest Jurassic or Early Cretaceous thermal event affected the metamorphic complex most strongly in the west. Upper intercept U/Pb ages of zircon from granite gneiss and tonalite gneiss cluster around 2.05 Ga (Turner and others, 2009; Box and others, 1990; Moll-Stalcup and others, 1996). Turner and others (2009) also report multiple K/Ar and 40Ar/39Ar analyses of biotite from three pyroxene granulite samples that yielded ages as old as 2.5 Ga. Turner and others (2009) suggested that these rocks may have excess argon, but no other technique has been used on the granulite-facies rocks to confirm that interpretation. Turner and others (2009) also reported a K/Ar age on hornblende from a garnet amphibolite that yielded an age of 1,770±53 Ma, identical to a sphene U/Pb age of 1,770 Ma, they interpreted these dates as a 1.8-Ga event that affected the Kanektok metamorphic complex. Marble in the complex is locally mapped separately as pCkm

Source map information

Source map Hoare, J.M., and Coonrad, W.L., 1978, Geologic map of the Goodnews and Hagemeister Island quadrangles region, southwestern Alaska: U.S. Geological Survey Open-File Report 78-9-B, 2 sheets, scale 1:250,000.
Symbol pCk
Unit name Metamorphic rocks of Kanektok River region
Description Gneiss and schist in the Kanektok River region. Sedimentary, volcanic, and plutonic rocks metamorphosed in the upper greenschist and lower amphibolite facies. Consists of medium- to coarse-grained, massive, and well-foliated biotite-hornblende gneisses, garnetiferous amphibolites, quartz-mica schists, and marble.
Lithology Metamorphic

Correlated geologic units

Label Xo
Description Amphibolite facies orthogniess and minor amphibolite and pelitic schist. Kanektok or Kilbuck terrane
Geologic age Paleoproterozoic to Proterozoic
Geologic setting Metamorphic, undivided
Lithology Form Importance
Amphibolite < Metamorphic Amphibolite Indeterminate, major
Orthogneiss < Metaigneous < Metamorphic Greenschist Indeterminate, major
Orthogneiss < Metaigneous < Metamorphic Amphibolite Indeterminate, major
Marble < Metacarbonate < Metasedimentary < Metamorphic Amphibolite Indeterminate, major
Pelitic-schist < Metaclastic < Metasedimentary < Metamorphic Amphibolite Minor