Felsic dikes, sills, and small stocks in southern Alaska

Unit symbol: Tehi
Age range Tertiary and older? (66 to 33.9 Ma)
Lithology: Igneous - Plutonic
Group name: Felsic dikes, sills, and small stocks in southern Alaska
Dikes, sills, and small intrusive bodies of mafic to felsic composition in the Anchorage and Seward quadrangles. These bodies yield a wide range of ages, representing multiple intrusive events throughout the region. Hypabyssal rocks of felsic and intermediate composition from the Anchorage quadrangle yielded six whole-rock K/Ar ages, from 54.8±2.7 to 40.0±1.6 Ma, and three zircon fission track ages, from 41.3±6.0 to 36.8±4.8 Ma (Winkler, 1992). Also in the Anchorage quadrangle, a basalt sill yielded a whole-rock K/Ar age of 40.9±1.6 Ma (Silberman and Grantz, 1984). A 40Ar/39Ar plateau age on amphibole from an andesite dike yielded an age of 58.64±0.52 Ma. Unit also includes unit TJds of Winkler (1992) that yielded widely different whole-rock K/Ar ages of 130±6 and 38±2 Ma. A basaltic-andesite dike intruding McHugh Complex yielded an 40Ar/39Ar hornblende plateau age of 115±2 Ma, whereas an intermediate composition dike yielded an 40Ar/39Ar isochron(?) age of 57.0±0.22 Ma (W. Clendenin, South Carolina Department of Natural Resources, Geological Survey, written commun., cited in Bradley and others, 1999). Lytwyn and others (2000) described a suite of dikes that range in composition from basalt to rhyolite that they associated with near/trench intrusions commonly related to the subduction of Kula-Farallon spreading center

Source map information

Source map Nelson, S.W., Miller, M.L., Haeussler, P.J., Snee, L.W., Phillips, P.J., Huber, Carol, 1999, Preliminary geologic map of the Chugach National Forest Special Study Area, Alaska, U.S. Geological Survey Open-File Report 99-362, scale 1:63,360.
Symbol Tf
Unit name Felsite Dikes
Description Includes two types: (1) leucocratic porphyry dikes with 20%-30% euhedral phenocrysts of potassium feldspar and quartz in a fine grained allotriomorphic groundmass. Intrudes only the Orca Group; (2) allotriomorphic granular, white weathering, fine grained dikes with approximately 30% quartz, 60% feldspar, and 20% altered mafic minerals. Thickness ranges from 0.3 m - 2 m. Intrudes the Orca and Valdez Group
Lithology Igneous

Correlated geologic units

Label Tfd
Description Felsic intrusions, leucocratic dikes, sills and small stocks, south of Border Ranges fault
Geologic age Paleocene to Eocene
Geologic setting Intrusive
Lithology Form Importance
Felsic-hypabyssal < Hypabyssal < Igneous Dike or sill Indeterminate, major