Hypabyssal intrusions

Unit symbol: Thi
Age range Tertiary (66 to 1.806 Ma)
Lithology: Igneous - Volcanic
Group name: Hypabyssal intrusions
Typically shallow dikes, sills, and stocks that range in composition from basalt to porphyritic, hornblende-bearing andesite to rhyolite, widely distributed in southern and central Alaska and the Aleutian Islands. Minor propylitic alteration is characteristic of these rocks. In the Middleton Island quadrangle, a prominent very pale-gray, very dense, conspicuously jointed dacite plug complex forms Cape Saint Elias and Pinnacle Rock at the southwestern end of Kayak Island (Plafker, 1974). On southern Adak and Kagalaska Islands dikes and small intrusive bodies consist largely of basalt and andesite (Coats, 1956a; Fraser and Snyder, 1959). Unit also includes a basalt dome older than caldera of ancestral Mount Kanaton on Kanaga Island (Coats, 1956b). On Kavalga, Ogliuga, and Skagul Islands, the unit consists of columnar-jointed porphyritic andesite sills and andesite and basalt dikes (units Tp, Tpi, and Tab of Fraser and Barnett, 1959). Also includes rocks associated with the Kiska Harbor and Vega Bay Formations on Kiska Island (Coats and others, 1961). Includes, on the eastern third of the Agattu Island, dacite porphyry cutting both the basement rocks and the Krugloi Formation and several of the diabase and gabbro bodies (Gates and others, 1971). Dikes of dacite, rhyolite, andesite, and rare basalt intrude rocks of the Valdez Group (unit Kaf) and McHugh Complex (unit Kumc) (Bradley and others, 1999). In the Dillingham quadrangle, this unit includes the larger of scattered occurrences of felsic hypabyssal rocks that occur as dikes and small plugs. In the Taylor Mountains quadrangle, unit includes felsic porphyritic rocks that occur in the northwest corner and along the southeastern margin. Analyzed samples of dikes throughout Alaska yield a wide range of ages. A basaltic-andesite dike intruding McHugh Complex yielded an 40Ar/39Ar hornblende plateau age of 115±2 Ma, but either the age or the sample setting must be considered suspect because this is significantly older that the unit it intrudes; an intermediate-composition dike yielded an 40Ar/39Ar isochron(?) age of 57.0±0.22 Ma (C.W. Clendenin, South Carolina Geological Survey, written commun., cited in Bradley and others, 1999) and felsic dikes mapped by Nelson and others (1999) yielded 40Ar/39Ar isochron ages as young as 31.1±0.2 Ma on potassium feldspar. Hypabyssal rocks from the Anchorage quadrangle yielded whole-rock K/Ar ages of 54.8±2.7 to 40.9±1.6 Ma and zircon fission track ages between 41.3±6.0 and 36.8±4.8 Ma (Silberman and Grantz, 1984; Winkler, 1992). 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), which yielded widely different whole-rock K/Ar ages of 130±6 Ma and 38±2 Ma. The subvolcanic hornblende dacite of Jumbo Dome in the Healy quadrangle yielded a K/Ar age as young as 2.79±0.25 Ma (Csejtey and others, 1992). Other determinations yielded ages throughout the Tertiary

Source map information

Source map Riehle, J.R., Detterman, R.L., Yount, M.E., and Miller, J.W., 1993, Geologic map of the Mount Katmai quadrangle and adjacent parts of the Naknek and Afognak quadrangles, Alaska: U.S. Geological Survey Miscellaneous Investigations Series Map I-2204, 1 sheet, scale 1:250,000.
Symbol Thi
Unit name Hypabyssal intrusive rocks
Description Light- to medium-gray or grayish-brown intrusive rocks that occur northwest of active volcanoes of Aleutian volcanic arc. Rocks are commonly porphyritic, have fine- to medium-grained groundmass, and consist chiefly of quartz diorite or tonalite. Green veins and patches of secondary quartz, epidote, chlorite, and albite suggesting deuteric alteration are common. Unit includes small plugs and sills as well as plutons exposed over as much as 30 km2. Intrudes rocks as young as unit Kk
Lithology Igneous

Correlated geologic units

Label Thm
Description Hypabyssal mafic intrusions. Age range varies, includes Miocene to Paleocene
Geologic age Tertiary
Geologic setting Extrusive
Lithology Form Importance
Andesite < Mafic-volcanic < Volcanic < Igneous Flow Indeterminate, major
Basalt < Mafic-volcanic < Volcanic < Igneous Flow Indeterminate, major
Lamprophyre < Hypabyssal < Igneous Dike or sill Incidental
Gabbro < Gabbroic < Plutonic < Igneous Dike or sill Incidental
Dacite < Felsic-volcanic < Volcanic < Igneous Dike or sill Incidental