Young shallow intrusive rocks

Unit symbol: QTi
Age range Quaternary or late Tertiary (5.333 to 0 Ma)
Lithology: Igneous - Plutonic
Group name: Young volcanic and shallow intrusive rocks
Hypabyssal plugs, domes, and subordinate flows that range in composition from rhyolite to basalt at Quaternary volcanic centers along the Aleutian magmatic arc and are associated with the volcanoes of the Wrangell Mountains. In the Aleutian Islands, compositions tend to be dacite to basaltic andesite; along the Alaska Peninsula the full range in composition is present; and in the Wrangell Mountains, compositions more typically range from rhyolite to dacite. Included are quartz keratophyre dikes on widely separated parts of Attu Island. Dikes consist of chalky white euhedral feldspar (albite) crystals and glassy quartz in a bluish-gray to light-gray groundmass of fine-grained albite, quartz and minor sericite, calcite, epidote, chlorite, magnetite, and apatite (Gates and others, 1971, p. 748). Unit also includes rhyolite domes at Mount Edgecumbe in the Sitka quadrangle (Riehle and others, 1989). Unit tends to be best exposed at older volcanic centers

Source map information

Source map Gates, Olcott, Powers, H.A., and Wilcox, R.E., 1971, Geology of the Near Islands, Alaska, with a section on Surficial geology by J.P. Schafer: U.S. Geological Survey Bulletin 1028-U, p. 709-822, 3 maps, scale 1:63,360.
Symbol QTqk
Unit name Quartz keratophyre dikes
Description Quartz keratophyre dikes occur on widely separated parts of Attu Island. The dikes consists of chalky white euhedral feldspar (albite) crystals and glassy quartz in a bluish-gray to light-gray groundmass, which is a felt of fine-grained albite and quartz and minor sericite, calcite, epidote, chlorite, magnetite, and apatite (Gates and others, 1971, p. 748)
Lithology Igneous

Correlated geologic units

Label QTqk
Description Quartz keratophyre dikes
Geologic age Pliocene to Pleistocene
Geologic setting Extrusive
Lithology Form Importance
Felsic-hypabyssal < Hypabyssal < Igneous Dike or sill Major