Diabase and gabbro, Aleutian Islands

Unit symbol: Tgba
Age range Tertiary, Miocene to Paleocene? (66 to 5.333 Ma)
Lithology: Igneous - Plutonic
Group name: Gabbroic rocks in southern Alaska
Diabase and gabbro dikes, sills, and small plutons are widespread and form two-thirds of the intrusive rocks on Attu Island and are very common on Agattu Island (Gates and others, 1971). These dikes and sills intrude all Attu Island units as old as the Chuniksak and Chirikof formations, but do not cut the Massacre Bay (included in unit QTv, here) and Faneto Formations (included in unit QTs, here). Hence, they are considered Miocene in age, although some of these rocks may be older (Gates and others, 1971, p. 746). Placed in the Attu Basement Series by Yogodzinski and others (1993). Similar gabbroic bodies in two large intrusions also occur on Kiska Island (Coats and others, 1961) and in a large mass of gabbro that locally includes hornblende gabbro,as well as augite-quartz syenite that intrudes the Finger Bay Volcanics on Adak Island (Coats, 1956a). On Attu Island, diabase and gabbro that intrude map unit Tba have yielded K/Ar whole-rock ages of 27.2±1.4 and 32.7±1.4 Ma (DeLong and McDowell, 1975). On Amlia Island, gabbro has yielded a K/Ar age of 39.8±1.2 Ma (McLean and others, 1983). Drewes and others (1961) mapped a gabbro body they inferred was a border facies of the granitic Shaler batholith on Unalaska and Sedanka Island; however, the gabbro is spatially distinct from the Shaler batholith and radiometric dates indicate that the gabbro is significantly older— about 30 Ma—whereas the other parts of the Shaler batholith are about 12 Ma. Unit also includes the Sled Pass gabbro of the eastern Lime Hills quadrangle (Gamble and others, 2013) for which Jones and others (2014) have reported a U/Pb zircon age of 28.5 Ma, which is unique for gabbro in that part of Alaska

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 Td
Unit name Diabase and gabbro dikes, sills and small plutons
Description Diabase and gabbro dikes, sills and small plutons, Attu Island
Lithology Igneous

Correlated geologic units

Label Tgb
Description Gabbro and diorite, undivided
Geologic age Paleocene to Miocene
Geologic setting Intrusive, gabbro
Lithology Form Importance
Diorite < Dioritic < Plutonic < Igneous Pluton Major
Gabbro < Gabbroic < Plutonic < Igneous Pluton Major
Gabbro < Gabbroic < Plutonic < Igneous Dike or sill Indeterminate, major