Gabbroic rocks

Unit symbol: TKgb
Age range Tertiary to Late Cretaceous or older (83.6 to 56 Ma)
Lithology: Igneous - Plutonic
Group name: Mafic intrusive rocks
Fresh to altered gabbroic and diabasic intrusive bodies including small plutons, dikes and sills. Widespread throughout western Alaska; distribution of unit mimics distribution of the more granitic rocks of units TKg and TKgd. Age control is limited relative to the granitic rocks and, in many cases, the age assigned by local geologic mappers is Devonian to Tertiary, hence, the rocks included here may, in part, be significantly older that the granitic rocks of units Tpgr, TKg, and TKgd. In the Ophir quadrangle, medium- to coarse-grained gabbro that has minor associated diorite and basalt intrudes rocks of inferred Mississippian to Triassic age and, locally, Cretaceous age. Contact relations are uncertain due to poor exposure, but Chapman and others (1985) indicate at least one gabbro body has a hornfels aureole. A similar unit of gabbroic sills, dikes, and small plugs in the adjoining Medfra quadrangle was given a provisional Devonian to Tertiary age (Patton and others, 1980). Unit also includes poorly exposed small bodies of mafic rocks in Talkeetna Mountains and Tyonek quadrangles. In the Talkeetna Mountains, Csejtey and others (1978) described a medium- to light-gray, coarse- to medium-grained plagioclase and pale-green hornblende-bearing leucogabbroJ.M. Schmidt (written commun., 2007) also described diorite associated with the gabbroic rocks. Reed and Elliot (1970) described diorite and olivine and (or) hornblende gabbro that occur as intrusive bodies and inclusions in other Tertiary intrusive rocks in Tyonek quadrangle. Age is poorly constrained, but samples of hornblendite and gabbro from Tyonek quadrangle yielded 40Ar/39Ar plateau ages of between about 84 and 69 Ma (P.J. Haeussler, USGS, unpub. data). Altered gabbro and diorite bodies crop out as small scattered bodies in the Russian Mission and southern Holy Cross quadrangles; the age of these bodies is uncertain, but not older than Jurassic. Unit TKgb includes dark-colored dikes and sills of diabase, basalt, and dioritic, gabbroic, and biotite lamprophyre that yield a K/Ar age of 64.6±2 Ma on biotite from the western Nushagak Bay quadrangle (Hoare and Coonrad, 1978). Unit also includes a medium- to coarse-grained pyroxene gabbro dike(?) on Saint Matthew Island (Patton and others, 1975)

Source map information

Correlated geologic units

Label TKl
Description Lamprophyre, alkali gabbro, and alkali diorite
Geologic age Late-Cretaceous to Paleocene
Geologic setting Intrusive
Lithology Form Importance
Lamprophyre < Hypabyssal < Igneous Dike or sill Indeterminate, major
Diorite < Dioritic < Plutonic < Igneous Dike or sill Indeterminate, major
Gabbro < Gabbroic < Plutonic < Igneous Dike or sill Indeterminate, major