Granodiorite to quartz monzodiorite

Unit symbol: TKgd
Age range Tertiary, Paleocene, or Late Cretaceous, Maastrichtian (72.1 to 56 Ma)
Lithology: Igneous - Plutonic
Group name: Granitic rocks of southern and interior Alaska
Typically described as granodiorite, quartz diorite, tonalite, monzonite, and quartz monzodiorite, these plutons compositionally overlap with unit TKg, described above, but are the more mafic plutons in this age range. Also widespread, they tend lie at the periphery of the TKg belt of plutons. They are locally weakly foliated and hypidiomorphic-granular or seriate in texture. K/Ar ages on biotite and hornblende range between 73.5 and 60.0 Ma—very slightly older than unit TKg. An outlier to the belt, a fine-grained hornblende granodiorite, is found on Saint Matthew Island and yielded a K/Ar age on hornblende of 62.3±2.0 Ma (Patton and others, 1975)

Source map information

Source map Csejtey, Bela, Jr., Nelson, W.H., Jones, D.L., Silberling, N.J., Dean, R.M., Morris, M.S., Lanphere, M.A., Smith, J.G., and Silberman, M.L., 1978, Reconnaissance geologic map and geochronology, Talkeetna Mountains quadrangle, northern part of Anchorage quadrangle, and southwest corner Healy quadrangle, Alaska: U.S. Geological Survey Open-File Report 78-558-A, 60 p., 1 sheet, scale 1:250,000.
Symbol TKt
Unit name Tonalite
Description Dominantly biotite-hornblende tonalite, locally grades into quartz diorite. The tonalite is medium-gray, coarse- to medium-grained, has a granitic texture and a fairly well-developed primary foliation.
Lithology Igneous

Correlated geologic units

Label TKgd
Description More mafic granitic rocks, tonalite, monzonite
Geologic age Maastrichtian to Paleocene
Geologic setting Intrusive
Lithology Form Importance
Granodiorite < Granitic < Plutonic < Igneous Pluton Major
Monzonite < Syenitic < Plutonic < Igneous Pluton Major
Diorite < Dioritic < Plutonic < Igneous Pluton Indeterminate, major
Monzodiorite < Dioritic < Plutonic < Igneous Pluton Indeterminate, major
Quartz-monzodiorite < Dioritic < Plutonic < Igneous Pluton Indeterminate, major
Quartz-monzonite < Syenitic < Plutonic < Igneous Pluton Indeterminate, major
Tonalite < Granitic < Plutonic < Igneous Pluton Minor