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 Patton, W.W., Jr., Wilson, F.H., Labay, K.A., and Shew, Nora, 2009, Geologic map of the Yukon-Koyukuk basin, Alaska: U.S. Geological Survey Scientific Investigations Map SIM-2909, 26 p., 2 sheets, scale 1:500,000.
Symbol TKm
Unit name Monzonite
Description Medium- to coarse-grained, equigranular, biotite and hornblende monzonite, quartz monzonite, monzodiorite, and quartz diorite. Subordinate monzogabbro and gabbro. Forms small plutonic bodies in Medfra and Ophir quadrangles
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