Plutons of the Yukon-Tanana Upland

Unit symbol: Jg
Age range Early Jurassic (201.3 to 174.1 Ma)
Lithology: Igneous - Plutonic
Group name: Intermediate to mafic plutonic rocks
The plutons range from granite to granodiorite and also include more potassic phases that are exposed primarily in the Eagle, and to a lesser extent, the Tanacross quadrangles of east-central Alaska. Includes the hornblende syenite porphyry of Mount Veta of Foster (1976), which has been shown by Day and others (2014) to include quartz monzonite and diorite phases. Radiometric ages, including K/Ar, 40Ar /39Ar plateau, and U/Pb zircon, are generally between 193.2 and 178.7 Ma, but several other U/Pb ages have been published, both older and younger than this range (Foster, 1976; Newberry and others, 1998b; Dusel-Bacon and others, 2002; Werdon and others, 2001; Szumigala and others, 2002; Dusel-Bacon and others, 2009; Day and others, 2014). Unit includes rocks mapped as “gabbro, associated basalt and andesite(?), minor diorite and diabase, and undifferentiated mafic- and intermediate composition igneous rocks” by Foster (1992) and since shown to include a significant proportion of granodiorite of Early Jurassic age. Triassic parts of Foster’s unit are included in unit Trgd. In the Eagle quadrangle, unit includes migmatitic border phases of the plutons as mapped by Day and others (2014)

Source map information

Source map Werdon, M.B., Newberry, R.J., Szumigala, D.J., and Pinney, D.S., 2001, Geologic map of the Eagle A-2 quadrangle, Fortymile mining district, Alaska: Alaska Division of Geological and Geophysical Surveys Preliminary Interpretive Report 2001-3A, 1 sheet, scale 1:63,360.
Symbol Jg
Description Numerous small and elongate Jurassic (?) plutons scattered throughout the northern half of the map area compositionally and texturally resemble the Chicken and Uhler plutons. May or may not be foliated, commonly porphyritic, medium-grained, tend to be elongated in east–west direction parallel to fold axial traces in surrounding metamorphic rocks, and mostly range between granite and quartz monzodiorite in composition. Typically contain 15–30 percent quartz, 15–35 percent K-feldspar, 35–45 percent plagioclase, 0–25 percent hornblende, 2–10 percent biotite, and minor sphene, opaques, and magmatic? epidote as overgrowths on trace allanite. Magnetic susceptibilities vary greatly, from 0.1–0.3 to 1–2.5 x 10x-3 SI. Presence of magmatic epidote indicates the bodies were emplaced at mesozonal depths. Locally spatially associated with small base-metal skarns (Werdon and others, 2000) south of Buckskin Creek, and east of the South Fork of the Fortymile River just north of the mouth of Napoleon Creek. One of these dikes from just east of the map area yielded a Ar/ Ar age of 188 Ma (Layer and others, 2001).
Lithology Igneous

Correlated geologic units

Label Jegr
Description Early Jurassic plutons of the Yukon-Tanana Upland
Geologic age Early-Jurassic to Toarcian
Geologic setting Intrusive
Lithology Form Importance
Granodiorite < Granitic < Plutonic < Igneous Pluton Major
Quartz-monzonite < Syenitic < Plutonic < Igneous Pluton Indeterminate, major
Gabbro < Gabbroic < Plutonic < Igneous Pluton Minor
Syenite < Syenitic < Plutonic < Igneous Pluton Minor