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 Jn
Description Sub-equigranular, medium- to coarse-grained, weakly- to nonfoliated hornblende quartz monzonite and lesser monzonite and quartz monzodiorite, herein called the Napoleon Creek pluton. Typical primary modal mineralogy is 3–8 percent quartz, 40–50 percent plagioclase, 20–40 percent K-feldspar, 15–20 percent hornblende, 1 percent sphene, 1 percent opaques, 0–1 percent myrmekite, and trace apatite and allanite. Generally characterized by large K-feldspar and obvious hornblende phenocrysts, which define the foliation, if present. Quartz is invariably strained, and crystal margins are commonly granulated. Most contacts with country rocks are linear zones lacking outcrops and displaying no obvious hornfels aureole, suggesting post-intrusive faulting. Magnetic susceptibility is typically high (.001–.027 SI), but the core of the body is significantly less magnetic (0.00031–0.00061 SI). Includes small bodies of biotite clinopyroxenite and coarse-grained hornblende gabbro, which may be inclusions, dikes, and (or) marginal phases. Both the Napoleon Creek pluton and the mafic phases are cut by minor late granite aplite–pegmatite dikes. Extensive, sheared zones of gold-bearing quartz–carbonate–sericite–chlorite–pyrite alteration are present within the pluton, yield an early Cretaceous Ar/ Ar isochron age (sample 4; table 1), and are under current industry investigation. Plutonic hornblende yielded a Ar/ Ar plateau age of 186 Ma (sample 7; table 1)
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