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 Jc
Description Medium- to coarse-grained, tan to white, porphyritic to equigranular, weakly- to non-foliated, biotite granite and lesser hornblende biotite granodiorite, herein called the Chicken pluton. Previously mapped as part of the Taylor Mountain batholith (Foster, 1976), but displays significant contrasts in mineralogy, texture, major oxide composition, and age. Typical primary modal mineralogy is 20–25 percent quartz, 30–40 percent plagioclase, 20–30 percent K-feldspar, 0–5 percent hornblende, 5–10 percent biotite, 1–2 percent myrmekite, <1 percent sphene, magmatic? epidote and opaques, and trace apatite and allanite. K-feldspar megacrysts are common; aplite dikes are locally abundant, especially in upper Stonehouse Creek. Quartz is commonly strained and mortar texture is prevalent. Usually exhibits little alteration, but where alteration is present, it consists mostly of hornblende replaced by epidote + chlorite. Magnetic susceptibility is usually high, typically .004–.012 SI. Contacts with surrounding rocks are mostly obscure. Lack of obvious hornfels or skarn in surrounding calcareous metamorphic rocks suggests contacts are largely faulted. No significant mineralization is known to occur in this pluton, although quartz–pyrite and K feldspar–quartz veins occur at the head of Chicken Creek and gold prospects are present near the Chicken pluton at Purdy and Lilliwig Creek. Hornblende from the Chicken pluton yields a Ar/ Ar plateau age of 187.8 Ma (sample 8; table 1), and a Ar/ Ar isochron age of 54 Ma for the biotite indicates significant heating in early Tertiary time, presumably associated with nearby gabbro. The presence of apparently magmatic epidote in this pluton suggests it crystallized at elevated pressure, probably >5 kbar
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