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 Foster, H.L., 1970, Reconnaissance geologic map of the Tanacross quadrangle, Alaska: U.S. Geological Survey Miscellaneous Geologic Investigation Map I-593, 1 sheet, scale 1:250,000.
Symbol Mzs
Unit name Hornblende syenite porphyry
Description Hornblende syenite porphyry. (Now commonly referred to as Mt. Veta syenite, dated at about 180 Ma)
Lithology Igneous

Correlated geologic units

Label Jsy
Description Hornblende syenite porphyry
Geologic age Early-Jurassic
Geologic setting Intrusive
Lithology Form Importance
Syenite < Syenitic < Plutonic < Igneous Pluton Major