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 Day, W.C., O’Neill, J.M., Dusel-Bacon, Cynthia, Aleinikoff, J.N., Siron, C.R., 2014, Geologic map of the Kechumstuk fault zone in the Mount Veta area, Fortymile mining district, east-central Alaska (ver. 1.1, March 12, 2014): U.S. Geological Survey Scientific Investigations Map 3291, scale 1:63,360,
Symbol Jmig
Unit name Migmatite
Description Foliated migmatite with the paleosome a mixture of Paleozoic metasedimentary country rocks (Pzms), hornfels, amphibolite (Pzam), laminated marble (Pzm) invaded by neosome of foliated, medium-grained, biotite-hornblende quartz monzonite of Mount Veta (Jgmv). Neosome is foliated and forms sills within Paleozoic units parallel to strong penetrative ductile metamorphic fabric and compositional layering and is locally enfolded with the paleosome. Unit structurally underlies Jurassic quartz monzonite, forming migmatitic footwall zone of the Jurassic intrusion (Jgmv)
Lithology Metamorphic

Correlated geologic units

Label Jpmu
Description Migmatite border zone of granodiorite
Geologic age Jurassic to Middle-Jurassic
Geologic setting Metamorphic, undivided
Lithology Form Importance
Migmatite < Metamorphic Major