Old Crow suite of Gordey and Makepeace (2003) and other granitic rocks of northeast Alaska

Unit symbol: MDgi
Age range Mississippian to Devonian? (393.3 to 323.2 Ma)
Lithology: Igneous - Plutonic
Group name: Granitic rocks and orthogneiss
Granite and quartz monzonite, exposed in two large batholithic bodies in northeast Alaska and in two smaller plutons. The northernmost, the granite of the Romanzof Mountains, straddles the boundary of the Mount Michelson and Demarcation Point quadrangles (Reiser and others, 1971, 1980). Predominantly muscovite- or biotite-quartz monzonite, with hornblende that is restricted to a small stock on the south side of the batholith. Age control is generally lacking. The pluton may intrude rocks as young as the Lisburne Group of Carboniferous age. A satellite pluton to the south that intrudes rocks of the Proterozoic Neruokpuk Quartzite yielded a K/Ar date on hornblende of 439 Ma; this date is hard to evaluate because it may reflect a separate magmatic event or, alternatively, may reflect the presence of excess argon inherited from the country rock. Extensive fission-track analyses (O’Sullivan and others, 1995; Peapples and others, 1997) indicate rapid uplift in the Eocene. The southern part of the unit, the Old Crow batholith, straddles the Alaska-Yukon border in the Coleen quadrangle of Alaska and Old Crow sheet of the Yukon. In Alaska, batholith consists of biotite granite and quartz monzonite which locally contain muscovite. Northern contact with adjacent metamorphic rocks is intrusive; southern boundary is not well known and may be a fault in part. K/Ar ages on mica range from 354 to 302 Ma (Brosgé and Reiser, 1969) in Alaska. In northern Yukon, the Old Crow batholith and related plutons consist, in part, of syenite and yield radiometric ages between 382 and 362 Ma (Gordey and Makepeace, 2003). Plutons in the Table Mountain quadrangle were described as either Tertiary or Mississippian to Devonian (C.G. Mull, written commun., 2011). In Yukon, the plutons have been considered Devonian (Gordey and Makepeace, 2003); in the Table Mountain quadrangle, a rhyolite dike was dated at 56.4 Ma (Barker and Swainbank, 1986); whether this date reflects the age of any of the larger granitic plutons is uncertain. Devonian to Mississippian orthogneiss is also known farther west in the Brooks Range, in the Yukon-Tanana Upland and northern Alaska Range, and in the Tanana and Melozitna quadrangles of central Alaska (see units MDag, Dmi, and Dogn)

Source map information

Source map Brosgé, W.P., and Reiser, H.N., 1969, Preliminary geologic map of the Coleen quadrangle, Alaska: U.S. Geological Survey Open-File Report 69-25, 1 sheet, scale 1:250,000.
Symbol Cgr
Unit name Granite
Description Biotite granite and quartz monzonite with minor muscovite locally; partly porphyritic. Intrudes unit Pzm to north. Southern contact obscure; may be fault west of Spike Mountain.
Lithology Igneous

Correlated geologic units

Label Dgr
Description Old Crow Suite granitic rocks: medium to coarsely crystalline leucocratic biotite granite, Old Crow, Schaeffer, Sedgwick, Ammerman, and Fitton granites
Geologic age Middle-Devonian to Mississippian
Geologic setting Intrusive
Lithology Form Importance
Granite < Granitic < Plutonic < Igneous Batholith Major
Quartz-monzonite < Syenitic < Plutonic < Igneous Batholith Major