Quartz monzonite, monzonite, and syenite

Unit symbol: Kmqm
Age range Cretaceous, Coniacian to Albian (113 to 86.3 Ma)
Lithology: Igneous - Plutonic
Group name: Granitic rocks of central and southeast Alaska
Large quartz monzonite plutons occur in three general areas of the state. The largest exposures are found in the Ruby terrane north of the Kaltag Fault in west-central Alaska. Plutons, such as the large Melozitna pluton, are largely quartz monzonite, but also have granite and monzonite phases. Locally, the Melozitna pluton intrudes granitic augen gneiss that has yielded a protolith emplacement age of 117.5 Ma (Roeske and others, 1995). On the southeastern Seward Peninsula, an elongate pluton 80 km long and 3 to 8 km wide extends along the crest of the Darby Mountains in the southeast part of the peninsula (Till and others, 2011). Other plutons of this unit are exposed in the Yukon–Koyukuk Basin in the Candle, Selawik, and Shungnak quadrangles, spatially associated with similar age syenite and nepheline syenite of unit Ksy. Additional exposures occur on the islands offshore of the Seward Peninsula—Little Diomede, King, and Sledge Islands. Plutons in these two areas range in age between about 112 and 85 Ma. In the transition zone between the Tintina and Kaltag Fault Systems in north-central Alaska, a number of 92- to 88-Ma quartz monzonite plutons lie in a belt parallel to the structural trend. In eastern Alaska, a number of large quartz monzonite plutons are found in the Tanacross and Nabesna quadrangles and extend into the Yukon of Canada. Age determinations on these plutons, of which there have been very few, are more restricted in age, between about 98 and 91 Ma. Included here is a small syenite body located just a few miles north of Fairbanks in central Alaska, which has been described by Newberry and others (1998a) and yielded a nearly concordant TIMS U/Pb date of 110.6±0.6 Ma. Also included is the quartz monzonite phase of the Mount Kashagnak pluton of the Skagway quadrangle, which is undated

Source map information

Source map Reifenstuhl, R.R., Dover, J.H., Pinney, D.S., Newberry, R.J., Clautice, K.H., Liss, S.A., Blodgett, R.B., Bundtzen, T.K., and Weber, F.R., 1997, Geologic map of the Tanana B-1 quadrangle, central Alaska: Alaska Division of Geological and Geophysical Surveys Report of Investigations 97-15a, 17 p., 1 sheet, scale 1:63,360.
Symbol Kgs
Unit name Quartz syenite, granite, and granite pegmatite
Description Elephant Mountain pluton. Buff to light gray, medium- to coarse-grained, subequigranular holocrystalline rock. The rock exhibits varying degrees of hydrothermal alteration, with much of the feldspar and biotite converted to white mica. Alteration makes petrographic distinction between plagioclase and alkali feldspar impossible, but quartz versus quartz + total feldspar ratios vary from 10 to 30 percent, and the vast bulk of the feldspar appears to have been potassium feldspar. Altered granite or quartz syenite is yellowish green with a light- to bright-orange weathering rind and complete alteration of feldspar and biotite to white mica. Pyrite is disseminated as 1 to 2 percent fine-grained crystals, especially in former feldspar sites. Fine-grained, secondary rutile and arsenopyrite are common in the highly altered rock. H. Noyes (Doyon Ltd., personal commun.,1996) reports values up to 1,740 ppb Au and several thousand ppm As on drill-core analyses
Lithology Igneous

Correlated geologic units

Label Kmqm
Description Quartz monzonite, monzonite, and syenite 85-110 Ma
Geologic age Albian to Coniacian
Geologic setting Intrusive
Lithology Form Importance
Quartz-monzonite < Syenitic < Plutonic < Igneous Pluton Major
Monzodiorite < Dioritic < Plutonic < Igneous Pluton Minor
Quartz-monzodiorite < Dioritic < Plutonic < Igneous Pluton Minor
Granodiorite < Granitic < Plutonic < Igneous Pluton Minor
Monzonite < Syenitic < Plutonic < Igneous Pluton Minor
Syenite < Syenitic < Plutonic < Igneous Pluton Minor
Alaskite < Leucocratic-granitic < Granitic < Plutonic < Igneous Pluton Incidental