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., Newberry, R.J., Clautice, K.H., Liss, S.A., Blodgett, R.B., and Weber, F.R., 1998, Geologic map of the Tanana A-1 and A-2, central Alaska: Alaska Division of Geological and Geophysical Surveys Public Data File 98-37a, 19 p., 1 sheet, scale 1:63,360.
Symbol Kgs
Unit name Granite and quartz syenite
Description Roughtop Mountain pluton. Buff to light gray, fine to coarse grained, sub-equigranular, holocrystalline rock. The rock exhibits slight hydrothermal alteration, with feldspar partly converted to fine-grained white mica. Quartz/(quartz + total feldspar) ratios vary from 15 to 35 percent, and the bulk of the feldspar appears to be K-feldspar. Biotite (5 to 10 percent) is the sole mafic mineral. Granite/quartz syenite is locally yellowish with a light orange weathering rind. Contacts with the surrounding rock are only exposed in rubble, but the occurrence towards the center of the Roughtop intrusion and rare dikes in the syenite and monzonite units suggests that this unit represents the most fractionated portion of the body
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