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Research about Highwood Mountains

Source-linked reports with geographic coverage including Highwood Mountains.

5 recordsLinked to original sources
USGS2014

The igneous rocks of the Highwood Mountains of central Montana

The study of the Highwood Mountains was undertaken by a group of men from Harvard University under a grant from the Shaler Memorial Fund of the Department of Geology. The work was under the general direction of Larsen, who, with the assistance of Norman A. Haskell, mapped most of the volcanic rocks. Hurlbut and Griggs worked mostly on the laccoliths, Burgess on the stocks, and Buie on the dikes. The party spent two summers in the field. The laboratory work is far from complete. We cooperated closely in all the work.

Montana↗
USGS1941

Igneous rocks of the Highwood Mountains, Montana: Part II. The Extrusive Rocks

Early eruptions of quartz latites, rather rich in potash, built up a volcanic mountain over 30 miles across on an irregular surface of late Cretaceous sediments. Erosion then removed much of the quartz latite. Renewed volcanism formed a volcano made up of basaltic rocks (mafic phonolite) that covered the quartz latites. The phonolites vary in texture and in the kind and proportion of the felsic minerals present. All contain abundant phenocrysts of augite and some of olivine; some have phenocrysts of leucite, others analcime, and many have pseudoleucite. Some have phenocrysts of biotite and barium sanidine. The groundmasses contain about the same minerals as the phenocrysts with more felsic minerals and more biotite in the coarser-grained groundmasses.

Montana↗
USGS1941

Igneous rocks of the Highwood Mountains, Montana: Part V. Contact Metamorphism

Very near the contacts of the stocks the sediments have been replaced by sanidine and diopside, through magmatic reaction. An irregular zone of indurated sediments, produced largely by hydrothermal agents, extends outward from the stocks for as much as half a mile. Locally, more intense hydrothermal metamorphism has formed orthoclase and fibrous amphibole with some diopside and locally garnet, phlogopite, spinel, and cordierite. Along the dikes metamorphism extends out from the contacts for a distance of a few inches up to 5 feet. Thermal metamorphism produced recrystallization of the fine interstitial material of the sandstone. Aegirite, calcite, hematite, and pyrite were deposited by hydrothermal solutions. Magmatic reaction produced diopside. Reaction between sandstone and the magma of the headed dike caused a progressive replacement of plagioclase and quartz by potash feldspar. Granitic inclusions in the biotite phonolite dikes have a marginal shell in which plagioclase has been replaced by orthoclase. Surrounding each inclusion is a zone of mixed rock which contains quartz grains much embayed and surrounded by 0.1 mm. prisms of pyroxene.

Montana↗
USGS1941

Igneous rocks of the Highwood Mountains, Montana: Part VI. Mineralogy

The minerals of the igneous rocks of the Highwood Mountains are described. The primary hornblende of the quartz latites is basaltic and it has been partly replaced by a common green hornblende. Hornblende is rare in the alkalic rocks. Augite is an abundant mineral of the alkalic rocks; in the syenites it contains some acmite, especially on the borders of the crystals. Aegirite is a common metamorphic mineral. The olivine of the shonkinites contains from 12 to 22 mol per cent of fayalite, that of the syenite from Square Butte 50 per cent. Monticellite twinned into trillings on (031) is an abundant mineral in one small intrusive body. In the alkalic rocks barium sanidine is abundant and plagioclase rare; leucite, primary analcime, and pseudoleucite are common and sodalite rare. Natrolite, thomsonite, and other zeolites are widespread and very abundant.

Montana↗