Garnet peridotites from Williams kimberlites, north-central Montana, U.S.A

Abstract

Geology of the Williams diatremes Textures of garnet peridotite inclusions Mineral compositions Garnet Clinopyroxene Orthopyroxene 9 01 i vine Spi nel 9 Phi ogopite Thermometry and barometry of inclusions

Geophysics and petrology of the deep crust and upper mantle; a workshop

“…Thermobarometry for garnet peridotite xenoliths indicates a perturbed middle Eocene geotherm (Hearn and McGee, 1984). Textures in some xenoliths indicate partial to complete reaction of garnet to a garnet-spinel facies or spinel facies assemblage, with indications of either metasomatic introduction of K 2 o or redistribution of K 2 o already present in the form of phlogopite and (or) amphibole.…”

Geology and mineral resources of the North-Central Montana Sagebrush Focal Area: Chapter D in <em>Mineral resources of the Sagebrush Focal Areas of Idaho, Montana, Nevada, Oregon, Utah, and Wyoming</em>

“…Two G9 chromian pyrope garnets were found in panned concentrates from the Big Slide diatreme (Hearn and McGee, 1984). Ten purple chromium pyropes from Saskatchewan Butte 2 diatreme concentrates have been analyzed by B. Carter Hearn, Jr.; eight garnets are G9 compositions, and two are G10 compositions.…”

“…30). The largest occurrence, Williams 1, is 350 by 200 m, has an irregularly rounded shape, and shows a flat-topped positive magnetic anomaly (Hearn and McGee, 1984;Irving and Hearn, 2003). Williams 2 is a breccia body about 100 by 50 m with two short dike offshoots in uplifted Paleozoic limestone.…”

“…Temperatures and pressures estimated from mineral compositions plotted in comparison to the graphite-diamond stability boundary show that the Williams garnet peridotites overlap the graphite-diamond boundary ( fig. 32; Hearn and McGee, 1984). According to BLM records in 2016, 13 active lode claims cover the Williams kimberlite occurrences.…”

“…Williams 4 is a N. 32° E.-trending dike that is 380 m long and 10-30 m wide, which contains massive kimberlite and kimberlite breccia. Williams 1 and 4 contain xenoliths of garnet peridotite, garnet-spinel peridotite, spinel peridotite, and megacrysts (>1 cm) of garnet, chromian diopside, and magnesian ilmenite from the upper mantle (Hearn and McGee, 1984).…”