File:Wyomingitic volcanic breccia with xenoliths (Lower Pleistocene, 2.19-2.56 Ma; Boar's Tusk, Leucite Hills, Wyoming, USA) 4 (48943826933).jpg

Wyomingitic volcanic breccia with light-colored xenoliths in the Pleistocene of Wyoming, USA.

Southwestern Wyoming's Leucite Hills are a group of Pliocene to Pleistocene volcanic centers that erupted lamproite lavas, a rare extrusive igneous lithology. Based on chemistry and mineral content, the Leucite Hills lamproite lavas have been categorized as wyomingite, orendite, or madupite.

Volcanism in this area may possibly be due to Yellowstone Hotspot fringe melting of the mantle. The lamproites appear to be derived from lherzolite-harzburgite mantle rocks that were metasomatically enriched in phlogopitic veins at >1.2 Ga (the latter may be caused by Precambrian subduction along the Wyoming Craton margin).

Seen here is a volcanic breccia on the eastern slope of Boar's Tusk, a volcanic center in the northwestern-most Leucite Hills. The clasts are composed of wyomingite, the most common type of lamproite in the area, and the "state rock" of Wyoming. Chemical analysis shows that Boar's Tusk wyomingite is mafic (~50% wt.% silica), with ~7 wt.% magnesium, ~6 wt.% calcium, and ~9.8 wt.% potassium. Boar's Tusk wyomingite is darker in color and richers in phlogopite mica and diopside pyroxene than other wyomingite volcanic centers in the Leucite Hills.

Age: Early Pleistocene (sensu novum), 2.19 to 2.56 Ma

Locality: eastern slopes of Boar's Tusk, northwestern Leucite Hills Volcanic Province, ~16.3 miles southeast of the town of Farson, Sweetwater County, southwestern Wyoming, USA (41° 57' 46.53" North latitude, 109° 11' 54.79" West longitude)

Synthesized from info. in:

Lange, R.A., I.S.E. Carmichael & C.M. Hall. 2000. 40Ar/39Ar chronology of the Leucite Hills, Wyoming: eruption rates, erosion rates, and an evolving temperature structure of the underlying mantle. Earth and Planetary Science Letters 174: 329-340.