English: Vesicular olivine orendite lamproite lava (6.5 cm across at its widest) from 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.

This sample is a vesicular olivine orendite lamproite lava. This very rare rock comes from the Black Rock volcanic center, in the eastern Leucite Hills. Orendite is a diopside-sanidine-phlogopite lamproite having leucite, diopside pyroxene, phlogopite mica, sanidine, plus minor hornblende amphibole and rutile. Lavas at Black Rock include orendites and olivine orendites, the latter having noticeable to subtle, scattered olivine phenocrysts/xenocrysts. Either lithology may be non-vesicular or vesicular (with fairly small vesicles) or pumiceous. The scattered, shiny, golden-brown crystals on this rock are phlogopite mica phenocrysts.

Chemical analysis shows that orendite is mafic (50 to 56 wt.% silica), magnesian (<11 wt.%) and sodium-poor. Orendite at Black Rock has had its groundmass leucite partially altered to analcime. The orendites at this locality are inferred to have been derived from lherzolite-harzburgite mantle rocks that were metasomatically enriched in phlogopite veins at >1.2 Ga (the latter event may have been caused by Precambrian subduction along the Wyoming Craton margin).

Volcanism in the Leucite Hills may possibly be due to mantle melting by the subsurface fringe areas of the Yellowstone Hotspot track.

Age: middle Lower Pleistocene, 1.30 to 1.37 Ma

Locality: northern slopes of Black Rock, eastern Leucite Hills Volcanic Province, Great Divide Basin, northeast of the town of Superior, southwestern Wyoming, USA (41° 52' 26.02" North, 108° 47' 41.13" West)

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Example references on Leucite Hills geology:

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.

Schultz, A.R. & W. Cross. 1912. Potash-bearing rocks of the Leucite Hills, Sweetwater County, Wyoming. United States Geological Survey Bulletin 512. 39 pp.