File:Pilot Butte (Pliocene madupite lamproite volcanic center in the Leucite Hills, Wyoming, USA) 5.jpg

English: Pilot Butte - an eroded madupite volcanic center in the Pliocene 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).

The hill seen here is Pilot Butte, the southwestern-most volcanic center in the Leucite Hills. It is composed of madupite, the rarest type of lamproite. The rocks are referred to as diopside lamproites. Pilot Butte madupite is composed of phlogopite mica, diopside pyroxene, leucite, magnophorite (= titanian potassic richterite amphibole), apatite, magnetite, and perovskite. The diopside occurs as microphenocrysts. The phlogopite is poikilitic around the diopside phenocrysts. Some glassy material is also present. Chemical analyses show that madupite is ultramafic (~42.6 to ~43.6 wt.% silica), magnesium-rich (~11 wt.%), calcium-rich (~12 wt.%), potassium-rich (~7 to 8 wt.%), and sodium-poor.

Age: Late Pliocene, 3 Ma

Locality: Pilot Butte, southwestern Leucite Hills Volcanic Province, 8 miles northwest of the town of Rock Springs, Sweetwater County, southwestern Wyoming, USA (41° 38' 39.44" North latitude, 109° 20' 57.51" West longitude)

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Synthesized from info. in:

Barton, M. & D.L. Hamilton. 1979. The melting relationships of a madupite from the Leucite Hills, Wyoming, to 30 Kb. Contributions to Mineralogy and Petrology 69: 133-142.

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.