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K-rich mafic to ultramafic alkaline rocks, termed lamproites, are common in the Leucite Hills area of southwestern Wyoming. These lamproites are very young (0.8-2.0 Ma) and show beautifully preserved volcanic/subvolcanic structures, as well as spectacular mineralogy and textures. Shown here are:
[Below] The Black Rock rising above the surrounding clastic sedimentary rocks of the Rock Springs uplift (as seen from the northeastern face of Spring Butte). The Black Rock is essentially a lava sequence composed of olivine-bearing diopside-sanidine lamproite. The blocky rock in the near view is diopside-leucite-phlogopite lamproite (wyomingite) of Spring Butte;
[Upper right] Sanidine-leucite lamproitic lava (white) covering a paleosol surface (orange to red), Zirkel Mesa;
[Lower right] Transitional madupitic lamproite from Middle Table Mountain as seen under the microscope (plane-polarized-light image at left and view in crossed polars at right). This rock is made up of leucite (colorless, round, isotropic), phlogopite (orange, elongate), diopside (colorless, anisotropic) and glass (brown patches in the groundmass).
Zirkel Mesa, Leucite Hills
Black Rock viewed from Spring Butte, Leucite Hills Transitional madupitic lamproite under the microscopeTransitional madupitic lamproiteunder the microscope