PETROGRAPHIC ANALYSIS OF LAMPROITES FROM ZIRKEL MESA, LEUCITE HILLS DISTRICT IN SWEETWATER COUNTY, WYOMING, USA

Zirkel Mesa is a structure which was capped by lavas that erupted ̴ 0.96 Ma over flat lying sedimentary rock. It is the largest of the fourteen buttes and mesa located in the Leucite Hills of Wyoming (Coopersmith, Mitchell & Hausel, 2003). The mesa is located 3.3 miles northeast of the town of Superior, Wyoming in the Leucite Hills igneous province. A part of Zirkel Mesa is quarried for road gravel, and listed as a potassium deposit. Specimens were collected atop a cinder cone on Zirkel Mesa at an elevation of 7,800ft. by geology students from the University of West Georgia on an experiential learning field trip in June 2015. Samples from this location are a diopside-sanidine-phlogopite lamproite. The purpose of this research is to better understand the formation of lamproite by examining the complex mineralogical and geochemical characteristics of continental alkaline rocks from Zirkel Mesa.

The hand samples collected are a peach-tan color on unweathered portions of the rock. Small gas vesicles (≤1mm) can be observed throughout the rock samples and vugs (≤1cm in diameter) are present as well. The samples have a porphyritic-aphanitic texture with phlogopite phenocrysts. The mica crystals are predominantly subhedral to anhedral, yellowish-reddish brown in color and have platy crystal habit. These crystals are nearly equigranular generally ranging from 0.5-1mm in size. Larger crystals are 6-7mm in size and some are zoned by color. Phlogopite constitutes ̴ 90% of the rock. Small blackish-green, stubby, anhedral crystals of pyroxene were observed.

In thin section the phlogopite phenocrysts are a yellowish brown color under normal light and displays upper 3^rd order interference colors in polarized light. Interference colors are masked in darker crystals. Crystals are pleochroic, in shades of brown. The rock matrix in thin section is composed of small lath shaped crystals of sanidine. Stubby anhedral green-colorless crystals of pyroxene can be found in small abundance in thin sections. These crystals are not or are weakly pleochroic in shades of green and show upper 2^nd order interference colors. Some crystals are compositionally zoned observable by varying birefringence. Apatite is observable as microphenocrysts in the rock matrix as an accessory mineral. Diamonds have yet to be observed in the samples collected from Zirkel Mesa.