Abstract

The ultrapotassic rocks of the Leucite Hills, Wyoming consist of silica-undersaturated madupites; silica-saturated to oversaturated wyomingites, orendites and olivine orendites; and “mixed” rocks with characteristics of both wyomingites and madupites. Based on textural relations and on major and trace element analyses of the rocks and their major minerals, phlogopite-nmmed olivines, some of the spinel group minerals and the salites are considered to be xenocrysts. Although consistent trends in the major elements suggest a genetic relationship among the rock types, the high Cr, Ni and 100 Mg/(Mg+Fe) ratios preclude fractional crystallization as a major mechanism in their genesis. Comparison of the compositions of the rocks with high pressure phase relations in the systems Mg2SiO4–KAlSiO4–SiO2 and KAl SiO4–MgO–SiO2–H2O–CO2 indicates that madupite liquid can be generated at pressures between about 24–34 kbar (94–120 km) depending on conditions of melting from a K-enriched peridotitic mantle source. The wyomingite and orendite magmas may be produced from the same source at pressures between approximately 14 and 19.5 kbar (49–68 km) depending on conditions of melting. By fractionation of olivine, diopside and phlogopite, an olivine orendite cumulate may be produced from an orendite liquid.

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