Geology, mineralogy, and geochemistry of the McClure Mountain alkalic complex, Fremont County, Colorado

The McClure Mountain alkalic complex is evolved from a consanguineous suite of gabbroic, syenitic, and lamprophyric magmas. Field, mineralogical, and chemical data suggest several differentiation hypotheses which are tested by means of linear least squares mixing models. Initially, partial melting of alkali peridotite produced parental nepheline-normative alkali basalt magma. A part of this magma ascended to shallow crustal levels along deep-seated faults and differentiated to a stratiform body of cumulitic olivine gabbros, gabbros, anorthosites, and magnetite-ilmenite ores. Petrographic studies, graphical analyses, and modeling support the hypothesis that the late lamprophyres were formed by mixing of a partly crystalline alkalic basalt magma with syenitic magma. Models rule out the hypothesis that the lamprophyres can be parental to syenite and gabbro. Progressive changes in the petrography along the strike of individual lamprophyres can be explained by differentiation during transport.