Petrology and geochemistry of the Red Mountain pluton, Laramie Anorthosite Complex, Wyoming

The Red Mountain pluton (RMP) is the youngest and smallest major intrusion in the Laramie Anorthosite Complex (LAC) of southeastern Wyoming. The RMP is zoned from monzonite to quartz monzonite and quartz syenite. Related monzonite and quartz monzonite dikes cut the main body and concordant, ring-like granite dikes (Red Mountain granite) occur along the northern and northeastern margins of the pluton.;LAC monzodiorites, which may be comagmatic with anorthosite, are postulated to be the RMP parent. RMP major, minor and trace element and isotopic data document fractional crystallization as the primary process to produce the observed liquid-line-of-descent, but shallow level assimilation of crustal material is also evident. Rare earth element concentrations are controlled by the abundance of allanite in the RMP igneous facies.;Comparisons between the Sherman batholith and the Red Mountain granite suggest the Sherman batholith near the LAC could be produced by LAC monzonitic to syenitic liquids assimilating crustal material. The RMP possesses many characteristics that are similar to syenite/mangerite intrusions from other Proterozoic anorthosite complexes.;The RMP is enriched in alkalis, iron and the REE elements, but is depleted in calcium and magnesium. Mafic minerals evolve to very iron-rich compositions; the feldspars are Ab- and Or-rich. These compositional trends are a continuation of similar trends noted in the Sybille Monzosyenite and Maloin Ranch pluton of the LAC. While the RMP is petrographically and geochemically similar to the Sybille Monzosyenite and Maloin Ranch, the RMP has been produced by a separate and distinct path of liquid evolution.