Petrogenesis of the Red Mountain pluton, Laramie anorthosite complex, Wyoming: implications for the origin of A-type granite

The Red Mountain pluton, the youngest intrusion in the Laramie anorthosite complex, is one of five monzonitic to granitic A-type plutons intruded at 1.43 Ga in the Laramie Mountains of southeastern Wyoming. The Red Mountain pluton ranges in composition from fayalite monzonite, clinopyroxene quartz monzonite, and biotite-hornblende quartz syenite to granite. The Red Mountain pluton is petrologically and geochemically distinct from the northern and southern Sherman batholith, the Sybille intrusion, and the Maloin Ranch pluton. It has higher FeO1/(FeO1 + MgO) and higher K2O, is more strongly enriched in REEs, and at any given silica content it has lower abundances of TiO2, FeO, MgO, CaO, and P2O5. The fayalite monzonite in the Red Mountain pluton crystallized at temperatures around 1000 degreesC and oxygen fugacities 1-2 log units below that of the FMQ buffer, similar to the crystallization conditions of the Sybille and Maloin Ranch plutons. The biotite-hornblende quartz syenite crystallized at oxygen fugacities near or slightly below those of FMQ. Most of the units in the pluton carry mantle-like Nd, Sr, and Pb isotopic compositions similar to the least contaminated anorthositic and ferrodioritic rocks of the LAC, indicating that the pluton evolved mainly via differentiation. Substantial crustal assimilation occurred only in the late dikes and the granite. We suggest that all of the 1.43 Ga A-type intrusions in southeastern Wyoming are ultimately derived from tholeiites. Extreme differentiation or partial melting produced monzonitic magmas that have assimilated varying amounts of siliceous crust. The differences between the Red Mountain pluton and other A-type granitoids in southeastern Wyoming are related to the extent of differentiation prior to crustal assimilation. The compositional ranges of A-type granitoids in southeastern Wyoming suggest that the geochemical spectrum of A-type granites worldwide similarly are produced from evolved mantle-derived melt that has assimilated varying proportions of crustal material. (C) 2003 Elsevier Science B.V. All rights reserved.