High-potassium intrusive rocks of the Crandall ring-dike complex, Absaroka Mountains, Wyoming

Abstract

The Crandall ring-dike complex has a gabbro-diorite core intruded by a shoshonitic ring dike. Both have been cut by granular and porphyritic quartz monzonite dikes. The most abundant rock is medium-grained diorite. In contrast, the ring-dike shoshonite has large phenocrysts of plagioclase and augite in a microcrystalline ground-mass of biotite, sanidine, plagioclase, amphibole, magnetite-ilmenite, and variable amounts of glass. Texturally, the plagioclase and pyroxene appear out of equilibrium with the surrounding matrix in both the ring dike and the intrusive core rocks. The gabbro, diorite, and porphyritic quartz monzonite are characterized by plagioclases with bimodal core and rim compositions, indicating that one group is possibly xenocrystic. Trends on variation diagrams for both minor and trace elements cannot be traced from the diorite to the quartz monzonite. On variation diagrams, the shoshonite rocks have the widest scatter of data points, which can be explained by the addition of plagioclase and augite phenocrysts. Geochemical modeling is successful in demonstrating that the gabbro can be formed as a cumulate from the diorite by fractional crystallisation but is unsuccessful in relating the diorite and the quartz monzonite by this process. The strongly positive Eu anomaly in the gabbro supports the cumulate origin; the diorite and shoshonite have chemical and textural signatures which strongly indicate contamination by plagioclase and pyroxene accumulation. Even the porphyritic quartz monzonite has experienced plagioclase accumulation. The nature of the uncontaminated magma is uncertain. Chemically, the diorite is a plutonic equivalent of shoshonite, and support is given to the hypothesis that shoshonites are formed by contamination with plagioclase and pyroxene (Prostka, 1973).