PETROGENESIS OF INTERMEDIATE STOCKS AND DIKES ON BONANZA RIDGE, SUMMIT AND WASATCH COUNTIES, UTAH

The stocks and associated intrusive bodies exposed on Bonanza Ridge are part of the larger Wasatch Igneous Belt, an Eocene (30-36 Ma) suite of calc-alkaline igneous bodies (Vogel, 2001). The petrogenesis of these related upper crustal intermediate stocks may provide insight to details of the emplacement of the Wasatch Igneous Belt. Bonanza Ridge lies along the eastern end of the Wasatch Igneous Belt 7 km south of Park City, Utah. Samples of the intermediate hypabyssal stocks and dikes of Bonanza Ridge were prepared for whole rock analysis and thin-sectioned for petrographic and scanning electron microscopy. Whole rock composition data was collected using standards based Energy Dispersive X-ray Spectroscopy (EDS) on Li metaborate glass beads. Petrographic analysis shows all samples are porphyritic with an aphinitc groundmass and share a common phenocryst assemblage of plagioclase, amphibole, +/- biotite, +/- quartz. The plagioclase phenocrysts range in composition between Ab₆₉ and Ab₈₂ and often show complex oscillatory zoning patterns. Whole rock major element analyses reveal compositional similarities between the stocks and dikes as well. All the stocks and dikes are dacites, except one andesitic dike. Compositions range from 62 -70% SiO₂ with the stocks generally having greater SiO₂ relative to the dikes_. On Harker variation diagrams the stock and dike samples lie along fractionation trends of a calc-alkaline magma series, suggesting they may be related to the evolution of a single magmatic system. Fractionation modeling using PELE (Boudreau, 1999) suggests these igneous bodies were emplaced at a depths of 1-2.5 km and contained between 1-3% dissolved H₂O.The model relates the samples using the fractionation of plagioclase, biotite, clinopyroxene, amphibole, as well as other minor mineral phases and correlates to the samples with a R² =.81 for SiO₂ vs CaO, R²=.71 for SiO₂ vs MgO, R²=.68 for SiO₂ vs Fe_T. Future analysis will investigate the details of this co-genetic evolution using trace elements and strontium isotopes.

Boudreau, A.E., (1999). PELE-a version of the MELTS software program for the PC platform, Computers & Geosciences, v.25 n.2, p.201-203

Vogel, T.A., Cambray, F.W, Constenius, K.N.(2001). Origin and emplacement of igneous rocks in the central Wasatch Mountains, Utah. Rocky Mtn Geology 36, 199-162