FIELD AND PETROGRAPHIC STUDY OF RADIAL DIKES AT SINKER BUTTE HYDROVOLCANO, WESTERN SNAKE RIVER PLAIN, IDAHO

Sinker Butte volcano is an emergent hydrovolcano with an approximate age of 1 Ma. Deposits at Sinker Butte consist primarily of massive and layered lapilli tuffs overlying older basalts and capped by a layer of basaltic spatter. We investigated 10 dikes that radiate outward from the buried Sinker Butte crater. The dikes cut the entire section from the lowermost tuffs through the spatter cap, but do not cut older, pre-Sinker Butte basalts. Several dikes are composite, exhibiting zonation in both vesicularity and phenocryst abundance. A few of the dikes connect to thin sheets that intruded parallel to the bedding of the tuffs.

All of the dikes contain abundant small tabular crystals of plagioclase and less abundant crystals of olivine. The plagioclase crystals in most of the dikes display varying degrees of flow alignment. In addition to discussing the field relations, this paper presents the results of a petrofabric analysis based on thin sections of 20 oriented samples. The modal distributions and shape-preferred orientations of plagioclase crystals are used to differentiate pulses in the composite dikes and study movement paths in the sill-dike complexes.