PETROGENESIS OF XENOLITHS IN MIOCENE BASALTS OF NORTHEASTERN OREGON

This is the first in depth study of xenoliths contained within basalt flows of the Powder River Volcanic Field of northeastern Oregon. During this study, a geologic map was produced illustrating the different lithologic units and their geographic distribution in the field area, which is located ~25 km due east of Baker City. Samples of three different types of rocks and two types of xenoliths were collected. The rock and xenolith samples were analyzed in terms of petrography, mineral chemistry, and whole-rock chemistry.

Miocene age volcanic rocks were erupted onto the Triassic-Permian metamorphic bedrock of the accreted island arc terrane of northeastern Oregon. Two different types of volcanic rocks are exposed in the study area. The first is a high-magnesium basalt which contains ultramafic and siliceous xenoliths. The second is an alkali-rich trachybasalt.

The siliceous xenoliths are very fine-grained and are composed of >95% silica. They are white/tan in hand sample, have average diameters of 4 cm, have rounded margins with distinct reaction rims, and comprise 90% of the xenoliths contained within the basalt. The siliceous xenoliths were identified as fragments of the underlying chert bedrock.

The ultramafic xenoliths are medium-grained and are almost entirely composed of olivine (average composition Fo₈₃) with varying degrees of alteration. They are orange/brown in hand sample, have average diameters of 4 cm, have angular margins with no distinct reaction rims, and make up 10% of the xenoliths. The ultramafic xenoliths are all dunites, and are interpreted as cumulates which formed along the wall of a crustal magma chamber.