GEOCHEMICAL STUDY OF THE TIMBER HILL BASALT AND ADJACENT BASALT PLUGS WITHIN THE BLACKTAIL AND RUBY MOUNTAINS, SOUTHWEST MONTANA

We present major and trace element whole-rock geochemistry for several basalt plugs and associated flows that occur in the Blacktail and Ruby Mountain ranges east of Dillon, Montana. The origin, nature, and timing of basalt magmatism in this region is not precisely known. The Blacktail and Ruby ranges consist predominantly of Precambrian crystalline rocks overlain by Paleozoic-Cenozoic sedimentary rocks and felsic intrusions at their northwestern and northern portions, respectively. On the eastern side of these ranges within the Neogene Ruby rift graben are several isolated plateau outcrops of the 6.0 Ma Timber Hill basalt that extend for approximately 50 km (Lonn et. al., 2000). Previous workers interpret the Timber Hill basalt as having originated from the Heise volcanic field in eastern Idaho and flowed northwards over 150 km up the Ruby graben. Geologic mapping has identified several extrusive basalt plugs within the Blacktail and Ruby ranges that could possibly have been alternate source vents for the Timber Hill basalt flow. The age of these extrusive plugs is currently uncertain and is an objective of on-going investigation. To better investigate this hypothesis, we performed whole-rock major and trace element geochemical analyses of 39 samples from the basaltic plugs as well as the Timber Hill basalt flows to see if they display similarities in their chemistry.

This study focuses on characterizing the geochemistry of the Blacktail and Ruby range basalts with those of the Timber Hill flow. Preliminary results indicate the Timber Hill samples have Mg# of 33-40, whereas the Blacktail and Ruby basalts record Mg# predominately between 50 and 85. Chemical variation diagrams show a consistent cluster of Timber Hill data that generally do not coincide with the Blacktail or Ruby mountain basalts. Limited geochemical data for basalts from the Heise volcanic field do not match with the Timber Hill basalt either. Therefore, while we believe the Timber Hill is not derived from the extrusive vents in the Blacktail or Ruby ranges, it is possible that it is derived from a yet to be discovered source.