TRACING THE EXTENT OF THE 6.0 MA TIMBER HILL BASALT INTO THE HEISE VOLCANIC FIELD: AGEOCHEMICAL APPROACH TO CORRELATING YOUNG BASALTIC LAVA FLOWS WITH THE YELLOWSTONE HOTSPOT TRACK

This study investigated the petrography and geochemistry of the 6.0 million year old

Timber Hill basalt flow in the eastern part of the Ruby Mountains of southwest Montana. It has been suggested that the Timber Hill basalt flow originated within the Heise Volcanic Center (HVC) and traveled northward into the Ruby Graben. The HVC is located over 150 km away within the Snake River Plain of the Yellowstone hotspot track in eastern Idaho.

Samples were collected from outcrops along the mapped trace of the Timber Hill basalt in the Ruby Valley as well as outcrops of similar age basalts south of the Blacktail/Ruby mountains at Lima, Montana, the 6.0 Ma Lone Pine basalt in Idaho, and south of Humphrey, Idaho on the Snake River Plain. If the Timber Hill originated from the HVC we would expect similar geochemical signature of the basalts along the trace of the Timber Hill. However, if the geochemistry is significantly different it might suggest an alternative model for the origin of the Timber Hill basalt flow is warranted.

Geochemical results show differences between the Timber Hill and the associated basalts. Timber Hill basalts are characterized by Mg#’s between 35-37, whereas basalts from the Lima area and Spencer Idaho range from 47-55. SiO2 content is also higher in the Timber Hill basalts than the other samples analyzed. Variation diagrams consistently show Timber Hill signatures that are different than all other basalts analyzed. REE analysis of Timber Hill reveals LREE enriched by over 200 times compared to chondrites. Basalts from other locations are enriched 45-120 times. Chondrite-normalized trace element diagram indicates the Timber Hill basalts have enriched trace elements distributions compared to basalts from the other areas.

The lack of similarities across a spectrum of geochemical data between the Timber Hill and a series of Tertiary basalt flows along the projected path of the flow from the HVC suggests an alternative model for the origin of the Timber Hill basalt is needed. Perhaps a more localized source of the eruption of the Timber Hill is a better explanation.