PETROGRAPHIC AND GEOCHEMICAL INVESTIGATIONS OF THE MCCARTNEY MTN. STOCK, SW MONTANA

Southwestern Montana contains an extensive record of Late Cretaceous to Eocene-aged magmatism that is well-preserved within a suite of granitoid batholiths, namely the Boulder, Pioneer, Mount Powell, Philipsburg, and Tobacco Root batholiths. Located approximately 15 miles north of Dillion, MT, is the McCartney Mtn. stock. This granitoid intrusion, along with a series of smaller unnamed intrusions, was emplaced between the Pioneer and Boulder batholiths ~75 million years ago. Although it has been hypothesized that the stock is related to the Pioneer batholith, this correlation is not well-established. To address this issue, this study focuses on characterizing the petrography and geochemistry of the McCartney Mtn. stock in order to evaluate this hypothesis.

Preliminary field observations indicate that the intrusion is predominantly composed of hornblende-biotite granodiorite with minor quartz diorite. Petrographic observations indicate that quartz, plagioclase, alkali feldspar, hornblende, and biotite are dominant mineral phases. New major and trace element data reveal that the McCartney Mtn stock has a narrow compositional range (58-66 wt% SiO₂). Samples are calcic to calc-alkalic, metaluminous (0.72 – 0.95), contain high concentrations of Sr (505 – 529 ppm) and Ba (622 – 1054 ppm), low Rb/Sr ratios (0.08 – 0.18), and high Na₂O/K₂O ratios (>1).

Combined with petrographic observations, these new geochemical data support that the McCartney Mtn. stock is broadly similar in composition to intrusions exposed in the Pioneer batholith, which have been interpreted to be partial melts from a deep crustal mafic source. Ongoing work will continue to elucidate these fundamental relationships and better characterize the petrogenesis of the McCartney Mtn. stock.