Major element, Rb, Sr, and Sr isotopic contents have been determined in predominantly chilled margin samples of a series of mafic dikes and plugs from the Beartooth Mountains, Montana and Wyoming. These rocks range in age from 2,550 to 740 m.y., and are predominantly continental tholeiites with low Al2O3 contents (∼12 percent), low K/Rb ratios (85 percent <250), and high Rb/Sr ratios (av = 0.17). The rocks can be separated on the basis of chemical criteria, principally TiO2 content, into four groups. Rb-Sr and K-Ar age determinations indicate that each group has a single, consistent age of formation.
Comparison of average elemental abundances and ages of individual dike groups and the Stillwater chill zone indicates continuous secular decrease in MgO and increase in K2O and TiO2 from 2,750 to 740 m.y. These patterns are interpreted to indicate a regular increase in the pressure of partial melting for successively younger magmas, possibly because of a world-wide decrease of the geothermal gradient during the Precambrian.