PETROLOGY AND GEOCHEMISTRY OF THE JUDITH MOUNTAINS ALKALIC IGNEOUS CENTER, MONTANA, USA

Magmatism in the Judith Mountains, central Montana, USA, was calc-alkalic from ~69 to 67 Ma and alkalic from ~67 to 62 Ma (K/Ar; Marvin et al., 1980). The earliest pulse included subalkalic rhyolite and quartz monzonite, with ensuing alkalic trachyandesite, trachyte, and phonolite (~67-65 Ma), and a final pulse of alkali rhyolite (~62 Ma). Twelve whole-rock geochemical analyses reveal similarities to nearby Highwood and Bearpaw Mountains, but differ from the Crazy Mountains and Black Hills in their lower total REE, Th/U and Ce/Yb, and higher Ba/Ce, characteristics that, in conjunction with published radiogenic isotopic data, have been associated with a lithospheric signature of the Wyoming Archean craton in these other centers.

Ba concentrations are highest in alkali trachytes (2240-2968 ppm), and lowest in subalkalic rhyolites (531-981 ppm), suggesting that the earlier subalkalic phase resulted from assimilation of lower crustal rocks with low contents of Ba, U, Th, and REE. The U and Th contents of the Judith Mtns are low relative to those of other centers in the region: U content is 1-12 ppm, with 1 ppm in subalkalic rhyolite and 12 ppm in phonolite, and Th content ranges from 2 to 27 ppm, with lowest concentrations in rhyolites and highest concentrations in phonolites. REE patterns of alkalic rocks are similar to those of the Bearpaw Mtns alkalic samples, but Judith Mtns subalkalic volcanics have lower total REE contents and LREE/HREE ratios.

Harker diagrams (major oxides vs. silica) reveal that fractional crystallization played a significant role in petrogenesis (except for K and Ca). La, Ce, Nd, Yb, Y, Zr, Ni, V, Ti, and Nb decrease fairly consistently with increasing silica content, but as with K and Ca, Ba and Sr vary inconsistently with silica, as do U and Th. Potassium and carbonate metasomatism may explain some of this variability; additionally, late-stage silica flooding occurred in rocks containing pseudo-leucite. Calcite infiltration, potassium metasomatism, and trace-element geochemistry support previous suggestions of carbonatite presence at depth, although no carbonatite was intercepted at a depth of 2000 feet. The magmatic evolution of the Judith Mtns is similar to that of other centers in the region, with early subalkalic magmatism and crustal assimilation followed by mantle-derived alkalic magmas.