Petrology and geochemistry of the Judith Mountains alkalic igneous center, Montana, USA

The Judith Mountains, of the central Montana alkalic province, are characterized by alkalic igneous rocks, intersecting fault zones, and two proposed pulses at ~69-67 Ma (calc-alkalic) and ~65-62 Ma (alkalic).  Limited studies of petrology and geochemistry of this specific center have been conducted.  We propose the Judith Mtns have geochemical attributes similar to those of other regional alkalic intrusive centers containing carbonatites.  In our study, a suite of 12 rock samples was collected and analyzed for major and trace-element geochemistry and compared to other regional centers.  The suite, excluding four samples, is silica-undersaturated, containing nepheline and/or leucite.  Two of the silica-oversaturated samples contain both quartz and leucite, indicating that the interstitial, late-stage quartz may represent sub-solidus alteration.   In the silica-undersaturated rocks, titanium (1487-4566 ppm) is concentrated in minerals including melanite garnet, titanite, rutile, and perovskite.  Ba concentrations are highest in trachytes (2240-2968 ppm), indicating that previously metasomatized lithopheric mantle was assimilated in forming these magmas.  La/Nb ratios (0.295-3.21) indicate a similar assimilation of lithospheric mantle.  Harker diagrams (major oxides vs. silica) reveal that fractional crystallization played a major role in the Judith Mtns.  Zr and Nb contents do not follow fractionation trends, however, indicating that metasomatic processes, assimilation, and/or carbonatite-silicate immiscible separation may have occurred.  REE patterns are similar to those of other centers in the province, pointing to similar magmatic origins throughout the region.  The magmatic evolution is similar to that of other centers in the region, with early subalkalic magmatism and crustal assimilation followed by mantle-derived alkalic magmas.