REEXAMINING THE GEOCHEMISTRY AND GEOCHRONOLOGY OF THE LATE CRETACEOUS BOULDER BATHOLITH, MT

The Late Cretaceous Boulder Batholith of southwestern Montana represents a voluminous early phase of the eastward migration of calc-alkalic magmatism into the foreland of the preexisting Mesozoic arc. The relationship of this magmatism to adjacent, contemporaneous ‘thin-skinned’ fold-and-thrust deformation remains enigmatic. The intrusive complex comprises 15 major plutons and numerous satellite stocks. Early geochemical and isotopic studies of the intrusions distinguished two separate magmatic suites, a main potassium-rich series and a peripheral sodium-rich series. In an attempt to constrain the origin and evolution of the two series, we have undertaken a detailed geochemical and geochronological study of 45 samples from 14 separate plutons and 8 samples from 4 individual satellite stocks. Preliminary analyses of most major and trace elements show predictable magmatic fractionation trends that are continuous and overlap one another. Rather, the two series are only distinguishable in the abundance of elements that show significant mobility in subsolidus, hydrothermal processes.

U-Pb dating of the main ‘potassic’ phase of the batholith yields a crystallization age of 76.5 ± 0.4 Ma based on three concordant and overlapping multigrain zircon analyses. Four concordant and overlapping multigrain titanite analyses record high-T subsolidus cooling at 75.7 ± 0.8 Ma. Two plutons from the ‘sodic’ series were also dated, one each from the northern and southern edges of the complex. The southern pluton yields a preliminary age estimate of 78.0 ± 1.3 Ma based on one concordant multigrain zircon analysis. Three other zircon fractions give discordant results and are interpreted to contain inherited zircon. The northern ‘sodic’ pluton yields a crystallization age of 74.6 ± 0.6 Ma based on concordant and overlapping results for two multigrain zircon fractions. Three other fractions underwent minor Pb-loss or contain inheritance. Three concordant and overlapping multigrain titanite analyses for this rock record high-T subsolidus cooling at 73.0 ± 2.3 Ma.