Characterization of crustal xenoliths from the Bearpaw Mountains, Montana (USA), using U-Pb geochronology, whole-rock geochemistry and thermobarometry, with implications for lower crustal processes and evolution of the Wyoming Craton

A suite of crustal xenoliths from the Bearpaw Mountains, Montana, USA, was examined using zircon geochronology, whole-rock geochemistry and petrology in order to investigate mid- to lower crustal geological processes along the northern margin of the Wyoming Craton. The xenoliths are hosted within Eocene alkaline intrusive and volcanic rocks (“minettes”) and include mafic to intermediate granulites and amphibolites. Whole-rock geochemistry and geothermobarometry indicate igneous protoliths of alkaline to sub-alkaline composition, which recrystallized at lower to middle crustal conditions (642–817 °C and 3.5–9.4 kbar). Mantle-normalized trace element patterns for all xenoliths show a subduction signature, with extreme enrichments in large-ion lithophile (LILE) and fluid mobile elements (Cs, Rb, Ba, Pb, Sr) relative to high-field strength elements, together with relative Zr-Hf and Nb-Ta-Ti depletions. While late-stage modification of the xenolith compositions by the magma host cannot be excluded, simple binary mixing models between the host lava and xenoliths indicate that such a process was insignificant, and implies that the xenoliths preserved their original geochemical signatures. SEM-CL imaging of zircon reveal complex internal structures and indicates the presence of both magmatic and metamorphic zircon. In the simplest cases, magmatic grains are recognized by weakly preserved, altered oscillatory zoning. In more complex magmatic grains, high Th/U (x¯ = 1.2) zircon appear recrystallized in the presence of a melt phase, resulting in resorbed internal domains and exterior morphology. Metamorphic zircon with low Th/U (0.03–0.8, x¯ = 0.2) is present as whole grains or rims with uniform domains devoid of recognizable internal microstructures. Zircon U-Pb dating produces a spectrum of dispersed Late Archean to Early Paleoproterozoic ages (2000–2534 Ma), including a dominant mid-Proterozoic age population that records both magmatic (1834 ± 19 and 1874 ± 10 Ma) and metamorphic (1772 ± 5–1788 ± 4 Ma) events. Proterozoic ages are correlated with lithospheric processes along the cratonic north-western margin (Great Falls Tectonic Zone and Montana Metasedimentary Terrane), involving continental arc magmatism and collision-related metamorphism. The protolith to xenoliths containing mid-Proterozoic (ca. 1834–1874 Ma) magmatic zircon ages were formed by subduction-induced melting of the overlying mantle wedge. Xenoliths containing older, dispersed magmatic zircon ages (ca. 2000–2534 Ma) do not produce a significant age population that can be well correlated with events within the Wyoming Craton/GFTZ, and their protolith was likely produced by melting of a subduction-related metasomatized mantle in the Late Archean or earlier. Both Proterozoic and Late Archean protolith magmas are envisaged as new additions to the Wyoming Craton that resided within middle to lower crustal levels of the continental lithosphere before being subjected to collision-related metamorphism and recrystallization at ca. 1770 Ma.