CONTACT ANATEXIS AT CIRCA 2100 MA IN METAPELITES ADJACENT TO WOLF CREEK ULTRAMAFICS, SOUTHERN RUBY RANGE, SW MONTANA

The Ruby Range of southwestern Montana is a basement-cored uplift exposing Archaean to Paleo-Proterozoic rocks of the Wyoming Craton. In the southeastern part of the range, the Wolf Creek ultramafic bodies, coarse orthopyroxenite to harzburgite, intrude a 5 km long by 1.5 km wide belt of migmatitc metapelites and gedrite-garnet-quartz gneiss that do not occur elsewhere in the range. Pressure-temperature estimates from mineral assemblages and compositions indicate conditions of 750-800 °C at 700 to 900 MPA during anatexis.

The ultramafics typically retain igneous textures but may be locally serpentinized near post-metamorphic faults that bound the belt. The metapelites are especially well exposed in an outcrop adjacent to and structurally above the largest ultramafic body. Melt segregations, like the ultramafics, are largely undeformed and retain igneous textures despite their presence within the highly deformed terrain. From these relationships we infer that anatexis occurred in response to heating during emplacement of the ultramafics and that anatexis was the last significant metamorphic event to affect the rocks.

U-Pb zircon ages by laser ablation (University of Arizona LA-ICPMS) yield evidence for metamorphism affecting the metapelites at an age ≈ 2377 ± 46 Ma (2 sigma) and at ≈ 2081 ± 30 Ma (weighted mean Pb²⁰⁷/Pb²⁰⁶ ages). The latter ages are from zircons with high U/Th ratios, consistent with growth in metamorphic fluids. A few grains yield ages ≈ 2720 Ma and are inferred to be detrital in origin. Ages of granitic gneisses that occur structurally above and below the metapelite indicate a magmatic event or events at ≈ 2750 Ma. A metamorphic event at ≈ 2525 ± 26 Ma affected the lower gneiss but is not observed in the upper. Neither yielded ages ≈ 2081 Ma, and with the exception of ≈ 2066 Ma mafic dikes in the adjacent Tobacco Root Range, similar ages have not been reported for the region. We, therefore, interpret ≈ 2081 Ma age to date a localized anatectic event driven by intrusion of the ultramafics.