ZIRCON TEXTURES AND U-PB EVIDENCE FOR SYN-OROGENIC CRUSTAL MELTING IN THE PALEOPROTEROZOIC BIG SKY OROGEN, SW MONTANA

Syn-orogenic crustal magmatism in collisional orogens can provide information about petrogenetic processes, markers for evaluating the timing and kinematics of tectonic events, and can significantly influence crustal rheology and strain localization. The ca. 1.8-1.7 Ga Big Sky orogen records widespread granulite-upper amphibolite facies metamorphism and deformation overprinting mostly Archean (3.3-2.7Ga) protoliths in southwest Montana, yet there is little previously documented evidence of crustal melting within the collisional orogen. In the SE portion of the N. Madison Range, metamorphic zircon and monazite from multiple lithologies constrain the timing of Big Sky associated high-grade tectonism to between ~1.75-1.72 Ga at peak conditions of 0.9-0.8 GPa and 700 °C. Several small volume (10s of meter scale) granitic to monzodioritic bodies crop out in this part of the range and have field relationships suggesting that they are some of the youngest intrusive units.

One sample is from a white two-mica monzodiorite with sillimanite blades up to 1 cm long and anomalously high bulk aluminum content (19.3 wt. % Al₂O₃ and 63.2 SiO₂). Zircon is abundant and consists of dispersed single grains with low CL intensity cores (up to 100 μm) and bright CL rims (1-20 μm) and glomerocrystic clusters with up to ten distinct cores welded by CL-bright rim material. The zircon glomerocrysts are also spatially associated with other accessory phases including rutile, monazite and apatite. LA-ICP-MS U-Pb analysis yielded a weighted mean ²⁰⁷Pb/²⁰⁶Pb date of 3184 ± 63 Ma (2σ, MSWD =1.9, n =23) for concordant or near concordant analyses of core domains and a weighted mean ²⁰⁷Pb/²⁰⁶Pb date of 1745 ± 32 Ma (2σ, MSWD =0.67, n =15) for the rims. We interpret the zircon cores as inherited and the rims to have grown during partial melt crystallization. The mineralogy, anomalously high aluminum bulk composition, and commonly glomerocrystic zircon textures suggest that this granitoid represents a metasedimentary-sourced partial melt product that is contaminated with restite. Therefore, this and perhaps other small volume peraluminous intrusions in the range represent the product of Paleoproterozoic crustal melting during the Big Sky Orogeny.