INVESTIGATING THE AGE AND SIGNIFICANCE OF PERALUMINOUS LEUCOGRANITE IN THE N. MADISON RANGE, SOUTHWEST MONTANA

Syn-collisional orogenic magmatism provides important petrogenetic indicators, markers for evaluating the timing and kinematics of tectonic events, and can significantly influence crustal rheology and strain localization processes. The ca. 1.8-1.7 Ga Big Sky orogen records widespread granulite-upper amphibolite facies metamorphism and deformation in southwest Montana. However, documented magmatic activity in this time interval is limited to local 1.77 Ga incipient migmatization in the Tobacco Root Mountains. This contribution addresses the nature of numerous small volume leucogranite bodies found in the N. Madison Range that may be Paleoproterozoic syn-collisional melts.

The Precambrian rocks of the N. Madison Range are poly-deformed granitoids, gneisses, supracrustal schists, amphibolites and quartzites. The major igneous units are dominantly intermediate in composition and those that have been dated yield zircon crystallization ages of ~3.3-2.7 Ga. Metamorphic zircon and monazite from multiple lithologies in these deep to mid crustal rocks constrain the timing of most recent high-grade metamorphism to between ~1.75-1.72 Ga. We identified numerous m-scale tabular leucogranite bodies, locally foliated and concordant with known Paleoproterozoic fabrics, in the southern Spanish Peaks region. Qualitative evaluation suggests the units are peraluminous and have Ms - Bt – Kfs ± Grt ± Sil igneous assemblages that are consistent with leucogranites produced from melting of metasedimentary sources in hot, thick crust (e.g. Himalayan orogen). CL images of zircon from one leucogranite body illuminate distinct morphological populations that may represent inherited cores, igneous grains with euhedral concentric zoning, and locally thin metamorphic rims. Monazite EMP dates from this sample range from 1.75-1.67 Ga, with some evidence suggesting the earlier end of this spectrum may represent an igneous population. Thus, we speculate that these small volume igneous units are Paleoproterozoic crustal melts in the hinterland of the Big Sky orogeny. The small scale of these leucogranite bodies could explain why a potentially wider distribution may have gone unrecognized. Ongoing work includes U-Pb dates of zircon, geochemical characterization and investigating the spatial distribution of these units.