COMPARATIVE ION AND ELECTRON MICROPROBE DATING OF WYOMING PROVINCE MONAZITE, WITH TECTONIC AND ANALYTICAL IMPLICATIONS

Monazite (Mnz) and zircon (Zrn) in a granitic gneiss from the NW Wyoming province (Tobacco Root Mountains, Montana) have been analyzed in situ by ion microprobe (SHRIMP) in order to date Early Proterozoic thermotectonic events. For comparison the Mnz has also been dated chemically by electron microprobe (EMP). Matrix Zrn (5-17% discordant, n=5) occurring as subequant grains or as overgrowths on inherited (i.e., >3000 Ma) magmatic cores yields an upper-intercept age of 2465 ± 18 Ma (2s) when anchored to a lower intercept of 72 Ma (known age of nearby Cretaceous batholith). Anhedral matrix Mnz (1-2% discordant, n=10) yields ²⁰⁷Pb/²⁰⁶Pb model ages mostly within a ~2430-2470 Ma range corresponding to a weighted average of 2454 ± 8 Ma (2s), although two crystallization (or recrystallization) events are potentially discernible within this range. These Zrn and Mnz ages duplicate an independent ²⁰⁷Pb/²⁰⁶Pb age of 2474 ± 19 Ma (2s) for Mnz, obtained by stepwise leaching of host garnet and interpreted as dating the onset of major metamorphism and deformation in the region. Subordinate, but texturally-distinct Mnz domains yield isotopic age subpopulations of: 2785 ± 9 Ma (²⁰⁷Pb/²⁰⁶Pb model age of truncated core, 1s), ~1978 Ma (upper intercept of 5-12% discordant array), and ~1710-1734 Ma (concordant ages of two rims). Respectively, these ages probably indicate: inheritance from an older protolith (Bighorn-Beartooth magmatic terrain?), local heating during ~2100 Ma mafic magmatism (Paleoproterozoic extension?), and late-stage overprinting following 1820-1740 Ma thermotectonism (Wyoming-Hearne collision?). Preliminary EMP ages for Mnz are largely consistent with the SHRIMP ages, albeit systematically younger (i.e., ~2640, ~2380-2410, ~2310-2340, and ~1675-1695 Ma). The magnitude of the SHRIMP-EMP age discrepancy appears to increase with both ²⁰⁷Pb/²⁰⁶Pb age and Th + U content of the Mnz. Provisionally, these patterns reflect either: fluid-mediated Pb loss from the Mnz, such as affected the Zrn, or some artifact of the original EMP method of analysis. Subsequently, an improved method of Pb background correction, applied to EMP dating of eastern Wyoming province rocks (Black Hills, South Dakota), has resulted in Mnz ages of 1735 and 1710 Ma that duplicate independent ²⁰⁷Pb/²⁰⁶Pb ages previously obtained by stepwise leaching of host garnets.