Paper No. 4
Presentation Time: 1:30 PM-5:30 PM
GEOCHRONOLOGY OF U-BEARING ACCESSORIES FROM PRECAMBRIAN METAMORPHIC ROCKS, RUBY RANGE, SW MONTANA: DECIPHERING GEOLOGICAL EVENTS THAT SHAPED THE NW WYOMING PROVINCE
U-Pb accessory minerals in metamorphic rocks from the Ruby Range were analyzed by microprobe techniques in order to constrain the timing of Precambrian geological events. Dated samples represent the Cherry Creek metasupracrustal suite, CCMS (youngest); the Dillon quartzofeldspathic gneisses (DQG); and the Pre-Cherry Creek basement gneisses, PCCG (oldest). Intruding the CCMS and crosscutting its NE-trending fabric are posttectonic tourmaline pegmatite bodies. Coexisting monazite and xenotime from the pegmatite yield 207Pb/206Pb ages with 2σ uncertainties of 1762 ± 6 Ma (weighted mean) and 1754 ± 6 Ma (upper-intercept), respectively, as determined by ion microprobe (SHRIMP-RG). Monazite from adjacent CCMS schists yields maximum U-Th-Pb chemical ages of ~1765 Ma, as determined by electron microprobe (EMP). Together, these results constrain latest thermotectonism and magmatism to a 1765-1760 Ma time frame. Monazite in a metapelite within the DQG also reveals evidence for the ~1765 Ma event, as determined by EMP, whereas monazites preserved as inclusions in garnet record a much older ~2450 Ma event. One DQG sample yields 207Pb/206Pb weighted-mean ages (±2σ) of 1765 ± 10 and 2460 ± 4 Ma for metamorphic monazite, and of 2762 ± 5 Ma for magmatic zircon, whereas another contains 2464 ± 26 Ma (migmatitic; Th/U=0.0-0.1) and 2773 ± 7 Ma (magmatic; Th/U=0.5-0.7) zircon only. Likewise, zircon in one PCCG sample preserves mostly 2455 ± 13 and 2783 ± 14 Ma ages, but also ~3000-3500 Ma ages of subordinate cores. The ~1760 Ma ages documented in the Ruby Range date waning phases of thermotectonism related to Wyoming-Medicine Hat collision along the Great Falls tectonic zone. The older ~2460 Ma ages reflect an earlier, cryptic thermotectonic event that is temporally associated with incipient breakup of the supercontinent Kenorland. Finally, occurrences of ~2780 Ma ages in the DQG record the Beartooth orogenic signature, whereas preservation of ~3000-3500 Ma ages documents a yet-older history for the PCCG basement.