MONAZITE THERMOCHRONOLOGY OF THE TRANS-HUDSON OROGEN, CANADA: ID-TIMS U-PB, EMP TOTAL PB, AND AR-AR AGE COMPARISONS

We have collected peak-metamorphic and high-T cooling ages from metamorphic rocks along a transect perpendicular to the Paleoprotreozoic Trans-Hudson Orogen (Sask and Man), which amalgamated the Archean Superior and Hearne provinces. Earliest magmatism in the THO began ~1900-1880 Ma whereas terminal collision occurred ~1800 Ma. ID-TIMS monazite and titanite crystallization ages are all 1810 ± 10 Ma, but single crystals from the same rock commonly show an age variation of up to 10 m.y., well outside analytical uncertainity. Orogen-wide Ar-Ar hornblende (500°C) cooling ages are ~1760 Ma, and Ar-Ar mica (350°C) cooling ages are ~1760 Ma to the east and ~1720 Ma to the west, with a steep age gradient near the Tabbernor Fault. In-situ elemental mapping of monazite from 4 regionally spaced metapeltic rocks reveals complex zonation in Th and U concentration. Results of EMP total-Pb chemical dating on these monazite (140 spots on 30 identified domains) spread over 200 m.y. (1880-1680 Ma), indicating xenocrystic inheritance and new monazite growth. Further, the EMP dates cluster around 4 time periods (Ma): 1833 ± 10, 1797 ± 6, 1761 ± 6, and 1722 ± 6. There appears to be a slight correlation between small grain size and younger ages. The older metamorphic dates combined with the dates on inherited domains are most consistent with our single crystal TIMS analyses, and confirm early crustal reworking and peak metamorphism ca. 1800 Ma. The 1760 Ma domains are most common from the eastern samples, consistent with Ar-Ar hornblende and mica dates. Monazite inclusions in garnet from the east yielded EMP dates of 1765 Ma, suggesting the 1760 Ma event is a significant thermal event and not simple cooling from peak ca. 1800 Ma metamorphism. In this regard, we note that numerous undeformed post-orogenic leucogranites also yield 1760 Ma dates. The 1720 Ma domain is most evident in the western sample, again consistent with local Ar-Ar mica ages. We suggest that metapeltic monazite from the THO recorded protracted crustal reworking during the prolonged tectonothermal history of the orogen, most likely due to a repeated monazite dissolution and reprecipitation. Significant numbers of EMP analyses are required to elucidate such an evolution, and should be accompanied by additional thermochronologic data.