TIME SCALES OF LOWER CRUSTAL GRANULITE FACIES METAMORPHISM DURING THE TRANS-HUDSON OROGEN, NORTHERN MANITOBA

Four monazite U-Pb SHRIMP age populations from the Kisseynew Domain, northern Manitoba, can be related to multiple pulses of fabric development during final collision in the Trans-Hudson Orogen ~1.8 Ga. Alligned sillimanite + biotite inclusions in garnet, hercynite and cordierite and sillimanite + hercynite inclusions in garnet and cordierite preserve prograde fabrics that show an evolution from upper amphibolite through granulite facies metamorphism. We independently resolved pressures and temperatures of ~8 kbars and 850°C using TWEEQ thermobarometric software (Berman, 1991) and PERPLEX isochemical modeling software (Connelly, 1990).

One-dimensional heat-flow modeling that incorporates our monazite age data with published detrital zircon ages, and our thermobarometric reconstructions and isochemical modeling results shows a permissible tectonic reconstruction for final collision within the Trans-Hudson Orogen. The oldest monazite ages overlap with detrital zircon ages and suggest deposition within the Kisseynew Basin persisted until about 1830 Ma. An early monazite age population at ~1825 Ma may record basin-wide recumbent F1 folding. F2 and F3 folding are associated with peak metamorphism and development of sillimanite fabrics and are recorded by monazite age populations at 1815 and 1805 Ma. A final stage of monazite growth at 1795 Ma may record the retrograde growth of garnet and cordierite over peak metamorphic fabrics.

These results show that granulite facies metamorphism in the Kisseynew Domain occurred in the lower crust at moderate pressures of >~9Kbar and that granulite facies conditions persisted from ~1820 to ~1800 Ma. Decompression melting from peak pressures could have generated the felsic melts regionally deformed by F2-F3 folding to produce the exposed map pattern.