PUNCTUATED MONAZITE GROWTH: A KEY TO METAMORPHIC EVOLUTION AND OROGENIC RATES DURING THE 1.8 GA TRANS-HUDSON OROGENY, MANITOBA

Four distinct monazite U-Pb SHRIMP age populations are identified regionally in meta-arenitic to meta-argillaceous migmatites in the Kisseynew Domain in the eastern Trans-Hudson Orogen (THO) in northern Manitoba, Canada. These ages are interpreted to represent multiple pulses of fabric development during metamorphism that initiated in the northwestern Kisseynew Domain around 1.83 Ga and lasted until 1.78 Ga in the southeast. Chemical mapping of monazite in both grain mount and thin section reveals zoning patterns that record a complex and punctuated growth history characterized by embayed cores mantled by multiple rims. Quantitative analysis reveals higher Y-content in the older cores and the youngest outer-most rims than in the intervening mantles. This may indicate that monazite grew prior to and then contemporaneously with the growth of garnet during prograde metamorphism and then continued to grow during the dissolution of garnet during retrograde metamorphism. Peak metamorphism recorded by reconstructed equilibrium mineral assemblages was recovered through high-temperature thermobarometry and reached 850°C and 8 kbar in the north.

Metamorphic monazite ages combined with detrital monazite and zircon U-Pb SHRIMP ages (Percival et al., 2005 Manitoba Report of Activities) and ⁴⁰Ar/³⁹Ar hornblende and biotite cooling ages (Schneider et al, 2007, Precambrian Research) constrain the timing of the 850°C and 8 kbar peak metamorphism resolved from isochemical equilibrium models and high-temperature thermobarometry. One-dimensional thermal models incorporate these data and yield rapid average heating and loading rates of 30°C/m.y. and 1.5 mm/year respectively between 1.83 and 1.80 Ga.