Rapid Loading and Heating to Granulite Facies in the Eastern Trans-Hudson Orogen, Manitoba, Canada

Orthopyroxene and hercynitic spinel-bearing granulite facies assemblages are identified regionally in metagreywacke to metapelitic migmatites in the Kisseynew Domain, in the eastern Trans-Hudson Orogen (THO) in northern Manitoba, Canada. Using recovery techniques for high temperature rocks (Pattison, 2003 Journal of Petrology) in conjunction with mineral assemblage equilibrium models, we show that these rocks reached peak temperature conditions around 850°C and 8kbars. New U-Pb SHRIMP monazite ages show multiple pulses of metamorphic growth that initiated in the northern Kisseynew Domain around 1.83 Ga and extended until 1.79 Ga in the southeast. Detrital zircon ages as young as 1831±25 Ma (Percival et al., 2005 Manitoba Report of Activities), and amphibole cooling ages at 1770 Ma (Schneider et al, 2007, Precambrian Research) show that the metamorphism in the eastern THO was rapid and intense. These data, together with peak metamorphic conditions of 850°C and 8kbars at ca. 1.80 Ga, interpreted from the most substantial phase of monazite growth, yield heating and loading rates of 30°C/m.y. and 1 mm/year respectively between 1.83 and 1.80 Ga.

Granulite facies assemblages and evidence for partial melting are obvious at all scales in the field, but the cause of rapid heating to high-temperature conditions has not been resolved. Existing heat-flow models for the Kisseynew Domain suggest that internal heating of a thick pile of back-arc sediments was sufficient to cause partial melting and granulite facies conditions, however the presence of Archean granulite-facies gneisses in the cores of structural culminations suggests that much of the crust in this part of the Kisseynew Domain is depleted basement that would have yielded less radiogenic heat than a 40-km thick section of pelitic rocks. This observation requires consideration of additional heat sources including overthrust hot arc crust, mantle-derived magmas, and low-conductivity sedimentary “blankets,” in revised inverse thermal models.