A REMNANT OF HADEAN CRUST PRESERVED IN THE NUVVUAGITTUQ GREENSTONE BELT

The Nuvvuagittuq Greenstone Belt (NGB) is dominated by mafic and ultramafic rocks metamorphosed to at least upper amphibolite facies. Rare felsic intrusive rocks give a minimum age of ~3.8 Ga for the NGB. However, primary U-rich minerals that may provide reliable dates for rock formation have yet to be found in the dominant lithology called the Ujaraaluk unit. Metamorphic zircons, rutiles and monazites are, however, present in the unit and give variably discordant results with ²⁰⁷Pb/²⁰⁶Pb ages ranging from 2.8 Ga to 2.5 Ga. The younger ages overlap 2686±4 Ma zircon ages for intruding pegmatites and Sm-Nd ages for garnet formation in the Ujaraaluk rocks suggesting this era as the time of peak metamorphism and metasomatism in the NGB, coeval with regional metamorphism of the Superior craton. ¹⁴⁷Sm-¹⁴³Nd data for Ujaraaluk whole rocks provide a statistically poor isochron of 3814±300 Ma. However, this “isochron” is seen to consist of a series of ~2.7 Ga slopes for the different geochemical groups of rocks, emanating from a baseline distribution older than 4 Ga. The ¹⁴⁶Sm-¹⁴²Nd chronometer is less affected by metamorphism at 2.7 Ga because of ¹⁴⁶Sm extinction prior to ~4 Ga. Expansion of the ¹⁴²Nd dataset for the Ujaraaluk rocks and associated ultramafic continues to show a good correlation between Sm/Nd and ¹⁴²Nd/¹⁴⁴Nd that corresponds to an age of 4.332⁺²⁸_-36 Ga. The dataset now includes samples with superchondritic Sm/Nd ratios that extend the correlation to ¹⁴²Nd excess of up to 8 ppm compared to the terrestrial standard. The upper Sm/Nd end of this correlation is defined by rocks that are interpreted as cumulates to compositionally related extrusive rocks indicating that this crystal fractionation had to occur while ¹⁴⁶Sm decay was active, i.e. well before 4 Ga. Intruding gabbros give ¹⁴³Nd and ¹⁴²Nd isochron ages overlapping within error at 4.16 Ga also supporting an Hadean age. 3.6 Ga tonalites surrounding the NGB and 3.8 Ga felsic rocks show a deficit in ¹⁴²Nd compared to the terrestrial standard. These felsic rocks plot to the low Sm/Nd side of the Ujaraaluk isochron without a correlation between their Sm/Nd and ¹⁴²Nd/¹⁴⁴Nd suggesting that they are remelts of this type of mafic basement after ¹⁴⁶Sm was extinct. The NGB thus preserves over 1.6 billion years of early Earth history including an expanse of mafic crust formed in the Hadean.