Building Archean cratons from Hadean crust

English Abstract:

Archean cratons are dominated by felsic Tonalite-Trondhjemite-Granodiorite (TTG) rocks. These felsic rocks however, cannot be directly produced by melting of the mantle but must instead be derived from an older mafic precursor. The scarcity of Hadean zircons also suggests that felsic magmatism may have not been a prominent process involved in the formation of the earliest crust and raises questions about the nature of the primordial crust and how, or if, it was involved in the formation of stable Archean cratons. The Nuvvuagittuq greenstone belt (NGB) provides a glimpse of what may have been Earth’s primitive crust. The NGB mafic rocks exhibit a ¹⁴²Nd/¹⁴⁴Nd vs. Sm/Nd correlation suggestive of an igneous age of ~4.3 Ga. These Hadean NGB rocks are surrounded and intruded by multiple generations of Eoarchean TTG produced at 3.76 Ga, 3.66 Ga, 3.51 Ga and 3.35 Ga. This Eoarchean / Hadean terrane is included in the Hudson Bay terrane, a terrane mainly composed of 2.88 to 2.69 Ga TTG. Deficits in ¹⁴²Nd compared to the terrestrial Nd standard have been measured in a number of Eoarchean and Neoarchean TTG surrounding the NGB area. The ¹⁴²Nd/¹⁴⁴Nd of these TTG however do not correlate with their Sm/Nd ratios, and all TTG analyses fall to the low Sm/Nd side of the NGB mafic rock isochron consistent with their derivation by partial melting of Hadean mafic crust. This is also supported by the εHf-zircon vs. time trend for the NGB TTG. One TTG sample that displays a deficit in ¹⁴²Nd however is located ~150 km from the NGB, suggesting that the Hadean crustal precursor was much more widespread than the surviving NGB remnant. Deficits in ¹⁴²Nd in the Eoarchean and Neoarchean TTG imply that the crustal reworking processes occurred for at least a billion years, from 3.8 Ga to 2.7 Ga within the Northeastern Superior Province and crustal recycling may have been an important mechanism contributing to the stabilization of the craton.