Late-stage Archean continental growth: New insights from zircon Hf-isotopes of granitoids from the Slave Craton (Canada)

Intrusions of large suites of late-stage Archean granitoids (ca. 3.0-2.5 Ga) are found across most Archean cratons globally, with their appearance marking the onset of potassic granitoid magmatism on Earth and reflecting a key stage in craton stabilization. Geochemical interrogation of these granitoids can inform on the characteristics of their more ancient precursors, including chronological fingerprints of the magmatic source and the nature of any supracrustal inputs. By reconstructing the chemical evolution of the ancient continental crust, we can further assess the timing of new crustal growth from the mantle versus crustal reworking processes.

 

The Slave craton (Northwest Territories, Canada) hosts a remarkable record of pulsed crustal formation throughout the Archean, including the oldest known zircon-bearing rocks in the Acasta Gneiss Complex (ca. 4.02 Ga). Notably, suites of 2.62-2.58 Ga granitoids occur across the craton which have largely intruded the Mesoarchean basement. Previous whole-rock analyses of these granitoids showed distinct geospatial trends in their Pb and Nd signatures which has led to the recognition of distinct isotopic boundaries. Where the western granitoids exhibit patterns suggesting a derivation from older and more evolved precursors, while the eastern granitoids contain patterns associated with derivation from a far more juvenile source [1]. These granitoids thus offer an unique opportunity to explore the nature of both ancient and juvenile crustal growth and/or reworking in the Archean.

 

We evaluated zircon Lu-Hf and U-Pb isotopes by laser-ablation split-stream (LASS) ICP-MS from 11 granitoid samples across the Slave craton. Additionally, we report O-isotopes from SIMS analysis of the same zircon crystals. In contrast to the patterns of whole-rock εNd, we found no significant geospatial trend in their zircon εHf or δ¹⁸O signatures. The εHf values of these samples are positive (+0.7 to +6.2), and fall just below the modeled 4.4 Gyr depleted mantle evolution. The δ¹⁸O values range from those of typical mantle zircon to slightly elevated (+4.5 to +6.9 ‰). The whole-rock elemental data for these granitoids are consistent with the global classification for late-stage Archean biotite- and two-mica granites and hybrid granites [2], however our εHf and δ¹⁸O values suggest the granitoids of this study represent juvenile magmatism with limited interaction with evolved supracrustal material. This is consistent with previous interpretations suggesting that these Slave craton granitoids formed in a convergent margin setting [1]. We will place these geochemical interpretations in a global context of other Archean crust-forming environments, exploring the implications regarding our understanding of crustal growth through time and the depleted mantle evolution.

 

[1] Davis, et al. (1996) Chem. Geo., 130 (3-4), 255-269. [2] Laurent, et al. (2014) Lithos, 205, 208-235.