It is unclear how the earliest continental crust formed on an Earth that was probably originally surfaced with oceanic crust. Continental crust may have first formed in an ocean island-like setting, where upwelling mantle generates magmas that crystallize to form new crust. Of the oceanic plateaux, Iceland is closest in character to continental crust, because its crust is anomalously thick1 and contains a relatively high proportion of silica-rich (sialic) rocks2. Iceland has therefore been considered a suitable analogue for the generation of Earth’s earliest continental crust3. However, the geochemical signature of sialic rocks from Iceland4,5,6,7 is distinct from the typical 3.9- to 2.5-billion-year-old Archaean rocks discovered so far8. Here we report the discovery of an exceptionally well-preserved, 4.02-billion-year-old tonalitic gneiss rock unit within the Acasta Gneiss Complex in Canada. We use geochemical analyses to show that this rock unit is characterized by iron enrichment, negative Europium anomalies, unfractionated rare-earth-element patterns, and magmatic zircons with low oxygen isotope ratios. These geochemical characteristics are unlike typical Archaean igneous rocks, but are strikingly similar to those of the sialic rocks from Iceland and imply that this ancient rock unit was formed by shallow-level magmatic processes that include assimilation of rocks previously altered by surface waters. Our data provide direct evidence that Earth’s earliest continental crust formed in a tectonic setting comparable to modern Iceland.
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We thank R. Ickert and A. Burnham for advice on zircon strain partition modelling; J. Davies for critical discussions throughout; J. Ketchum and the Northwest Territories Geoscience Office staff for scientific and field support; and E. Thiessen for mapping assistance. We thank T. Iizuka and T. Kemp for constructive and informative reviews. This research was funded by National Science and Engineering Research Council of Canada grants to T.C. and L.M.H. as well as a Circumpolar/Boreal Alberta Research grant for fieldwork to J.R.R.
The authors declare no competing financial interests.
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Reimink, J., Chacko, T., Stern, R. et al. Earth’s earliest evolved crust generated in an Iceland-like setting. Nature Geosci 7, 529–533 (2014). https://doi.org/10.1038/ngeo2170
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