An Iceland-like Setting for Generation of a ~4.02 Ga tonalite, Acasta Gneiss Complex, Canada

Jesse Ray Reimink1, Tom Chacko2, Richard A Stern2, Larry Heaman2, Joshua Davies3, Graham Pearson4 and Robert A Creaser2, (1)Carnegie Institution for Science, Department of Terrestrial Magnetism, Washington, United States, (2)University of Alberta, Earth and Atmospheric Sciences, Edmonton, AB, Canada, (3)University of Geneva, Geneva, Switzerland, (4)University of Alberta, Earth & Atmospheric Sciences, Edmonton, AB, Canada

Contact First Author: Jesse Ray Reimink; jreimink@psu.edu

Previously Published Material: A portion of the data and conclusions in this talk were presented in poster form at the Goldschmidt Geochemistry conference (2013) and in talk format at the Northwest Territories Geoscience Forum (2013) as well as published in Nature Geoscience (2014).

Abstract ID#: 35048

 

English Abstract:
The Acasta Gneiss Complex (AGC) contains the oldest known continental rocks on Earth with U-Pb zircon ages indicating crust formation between 3.6-4.03 Ga [Stern and Bleeker, 1997; Bowring and Williams, 1999; Iizuka et al., 2007]. Here we discuss whole-rock elemental and isotope geochemistry along with SIMS U-Pb, trace element, Hafnium and O-isotope compositions of zircon from a >4.0 Ga tonalitic gneiss unit identified by Reimink et al., [2014].

Unlike typical Archean TTG magmatism [Moyen and Martin, 2012], this unit is characterized by moderate silica contents (58-62 wt % SiO2), strong Fe-enrichment (12-15 wt% FeO), and low Mg numbers (13-18). REE patterns are relatively flat with a significant negative Eu anomaly. These features strongly suggest that, unlike deep-seated Archean TTG magmas [Moyen and Martin, 2012], the evolution of this AGC tonalite was dominated by shallow-level fractionation processes involving plagioclase.

Zircons from this well-preserved unit document primary crystallization at ~4.02 Ga. Oxygen isotope compositions from zircon centers and mantles document a marked decrease in d18O from a mean of +5.6 ‰ to a mean of +4.7 ‰ that can be explained by processes occurring within a shallow magma chamber involving late-stage assimilation of hydrothermally altered crust.

The whole-rock and zircon data for the >4.0 Ga AGC tonalitic gneiss are similar to those of intermediate rocks from Iceland (e.g., icelandites), thought to be formed by shallow-level basaltic magma fractionation and assimilation of surface-water altered crust. Comparisons of zircon trace element data also compare favorably with Icelandic zircons [Carley et al., 2014]. We will present recently collected radiogenic isotope data and discuss these new findings in relation to the issue of crust formation on the Hadean Earth.

Bowring & Williams, (1999) Cont. Min. Petro. 134, 3-16

Stern & Bleeker, (1999) Geosci Can 25, 28-31.

Iizuka et al., (2007) Precambrian Research 153, 179-208

Reimink et al. (2014) Nature Geoscience 7, 529-533

Moyen & Martin, (2012) Lithos 148, 312-336.

Carley et al., (2014) EPSL 405, 85-97

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