2015 GSA Annual Meeting in Baltimore, Maryland, USA (1-4 November 2015)

Paper No. 99-15
Presentation Time: 11:45 AM

DETAILED PETROCHRONOLOGY OF THE 4.02 GA IDIWHAA TONALITIC GNEISS: EVIDENCE REGARDING AMOUNT OF PRE-EXISTING HADEAN CONTINENTAL CRUST


REIMINK, Jesse R1, CHACKO, Tom1, DAVIES, Joshua H.F.L.2, STERN, Richard3, PEARSON, D. Graham1, HEAMAN, Larry M.1 and CREASER, Robert A.1, (1)Earth and Atmospheric Sciences, University of Alberta, 1-23 Earth Sciences Building, Edmonton, AB T6G2E3, Canada, (2)Section of Earth and Environmental Science, Université de Genève, Genève, 1205, Switzerland, (3)Canadian Centre for Isotopic Microanalysis, Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB T6G 2E3, Canada, reimink@ualberta.ca

The Acasta Gneiss Complex (AGC) contains the oldest known evolved rocks on Earth with U-Pb zircon ages recording crust formation between 4.0-3.6 Ga. Here we discuss whole-rock elemental and isotope geochemistry along with U-Pb, trace element, hafnium and O-isotope compositions of zircon from the km-scale >4.0 Ga Idiwhaa tonalitic gneiss (ITG).

Unlike typical Archean TTG magmatic rocks, the well-preserved meta-igneous ITG 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, the evolution of the ITG was dominated by shallow-level fractionation processes involving plagioclase.

Zircons from the ITG document primary crystallization at ~4.02 Ga. Two distinct phases of 4.02 Ga zircon growth occur in this sample and document a marked decrease in d18O from +5.6 ‰ to +4.7 ‰ that can be explained by late-stage assimilation of hydrothermally altered crust. Despite the difference in d18O values, Hf-isotope compositions of these two phases of zircon are indistiguishable, with initial eHf values (normalized to chondrite at 4020 Ma) of approximately -2.

We discuss the implications of the negative initial Hf and provide a model for the formation of the ITG which is consistent with all geochemical and isotopic information. Our model has important implications for crust formation processes in the Hadean (>4.0 Ga) and the amount of continental crust present on the early Earth.