Zircon geochemistry from early evolved terranes records coeval stagnant- and mobile-lid tectonic regimes

Abstract

Determining the mechanisms by which the earliest continental crust was generated and reworked is important for constraining the evolution of Earth's geodynamic, surface, and atmospheric conditions. However, the details of early plate tectonic settings often remain obscured by the intervening ~4 Ga of crustal recycling. Covariations of U, Nb, Sc, and Yb in zircon have been shown to faithfully reflect Phanerozoic whole-rock-based plate-tectonic discriminators and are therefore useful in distinguishing zircons crystallized in ridge, plume, and arc-like environments, both in the present and in deep time. However, application of these proxies to deciphering tectonic settings on the early Earth has thus far been limited to select portions of the detrital zircon record. Here, we present in situ trace-element and oxygen isotope compositions for magmatic zircons from crystalline crustal rocks of the Acasta Gneiss Complex and the Saglek-Hebron Complex, Canada. Integrated with information from whole-rock geochemistry and zircon U-Pb, Hf, and O isotopes, our zircon U-Nb-Sc-Yb results reveal that melting of hydrated basalt was not restricted to a single tectonomagmatic process during the Archean but was operative during the reworking of Hadean protocrust and the generation of juvenile crust within two cratons, as early as 3.9 Ga. We observe zircon trace-element compositions indicative of hydrous melting in settings that otherwise host seemingly differing whole-rock geochemistry, zircon Hf, and zircon O isotopes, suggesting contemporaneous operation of stagnant-lid (oceanic plateau) and mobile-lid (arc-like) regimes in the early Archean.

Keywords: Archean; crustal evolution; isotope geochemistry; zircon.

Fig. 1.

(A and B) Primitive-mantle normalized (35) whole-rock TE plots for AGC (blue) and SHC (green) gneisses, showing heavy REE depletion relationships with age. Published AGC whole-rock data (32) and average Archean TTGs (5) are shown for comparison. AGC and SHC whole-rock data for samples from this study are found in Dataset S1. (C) Sample-averaged (all zircon analyses that pass screening criteria within a rock) Eu/Eu* for AGC and SHC (Materials and Methods, this study) with higher zircon Eu/Eu* values denoting higher pressure melting. Detrital zircon analyses from Green Sandstone Bed (GSB) and broader Barberton studies (–25) shown for comparison. Note that screening criteria for this study and previously published data differ. Zircon Eu/Eu* uncertainties are smaller than symbol size. (D) Map displays locations of the AGC (photo from Sam Bowring), SHC, and GSB relative to global Archean crust distribution.

Fig. 2.

Time series of zircon isotope and TE data. (A) Compiled global zircon ε_Hf data (34). Samples targeted by the current study are shown as diamonds. Juvenile compositions are interpreted as those closest to DM. (B) Zircon δ¹⁸O, with global compilation from ref. . (C) Zircon U/Nb. (D) Zircon Sc/Yb. Diamonds in Panels B, C, and D represent rock-averaged zircon data (all analyses that pass screening criteria from a given rock) for AGC and SHC gneisses, while colored circles show data range of individual zircon analyses (Materials and Methods). Colored bars (C and D) depict the first to third quartile of typical Phanerozoic arc-like and MORB/OIB sources (8). X’s in all panels denote detrital GSB data from ref. , and crosses denote detrital Barberton zircon from ref. .

Fig. 3.

Zircon TE bivariate plots after Grimes et al. (8). All AGC and SHC data presented pass screening criteria (Materials and Methods). AGC (blue) and SHC (green) data are displayed as rock-averaged zircon values for individual samples (ages shown in Fig. 2). Eoarchean samples are darker blue and darker green, respectively. Detrital GSB data (X and squares) from ref. and Barberton (+) from ref. shown for comparison, with squares representing zircons > 3.85 Ga. Colored fields and contour lines (95% level) represent tectonic affinities defined by the compilation from ref. .

Fig. 4.

Petrogenetic diagrams of individual zircon analyses from the AGC (blue) and SHC (green) and detrital GSB (X and squares) and Barberton zircon (+), showing influence of amphibole fractionation in later-formed zircon (trends from ref. 8). Magmatic generations are defined by (refs. , AGC) and (ref. , SHC) and are shown in SI Appendix, Table S1. Relatively elevated Sc/Yb of AGC and SHC zircon is inferred to result from zircon growth in equilibrium with amphibole-saturated melt, while decreasing Sc/Yb and Gd/Yb with increasing Ti for all three zircon localities (AGC, SHC, GSB/Barberton) likely reflects open-system fractional crystallization.