Research ArticleGEOLOGY

Remnants of Eoarchean continental crust derived from a subducted proto-arc

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Science Advances  14 Feb 2018:
Vol. 4, no. 2, eaao3159
DOI: 10.1126/sciadv.aao3159
  • Fig. 1 Location and geological map of the Eoarchean Aktash gneisses complex.

    (A) Global distribution of Eoarchean gneiss complexes and their oldest TTG components (ages in Ga). Nuv., Nuvvuagittuq. (B) Geological map of the Aktash Complex. The Eoarchean gneisses occur as tectonic enclaves in ~2.0-Ga gneisses. The inset shows the lower hemisphere equal-area stereoprojection of the regional foliation (S2) at the outcrop. See the Supplementary Materials and fig. S1 for detailed regional geological setting.

  • Fig. 2 Zircon structures, U-Pb ages, and Hf isotopic compositions of the Eoarchean Aktash tonalitic gneisses.

    (A) Representative CL images showing complex core (c), mantle (m), and rim (r) structures. Oscillatory zoned magmatic cores are surrounded by multiple recrystallized mantles that are separated by a thin bright seam (white arrows) with multiple metamorphic overgrowth rims. (B) Concordia diagram showing all U-Pb ages by secondary ion mass spectrometer (SIMS) and laser ablation (LA) (see Materials and Methods). The inset shows the 207Pb/206Pb ages of concordant analyses from magmatic cores and recrystallized mantles, with weighted mean ages of the oldest cores and youngest mantles interpreted as the best estimates of magma crystallization and metamorphic recrystallization, respectively. Other discordant analyses plot in the region defined by multiple radiogenic Pb (Pb*) loss events. (C) Measured 176Hf/177 Hf ratios versus 207Pb/206Pb ages for the magmatic cores and recrystallized mantles, showing the indistinguishable Hf isotopic compositions for both domains that do not change with apparent ages. The inset shows the near Gaussian distribution of εHf values calculated using the magmatic crystallization age.

  • Fig. 3 Geochemical composition of the Eoarchean Aktash tonalitic gneisses compared to Eoarchean TTGs from the Itsaq, Acasta, and Anshan gneiss complexes.

    Average compositions of individual samples from replicate analyses are used in the plots for the Aktash gneisses. Acasta 1 refers to the 4.02-Ga high-FeOT tonalites that are compositionally different from typical TTGs. See table S4 for data source. (A) Feldspar triangular classification diagram showing the tonalitic composition of the studied samples, except for sample 16ALT08-3. (B) Sr/Y diagram. (C) Nb/Ta diagram. The Aktash tonalitic gneisses mostly plot into the HP TTG fields in (B) and (C), whereas other Eoarchean TTGs mostly plot into the medium and LP TTG fields (13). (D) Cr/Mg# diagram showing the higher Cr contents and Mg# of the Aktash tonalitic gneisses relative to the lower crust–derived Eoarchean TTGs and experimental TTG melts (37). The most “mafic” sample (16ALT04-2) plots close to the average low-SiO2 Adakites and Archean Sanukitoids (3), which can be modeled by simple mixing with 3 to 5% mantle peridotite (DP1) (39). This provides a minimum constraint for mantle peridotite interacted with TTG melts because experiments show that this interaction produces peritectic mafic minerals (for example, garnet and pyroxene) that host most of the MgO, Cr, and Ni, so that even interaction with 30% peridotite produces melts (for example, CF-HP) similar to our most mafic sample (39).

  • Fig. 4 Trace element modeling.

    (A) Trace element patterns for the median composition of TTGs (13) and their potential source rocks (tables S6 and S7), highlighting the significant enrichment of LILEs (for example, Rb, Ba, Th, U, and K) in TTGs. The dashed lines represent the degree of LILE enrichment required for the source to produce TTGs by 10, 20, or 30% melting, assuming perfect incompatibility (bulk partition coefficient = 0). In modern oceanic settings, only IAB are enriched enough to be the source of TTGs; mid-ocean ridge and oceanic plateau basalts are too depleted. The median composition of Archean arc-like basalts used for thermodynamic modeling appear to be a suitable source of TTGs. NMORB, normal MORB. (B) Trace element patterns of TTG melts calculated at different P-T-X(H2O) conditions for the median composition of Archean arc-like basalts (table S9). The shaded area shows the overall compositional range of the ~3.7 Ga Aktash tonalites. The best fit for the median composition of the Aktash tonalites is obtained for water-fluxed melting [X(H2O) = 2 to 3 wt %] at 1.8 to 1.9 GPa and 800° to 830°C; dehydration melting [X(H2O) = 1.0 to 1.5 wt %] at higher temperatures (900° to 950°C) results in a poorer fit in light REE. Melts derived from lower pressures at 1.4, 1.0, and 0.6 GPa significantly deviate from the median Aktash tonalites in terms of Nb-Ta, Sr and Y, and heavy REE contents.

  • Fig. 5 Thermodynamic modeling.

    Simplified P-T phase diagram for the median Archean arc-like basalt (table S7), calculated with X(H2O) = 2.0 wt %, corresponding to at least 0.3 wt % water-fluxed melting. Long and short dashed lines show calculated degree of melting (weight % of melt) and melt Sr/Y ratios, respectively. Dotted line marks water saturation of the system. Also shown are P-T conditions estimated for the Aktash tonalitic gneisses (red dot), compared to those for the Itsaq (purple square), Anshan (green diamond), and Acasta (group 2, blue triangle) TTG gneisses. Error bars are 1σ based on uncertainties of observed Sr/Y ratios, Sr contents (for pressure), and LILE contents (for degree of melting and temperature) and should be considered as minimum estimates. Whereas most Eoarchean TTGs were likely derived from thickened lower crust (1.0 to 1.5 GPa), the composition of the Aktash tonalites (for example, Sr/Y ≥ 100) requires melting at mantle depths (>1.6 GPa).

  • Table 1 Median values of key elemental concentrations and ratios for TTGs and selected modeled melts.

    See tables S4, S6, S7, and S9 for details.

    K2O (wt %)Rb (ppm)Sr (ppm)Y (ppm)Sr/YNb (ppm)Nb/TaMg#
    TTGs derived from different depths
    LP TTG1.6768.127714.020.58.0010.637.7
    MP TTG1.5352.04437.6157.65.0010.941.5
    HP TTG1.5240.05763.831302.4113.342.5
    Eoarchean TTGs
    Itsaq1.4250.33274.7957.23.1112.242.4
    Anshan1.891112956.5047.24.1213.333.7
    Acasta (group 2)1.7643.82418.0037.19.1018.442.0
    Aktash1.4822.85614.461272.9723.048.9
    Selected modeled TTG melts of Archean arc-like basalts [X(H2O) = 2.5 wt %]
    T = 820°C, P = 0.6 GPa1.1129.321814.015.68.5418.830.2
    T = 820°C, P = 1.0 GPa1.2433.63049.9430.56.4018.133.6
    T = 820°C, P = 1.4 GPa1.2733.14305.8573.53.3516.737.8
    T = 820°C, P = 1.9 GPa1.4334.06164.531362.3216.047.5

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/2/eaao3159/DC1

    Supplementary Text

    fig. S1. Regional geological setting of the Aktash gneiss complex.

    fig. S2. Field photographs of the Eoarchean Aktash tonalitic gneisses and related rocks.

    fig. S3. Photomicrographs of the Eoarchean tonalitic gneisses and associated rocks.

    fig. S4. Concordia diagrams and representative zircon CL images of the Eoarchean tonalitic gneisses.

    fig. S5. Binary element variation diagrams of the Aktash tonalites.

    fig. S6. REE and trace element patterns for the Eoarchean TTG gneisses.

    fig. S7. Histograms of LILEs (for example, K, Rb, Ba, and Th).

    fig. S8. Phase diagrams calculated for the median composition of Archean arc-like basalts.

    fig. S9. Trace element patterns of modeled melts compared to the Eoarchean TTGs from the Itsaq, Anshan, and Acasta complexes.

    fig. S10. Zircon saturation temperature (TZr).

    table S1. SHRIMP zircon U-Th-Pb isotopic data for the Eoarchean gneisses in the Aktash Tagh area, southeast Tarim Craton, northwestern China.

    table S2. LA-ICPMS zircon ages for the Eoarchean gneisses in the Aktash Tagh area, southeast Tarim Craton, northwestern China.

    table S3. LA-MC-ICPMS zircon Lu-Hf isotopic data for the Eoarchean gneisses in the Aktash Tagh area, southeast Tarim Craton, northwestern China.

    table S4. Whole-rock geochemical data for the Eoarchean tonalitic gneisses in the Aktash Tagh area, southeast Tarim Craton, northwestern China, and literature data for the Eoarchean TTGs from the Itsaq, Acasta, and Anshan gneiss complexes.

    table S5. Major and trace element composition of standard materials analyzed along with unknowns during this study.

    table S6. Average composition of TTGs for different ages and assumed melting pressures.

    table S7. Average composition of potential source mafic rocks for Archean TTGs compiled from the GEOROC and PetDB databases.

    table S8. Mineral partition coefficients used in trace element modeling of partial melting of mafic rocks [after Bédard (8) and Martin et al. (7)].

    table S9. Thermodynamic and trace element modeling of partial melting of median Archean arc-like basalt.

    References (6173)

  • Supplementary Materials

    This PDF file includes:

    • Supplementary Text
    • fig. S1. Regional geological setting of the Aktash gneiss complex.
    • fig. S2. Field photographs of the Eoarchean Aktash tonalitic gneisses and related rocks.
    • fig. S3. Photomicrographs of the Eoarchean tonalitic gneisses and associated rocks.
    • fig. S4. Concordia diagrams and representative zircon CL images of the Eoarchean tonalitic gneisses.
    • fig. S5. Binary element variation diagrams of the Aktash tonalites.
    • fig. S6. REE and trace element patterns for the Eoarchean TTG gneisses.
    • fig. S7. Histograms of LILEs (for example, K, Rb, Ba, and Th).
    • fig. S8. Phase diagrams calculated for the median composition of Archean arc-like basalts.
    • fig. S9. Trace element patterns of modeled melts compared to the Eoarchean TTGs from the Itsaq, Anshan, and Acasta complexes.
    • fig. S10. Zircon saturation temperature (TZr).
    • References (61–73)

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    Other Supplementary Material for this manuscript includes the following:

    • table S1 (Microsoft Excel format). SHRIMP zircon U-Th-Pb isotopic data for the Eoarchean gneisses in the Aktash Tagh area, southeast Tarim Craton, northwestern China.
    • table S2 (Microsoft Excel format). LA-ICPMS zircon ages for the Eoarchean gneisses in the Aktash Tagh area, southeast Tarim Craton, northwestern China.
    • table S3 (Microsoft Excel format). LA-MC-ICPMS zircon Lu-Hf isotopic data for the Eoarchean gneisses in the Aktash Tagh area, southeast Tarim Craton, northwestern China.
    • table S4 (Microsoft Excel format). Whole-rock geochemical data for the Eoarchean tonalitic gneisses in the Aktash Tagh area, southeast Tarim Craton, northwestern China, and literature data for the Eoarchean TTGs from the Itsaq, Acasta, and Anshan gneiss complexes.
    • table S5 (Microsoft Excel format). Major and trace element composition of standard materials analyzed along with unknowns during this study.
    • table S6 (Microsoft Excel format). Average composition of TTGs for different ages and assumed melting pressures.
    • table S7 (Microsoft Excel format). Average composition of potential source mafic rocks for Archean TTGs compiled from the GEOROC and PetDB databases.
    • table S8 (Microsoft Excel format). Mineral partition coefficients used in trace element modeling of partial melting of mafic rocks after Bédard (8) and Martin et al. (7).
    • table S9 (Microsoft Excel format). Thermodynamic and trace element modeling of partial melting of median Archean arc-like basalt.

    Download Tables S1 to S9

    Files in this Data Supplement:

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