Mechanisms of Archean crust formation inferred from high-precision HFSE systematics in TTGs J. Elis Hoffmann a,b,⇑ , Carsten Mu ¨ nker b , Tomas Nraa c,d , Minik T. Rosing d , Daniel Herwartz a , Dieter Garbe-Scho ¨ nberg e , Henrik Svahnberg f a Rheinische Friedrich-Wilhelms Universita ¨ t Bonn, Steinmann Institut, Abt. Endogene Prozesse, Poppelsdorfer Schloss, 53115 Bonn, Germany b Universita ¨t zu Ko ¨ ln, Institut fu ¨ r Geologie und Mineralogie, Zu ¨ lpicher Str. 49b, 50674 Ko ¨ ln, Germany c Geological Survey of Denmark and Greenland (GEUS), Department of Geological Mapping, Øster Voldgade 10, 1350 Copenhagen K, Denmark d Museum of Natural History, Nordic Center for Earth evolution, Øster Voldgade 14, 1350 Copenhagen K, Denmark e Christian-Albrechts Universita ¨ t Kiel, Institut fu ¨ r Geowissenschaften, Ludewig-Meyn-Str. 10, 24118 Kiel, Germany f Stockholm University, Department of Geological Sciences, SE-106 91 Stockholm, Sweden Received 27 September 2010; accepted in revised form 26 April 2011; available online 8 May 2011 Abstract It has been proposed that Archean tonalitic–trondhjemitic–granodioritic magmas (TTGs) formed by melting of mafic crust at high pressures. The residual mineralogy of the TTGs (either (garnet)-amphibolite or rutile-bearing eclogite) is believed to control the trace element budget of TTGs. In particular, ratios of high-field-strength elements (HFSE) can help to discriminate between the different residual lithologies. In order to place constraints on the source mineralogy of TTGs, we performed high-precision HFSE measurements by isotope dilution (Nb, Ta, Zr, Hf) together with Lu–Hf and Sm–Nd measurements on representative, ca. 3.85–2.8 Ga TTGs and related rock types from southern West Greenland, W-India and from the Superior Province. These mea- surements are complemented by major and trace element data for the TTGs. Texturally homogeneous early Archean (3.85– 3.60 Ga old) and Mesoarchean (ca. 3.1–2.8 Ga old) TTGs have both low Ni (<11 ppm) and Cr contents (<20 ppm), indicating that there was little or no interaction with mantle peridotite during ascent. Ratios of Nb/Ta in juvenile Eoarchean TTGs range from ca. 7 to ca. 24, and in juvenile Mesoarchean TTGs from ca. 14 to ca. 27. Even higher Nb/Ta (14–42) were obtained for mig- matitic TTGs and intra-crustal differentiates, most likely mirroring further fractionation of Nb from Ta as a consequence of par- tial melting, fluid infiltration and migmatisation. In the juvenile TTGs, positive correlations between Nb/Ta and Gd/Yb, La/Yb, Sr/Y, Zr/Sm and Zr/Nb are observed. These compositional arrays are best explained by melting of typical Isua tholeiites in both, the rutile-bearing eclogite stability field (>15 kbar, high Nb/Ta) and the garnet-amphibolite stability field (10–15 kbar, low Nb/ Ta). With respect to the low end of Nb/Ta found for TTGs, there is currently some uncertainty between the available experimen- tal datasets for amphibole. Independent of these uncertainties, the TTG compositions found here still require the presence of both endmember residues. A successful geological model for the TTGs therefore has to account for the co-occurrence of both low- and high-Nb/Ta TTGs within the same geologic terrane. An additional feature observed in the Eoarchean samples from Greenland is a systematic co-variation between Nb/Ta and initial eHf(t), which is best explained by a model where TTG-melting occured at progressively increasing pressures in a pile of tectonically thickened mafic crust. The elevated Nb/Ta in migmatitic TTGs and intra-crustal differentiates can shed further light on the role of intra-crustal differentiation processes in the global Nb/Ta cycle. Lower crustal melting processes at granulite facies conditions may generate high-Nb/Ta domains in the middle crust, whereas mid-crustal melting at amphibolite facies conditions may account for the low Nb/Ta generally observed in upper crustal rocks. Ó 2011 Elsevier Ltd. All rights reserved. 0016-7037/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.gca.2011.04.027 ⇑ Corresponding author at: Rheinische Friedrich-Wilhelms Universita ¨t Bonn, Steinmann Institut, Abt. Endogene Prozesse, Poppelsdorfer Schloss, 53115 Bonn, Germany. Tel.: +49 228 737967; fax: +49 228 732763. E-mail address: hoffjoel@uni-bonn.de (J.E. Hoffmann). www.elsevier.com/locate/gca Available online at www.sciencedirect.com Geochimica et Cosmochimica Acta 75 (2011) 4157–4178