Application des traceurs isotopiques Pb-Pb Sm-Nd et Lu-Hf à la compréhension de l histoire archéenne du bouclier canadien et à la formation de la croûte continentale

Sm-Nd, Lu-Hf and Pb-Pb signatures of gneisses and granitoids from the La Grande belt: extent of late Archean crustal recycling in the northeastern Superior Province, Canada

“…8), the crustal end-member has Nd (15-22 ppm) and Hf (3-4 ppm) elemental abundances typical of those actually determined in whole-rock samples of the Langelier gneisses (Table 2), and Pb abundances (7-10 ppm) typical of average continental crust (Allègre et al, 1988). The Nd (2-7 ppm) and Hf (0.2-1.7 ppm) abundances in the mantle melt component are in the range of those determined for several basaltic suites of the La Grande and Nemiscau belts (Isnard, 2002(Isnard, , 2003, and comparable to those reported for present-day MORB (Sun and McDonough, 1989). The Pb abundances in the mantle melt component were varied between 0.5 and 3 ppm (Fig.…”

“…2.7 Ga, the Nd and Hf values reported for mafic and ultramafic lavas of the Abitibi belt (Cattell et al, 1984;Dupré et al, 1984;Walker et al, 1988;Vervoort al., 1994;Blichert-Toft and Arndt, 1999;Vervoort and Blichert-Toft, 1999). Also shown for reference on Figure 6 are the Nd (T) and Hf (T) signatures of La Grande komatiites and the Nd (T) values of Nemiscau basalts (Isnard, 2002(Isnard, , 2003 which plot close to the chosen DM models. The initial 207 Pb/ 204 Pb ratios of the samples are evaluated in Figure 7.…”

“…The Nemiscau belt (Fig. 2) also contains small allochtonous rafts of mafic volcanic rocks (Isnard, 2003). Synkinematic biotite-granites and extensive fields of pegmatites cover the whole belt.…”

“…Firstly, tectonically juxtaposed slabs of basalts in the Nemiscau belt have depleted Nd (T) values as high as ϩ4 (Fig. 6;Isnard, 2003); similarly, komatiites and basalts from the La Grande belt record Nd (T) values Ͼ ϩ3 and Hf (T) values Ͼ ϩ5 ( Fig. 6; Isnard, 2003).…”

“…6;Isnard, 2003); similarly, komatiites and basalts from the La Grande belt record Nd (T) values Ͼ ϩ3 and Hf (T) values Ͼ ϩ5 ( Fig. 6; Isnard, 2003). Secondly, the isotopic compositions of the TTG rocks within the Lac Taylor suite (Eastmain domain), even if solely derived from the mantle without any form of subducted sediment/crustal interactions, require depleted mantle isotopic characteristics with Nd (2.7 Ga) Ͼ ϩ2.5, Hf (2.7 Ga) Ͼ ϩ4 and 207 Pb/ 204 Pb ratios Ͻ14.5 (Figs.…”