Modern-style subduction during the late Neoarchean to early Paleoproterozoic? Geochemical evidence from ca. 2.45 Ga arc-type magmatism in the Feidong Complex, northeastern Yangtze Craton, South China

Abstract

The late Neoarchean to early Paleoproterozoic tectonic evolution of the Yangtze Craton remains poorly constrained as a result of the poor exposure of contemporaneous basement rocks. This study presents new zircon U–Pb–Hf isotopic data and whole–rock geochemical and Sr–Nd isotopic data for orthogneisses from the Feidong Complex of the northeastern Yangtze Craton. Two orthogneiss samples yielded weighted mean ages of 2416 ± 21 and 2436 ± 21 Ma, respectively, indicating the timing of protolith formation. These samples contain relatively high concentrations of SiO2 (54.04–63.25 wt%), but low concentrations of Na2O (3.29–4.12 wt%) and K2O (1.87–2.84 wt%), indicating they are calc–alkaline intermediate rocks. They are enriched in the light ion lithophile elements (LILE) and the light rare earth elements (LREE) but are depleted in Nb–Ta, Ti, the heavy REE (HREE), and Y. They also plot within the arc affinity fields of (La/Yb)N vs. (Yb)N and Sr/Y vs. Y diagrams, consistent with their relatively high Th/Yb and Ta/Yb and low Ce/Pb and Nb/U values, all of which are indicative of rocks formed by arc–type magmatism. The two orthogneiss samples have zircon εHf(t) values that vary between –6.83 and +2.54 with two–stage model ages (TDM2) of 3.35–2.79 (peak at 3.2 Ga) and with similar Nd isotopic compositions. The high total REE contents and Nb–Ta depletions within the orthogneiss samples may reflect derivation from magmas sourced from regions containing fluids released from a subducting slab. These data indicate that the calc–alkaline protoliths of these orthogneisses formed from magmas generated by the partial melting of mantle wedge material driven by the addition of fluid released from a subducted slab. This process induced the partial melting of ca. 3.2 Ga lower crustal material in a manner similar to that associated with modern arc–type magmatism. Taken collectively, the regional magmatism recorded by the ca. 2.50–2.47 Ga tonalite–trondhjemite–granodiorite (TTG) gneisses of the Douling Complex, the ca. 2.45 Ga calc–alkaline arc-type magmatic rocks in the Feidong Complex, and contemporaneous metamorphism within the north margin of Yangtze Craton all provide evidence that modern–style subduction occurred within the northeastern Yangtze Craton between the late Neoarchean and the early Paleoproterozoic.