科研之友
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摘要
The late Neoarchean to early Paleoproterozoic tectonic evolution of the Yangtze Craton remains poorly con-strained 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 lith-ophile 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 epsilon Hf(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.
