Nd isotopic evolution of Archean tonalitic-granodioritic rocks of the Napier Complex, East Antarctica

Abstract

We examined Sm-Nd isotopes of samples from the Archean tonalitic-granodioritic orthogneiss mass of Mt. Riiser-Larsen in the Napier Complex, East Antarctica. Analytical data show εNd ≈ 0 of ∼ 3.27 and ∼ 3.07 Ga at the time of protolith formation, as determined by SHRIMP zircon analyses. This differs from previously reported Sm-Nd whole-rock isotope data from the oldest tonalitic orthogneisses of Mt. Sones in the Napier Complex, which show εNd = 0 at 3.87-3.80 Ga (T_CHUR), which is coincident with ∼ 3.8 Ga from SHRIMP zircon analyses. These data suggest that the voluminous and homogeneous tonalitic-granodioritic rocks retained the εNd ≈ 0 signal throughout the protolith-metamorphic process and that the source materials of the rocks showed εNd ≈ 0 at ∼ 3.80, 3.27, and 3.07 Ga. Tonalitic-granodioritic orthogneisses from the Napier Complex may contain genetic information regarding Nd isotopic evolution from the Eoarchean to Mesoarchean.