Constraints on the stratigraphic age of metasedimentary rocks from the Larsemann Hills, East Antarctica: possible implications for Neoproterozoic tectonics

Abstract

In this study we investigate crystallization ages of detrital zircon from paragneiss from the Larsemann Hills, East Antarctica, using the single zircon PbPb thermal evaporation technique. The 207Pb/ 206Pb ages of eleven detrital zircons range from 1200 ± 6(σ) Ma to 766 ± 12(σ) Ma. Although these ages must be regarded as minimum crystallization ages, investigation of the U content and damage state of the internal crystal structure using fission track analysis suggests that they may approach crystallization ages. This is confirmed by zircons from a granitic orthogneiss (W20401), which show consistent radiogenic Pb ratios released during the final evaporation steps. The 207Pb/ 206Pb age of this orthogneiss is 940 ± 6(σ) Ma, similar to its whole-rock RbSr isochron age of 1024 ± 45(σ) Ma (IR = 0.7241 ± 40). Its SmNd model age ( T DM) is about 2.2 Ga. Further Sm Nd model ages ( T DM) include 2.1 Ga for the paragneiss, and 1.6–1.7 Ga for a 0.55 Ga granite. The presented field and isotopic chronological data suggest that: (1) the 1-Ga event represents emplacement of the orthogneiss protolith, but does not necessarily involve high-grade metamorphism; (2) deposition of the paragneiss protoliths occurred during the Neoproterozoic, which suggests that accretion of the East Antarctic craton had not been completed until the early Palaeozoic; (3) deformation and granulite facies metamorphism in the region can be attributed to the ∼ 0.5-Ga (‘Pan-African’) event.