CONSTRAINTS ON THE STRATIGRAPHIC AGE OF METASEDIMENTARY ROCKS FROM THE LARSEMANN-HILLS, EAST ANTARCTICA - POSSIBLE IMPLICATIONS FOR NEOPROTEROZOIC TECTONICS

In this study we investigate crystallization ages of detrital zircon f rom paragneiss from the Larsemann Hills, East Antarctica, using the si ngle zircon Pb-Pb thermal evaporation technique. The Pb-207/Pb-206 age s of eleven detrital zircons range from 1200+/-6(sigma) Ma to 766+/-12 (sigma) Ma. Although these ages must be regarded as minimum crystalliz ation ages, investigation of the U content and damage state of the int ernal crystal structure using fission track analysis suggests that the y 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 Pb-207/Pb-206 age of this orthogneiss is 940+/-6(sigma) Ma, similar to its whole-roc k Rb-Sr isochron age of 1024+/-45(sigma) Ma (IR =0.7241+/-40). Its Sm- Nd model age (T-DM) is about 2.2 Ga. Further Sm-Nd model ages (T-DM) i nclude 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) depositi on of the paragneiss protoliths occurred during the Neoproterozoic, wh ich suggests that accretion of the East Antarctic craton had not been completed until the early Palaeozoic; (3) deformation and granulite fa cies metamorphism in the region can be attributed to the similar to 0. 5-Ga ('Pan-African') event.