AGE LIMITS OF HIGH-GRADE METAMORPHIC SUPRACRUSTAL COMPLEXES IN THE CENTRAL ALDAN SHIELD: SM-ND ISOTOPIC DATA

Abstract

First investigation results on Sm-Nd isotope systems in clastic metasedimentary and metavolcanic rocks from the high-grade metamorphic supracrustal complexes of central Aldan Shield are used to approximately evaluate their maximum age boundary. It is established that the Upper Archean rocks (model age TNd(DM) = 3.3-2.6 Ga) served as a principal source material for metapelites of the Kurumkan Group; metapelites and metagraywackes of the Amedichi Group; and also for the metagraywackes of the Kurulta, Zverev, and Seim groups. Provenances with the Early Archean model age (TNd(DM) = 3.5 Ga) are found to be appropriate only for few rock samples from the Kurumkan and Kurulta groups. The Early Proterozoic age (TNd(DM) = 2.5-2.2 Ga) is inferred for the protolith material of supracrustal rocks of the Chuga, Kholbolokh, Kyurikan, Idzhek, and Fedorov groups. The data obtained suggest that supracrustal rocks of the Aldan granulite-gneiss and East Olekma granite-greenstone terranes were formed not in the Early Archean, as it was formerly believed, but at the Late Archean and/or Early Proterozoic stages of geologic evolution of the shield.