Early Precambrian granite-gneiss complexes in the Central Aldan Shield

Abstract

The central portion of the Aldan Shield hosts very widely spread Archean and Early Proterozoic granitoids, much of which are granitegneisses. Geochemical lines of evidence, data on inclusions in miner� als, and Sm-Nd isotopic geochemical data suggest that the protoliths of granitegneisses in the central part of the Aldan Shield were granitoids that had various composition, age, and were derived from distinct sources and under different parameters and were then emplaced in different geodynamic environments. The grani� toids belong to at least two types of different composition that occur within spatially separated areas. The pro� toliths of granitegneisses in the western part of the Western Aldan Megablock and the junction zone of the Chara-Olekma and Aldan geoblocks (granitegneisses of type I) had the same age and affiliated to the same associations as the withinplate granitoids of the Nelyukinskii Complex. Their parental melts were derived at 2.4-2.5 Ga by the melting of Archean tonalite-trondhjemite orthogneisses of the Olekma and Aldan com� plexes. The protolith of granitegneisses in the east ern portion of the Western Aldan Megablock (granite� gneisses of type II) can be subdivided into two groups according to their composition: granitoids with geochemical characteristics of subductionand collisionr elated rocks. The protoliths of the typeII granite� gneisses with geochemical characteristics of subduction granitoids were produced simultaneously with the development of the Fedorovskaya island arc (at 2003-2013 Ma), whereas the protoliths of the typeII gran� itegneisses with geochemical characteristics of collisio n granitoids were formed in the course of accretion of the Fedorovskaya island arc and the Olekma-Aldan continental microplate at 1962-2003 Ma, via the melt� ing of magmatic rocks of the Fedorovskaya unit and older continental crustal material.