THE ALDAN SHIELD OF SIBERIA, USSR - THE AGE OF ITS ARCHEAN COMPONENTS AND EVIDENCE FOR WIDESPREAD REWORKING IN THE MIDPROTEROZOIC

The western-central Archaean Aldan Shield of Siberia is divided from west to east into the Olekma and the West-Aldan Blocks. To the south lies the Stanovik Block, which also contains Archaean rocks. The Stanovik Block is separated from the other blocks by a fault, active since the Mesozoic, and the tectonic boundary between the Olekma and the West Aldan blocks has been interpreted as a Proterozoic thrust, now exposed at a deep crustal level. The Olekma Block is an amphibolite facies granite-greenstone complex, containing approximately 3000 Ma greenstone belts and dioritic-trondhjemitic intrusions, younger granites and fragments of an older gneiss complex. The West-Aldan Block is a partially-retrogressed granulite facies complex of gneisses, granites and supracrustal units dominated by metasediments. The Stanovik Block contains granulite-amphibolite facies gneisses and supracrustal units containing both metasediments and mafic rocks. SHRIMP ion-microprobe within-grain zircon geochronology has been used to search for the oldest intact rocks in the Olekma, West-Aldan and the Stanovik Blocks, and unravel their subsequent history. The oldest rocks (all orthogneisses) that have so far been identified using SHRIMP are approximately 3350 Ma (West Aldan Block), approximately 3250 Ma (Olekma Block) and approximately 2750 Ma (Stanovik Block). Dating of zircons from granitic partial melt segregations and metamorphosed mafic dykes, combined with field and petrographic studies, shows that in the period 1870-1960 Ma the Stanovik Block, West-Aldan Block and the eastern fringe of the Olekma Block were affected by amphibolite to granulite facies metamorphism, migmatisation and heterogeneous deformation, and are interpreted to lie within a Proterozoic mobile belt. The central-western portions of the Olekma Block were not reworked during the mid-Proterozoic, and are interpreted to belong to the foreland of the mobile belt exposed in to the east.