Precambrian granulites of the Aldan shield, eastern Siberia, USSR

Abstract Precambrian granulites of the Aldan shield in southern Yakutia, USSR, form a massif of 200,000 km 2 bounded by younger fold‐belts to the south, west and east. The massif consists of several blocks that reflect a primary heterogeneity of composition and differences in structural and thermodynamic evolution of different parts of the area. According to structural and petrological data the massif can be divided into two megablocks: eastern Aldan and western Aldan. They are separated by a narrow meridional fold‐belt. Structural evolution of this central zone was determined by the geodynamics of the mega‐blocks and was completed in the late Archaean. Towards the south, this central zone is ‘transformed’into the relatively small Sutam block adjoining the Stanovoy fold‐belt that bounds the Aldan shield on the south. The Sutam block is separated from the other structural units of the Aldan shield by a system of north trending grabens filled by post‐Archaean sediments. The Aldan shield is composed of Archaean high‐grade granulites, while the Stanovoy fold‐belt, to the south, consists of highly foliated Proterozoic rocks metamorphosed under relatively lower‐grade conditions. However, relics of the granulites are mapped within the fold‐belt. They contain high‐grade assemblages (e.g. Opx + Sil + Qz, Sap + Qz, Opx + Gr + Sil, etc.). One of the relics, the Tokskii block, which is only slightly touched by diaphthoresis, is located in the southeastern part of the Stanovoy fold‐belt. Metamorphic conditions of the Tokskii block are compared with those of the Sutam block and a similar evolution of the units is revealed. Mineral assemblages and mineral compositions do not vary within each unit, but they change in a north‐south direction. The Opx + Sil + Qz assemblage has been found only in Sutam and Tok, but not in eastern Aldan and western Aldan. The Sap + Qz assemblage has been found in the Tokskii block but has not yet been found in the Sutam block. The pyrope content in garnets, from metapelites of both blocks, is significantly higher than that from the Aldan (eastern and western blocks) rocks to the north. The most important assemblages from different units of the Aldan shield have been studied using the electron microprobe in order to unravel the metamorphic evolution of the granulites and thus to deduce the thermodynamic regime of this evolution. A geodynamic model for the Aldan shield is discussed in terms of Archaean island arc development.