Chapter 10 Tectonics and Petrogenesis of early Precambrian Complexes of the Aldan Shield, Siberia

Abstract

Abstract The early Precambrian of the Aldan Shield can be subdivided into at least two major tectonic units: (1) a gneiss-granulite basement; (2) greenstone and schist belts (trough structures). Basement complexes were formed in two stages: (1) accumulation of ultra-mafic volcanic and volcano-sedimentary series (proto-ophiolite associations); and (2) transformation of basic protocrust with the emplacement of granites, growth of granite-gneiss domes or ovals (ring structures) and development of interoval granulite-charnockite-anorthosite belts. These stages of nonlinear tectonics coincided with the end of the permobile regime of the earth's crust after which the first relatively rigid microplates were formed. Greenstone belts younger than about 2.6 Ga appear to have developed from rift (trough) depressions, similar to minor oceans at the initial stages of evolution, which subsequently developed not as spreading basins, but as marginal back-arc basins. The closing of these basins took place simultaneously with the continuing growth of granite-gneiss domes and vertical accretion of lithosphere, i.e. the tectonic conditions of primitive horizontal microplate motion were coupled with partial preservation of a permobile (nonlinear) geodynamic environment. Granitization and alkaline metasomatism (with a maximum at about 2 Ga ago in the Aldan Shield) resulted in folding and thrusting in greenstone belts, thereby obliterating the unconformity between the basement and greenstone sequences. This process also resulted in a total cratonization of the shield, i.e. it led to the formation of the Siberian rigid continental lithospheric plate. The time interval of development of greenstone belts is unique in the earth's history by its double-faced character of geodynamic conditions transitional from a permobile regime to plate tectonics.