Article

The Archean of the Baltic Shield: Geology, Geochronology, and Geodynamic settings

Geotectonics (Impact Factor: 0.75). 01/2006; 40(6):409-433. DOI: 10.1134/S001685210606001X

ABSTRACT The Archean provinces and lithotectonic complexes of the Baltic (Fennoscandian) Shield are considered. The supracrustal complexes
are classified by age: <3.2, 3.10–2.90, 2.90–2.82, 2.82–2.75, and 2.75–2.65 Ga. The data on Archean granitoid complexes and
metamorphic events are mentioned briefly, whereas the recently found fragments of the Archean ophiolitic and eclogite-bearing
associations are discussed in more detail. The Paleoarchean rocks and sporadic detrital grains of Paleoarchean zircons have
been found in the Baltic Shield; however, the relatively large fragments of the continental crust likely began to form only
in the Mesoarchean (3.2–3.1 Ga ago), when the first microcontinents, e.g., Vodlozero and Iisalmi, were created. The main body
of the continental crust was formed 2.90–2.65 Ga ago. The available information on the Paleoarchean complexes of the Baltic
Shield is thus far too scanty for judgment on their formation conditions. The geologic, petrologic, isotopic, and geochronological
data on the Meso-and Neoarchean lithotectonic complexes testify to their formation in the geodynamic settings comparable with
those known in Phanerozoic: subduction-related (ensialic and ensimatic), collisional, spreading-related, continental rifting,
and the setting related to mantle plumes.

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    ABSTRACT: The diamond-bearing mantle keels underlying Archean cratons are a unique phenomenon of Early Precambrian geology. The common stable assemblage of the Archean TTG early continental crust and underlying subcontinental lithospheric mantle clearly shows their coupled tectogenesis, which was not repeated in younger geological epochs. One of the least studied aspects of this phenomenon is concerned with the eclogitic xenoliths carried up by kimberlite pipes together with mantle-derived nodules. The eclogitic xenoliths reveal evidence for their subduction-related origin, but the Archean crustal counterparts of such xenoliths remained unknown for a long time, and the question of their crustal source and relationships to the formation of early continental crust remained open. The Archean crustal eclogites recently found in the Belomorian Belt of the Baltic Shield are compared in this paper with eclogitic xenoliths from kimberlites in the context of the formation of both Archean subcontinental lithospheric mantle (SCLM) and early continental crust. The crustal eclogites from the Belomorian Belt are identical in mineral and chemical compositions to the eclogite nodules (group B), including their diamond-bearing varieties. The eclogite protoliths are comparable in composition with the primary melts of the Meso- and Neoarchean oceanic crust, which was formed at a potential temperature of the upper mantle which exceeded its present-day temperature by 150–250 K. The reconstructed pathways of the Archean oceanic crust plunging in the upper mantle suggest that the Archean mantle was hotter than in the modern convergence settings. The proposed geodynamic model assumes coupled formation of the Archean diamond-bearing SCLM and growth of early continental crust as a phenomenon related to the specific geodynamics of that time controlled by a higher terrestrial heat flow.
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