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Olexandr Ponomarenko, Institute Geochem., Mineral. and Ore Formation N.P. Semenenko, Ukr. Ac. Sc. (Ukraine)
Konstantin Esipchuk, Institute Geochem., Mineral. and Ore Formation N.P. Semenenko, Ukr. Ac. Sc. (Ukraine)
Vitaly Kalinin, Institute Geochem., Mineral. and Ore Formation N.P. Semenenko, Ukr. Ac. Sc. (Ukraine)
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There are known some approaches to the construction of geochronological scale of the Precambrian. Most common approach is based on distinguishing in geological history of global tectonic events, the periods of tectonic-magmatic activity which are generally coeval within various geotectonic structures and even continents that allows to outline some global age boundaries in Precambrian.
To such boundaries should be related Archean - Proterozoic (2500-2600 Ma) and Paleo-Mesoproterozoic (1600-1700 Ma), age of intensive granite formation in Paleoproterozoic (1900-2000 Ma ), and boundary between Meso- and Paleoarchean (3400 Ma).
Lythologic or formational approach to subdivision of the Precambrian is based on composition of metamorphic rock masses. The available data on different composition of uncoeval Precambrian rock masses have allowed to develop the doctrine about the general evolution of lithogenesis in a history of the Earth in Precambrian sections. Usually, in preparation of Precambrian stratigraphic schemes and scales both these approaches are used.
International stratigraphic scale (International Stratigraphic Chart, 2004) was also based on these principles. This scale includes erathems with following (million years) duration: Eoarchean (?), Paleoarchean (400), Mesoarchean (400), Neoarchean (300), Paleoproterozoic (900), Mesoproterozoic (600), Neoproterozoic (460), Phanerozoic (540). Let's pay attention to different duration of some subdivisions that is excluded by the third approach to construction of the general geochronological scale which is referred as chronometric.
The most substantiated is a scale suggested by A.E.Kulinkovichem (1985, 2005). The largest unit of periodicity in this scale is the megacycle with duration of 528 million years that is equal to three galactic years and named as Trion. The Trion corresponds to era of traditional geochronological and stratigraphic scales. Accepting A.E.Kulinkovicha's basic idea, we can not agree with absolutely identical duration of divisions (528 million years for trions).
We propose the following geochronological scale, with indication of the top age boundaries (one million years): Katarchean (3850), Paleoarchean (3400), Mesoarchean (2950), Neoarchean (2500), Paleoproterozoic (2050), Mesoproterozoic (1550), Neoproterozoic - I (1050), a Neoproterozoic - II (540) and Phanerozoic.
For initial reference points in this scale such boundaries as Phanerozoic - Precambrian (540) and Proterozoic - Archean (2500) are accepted. Duration of Archean eras is 400-450 million years and Proterozoic - 450-510.
During the period of certain manifestation within the separate segments of earth's crust many other factors which either accelerated or slowed down these processes have occurred. With taking into account of the influence of these factors the position of outlined age boundaries can be shifted a little in different regions of about (± 50 million years).
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