Geodynamic regimes in the precambrian of the urals

Abstract

Of all Precambrian rocks in the Urals the late Proterozoic is best represented. The Archaean and lower Proterozoic are reconstructed from separate blocks (massifs) which are roughly correlated with the crystalline basement of the adjacent platforms. Radiometric dates for the majority of early Precambrian blocks exceed 2 Ga. Features of very old (2.9 Ga) protocrustal material have been identified with grey gneiss complexes. Archaean blocks represent remnants of charnockite-granulite and migmatite-amphibolite complexes, and the geodynamic regime of their generation is the most primitive and does not occur in later periods of geological history. The Karelian age blocks appear to be relics of nongeosynclinal mobile belts. Their petrochemical characteristics and REE patterns suggest rift-related conditions of their formation. Late Proterozoic deposits (1650-590 Ma) comprise thick terranes, both at the western slope and in the axial zone of the Urals. Revision of rock associations disclosed that these terranes were previously wrongly interpreted as geosynclinal formations. They consist of platform sediments and include magmatic rocks which are typical of zones of diaschysis (tectonomagmatic activation) and rifting. Recurrent rifting of the Elsonian, Grenville and Baikal (Assynthian) cycles of diastrophism was responsible for crustal extension and thinning, followed by complete continental breakup and development of oceanic basins in the early Palaeozoic only. Extension in the lower crust during rifting was responsible for dynamometamorphism under different temperature conditions and dropping lithostatic pressure. Blastomylonites and schists produced during this process were previously mistaken for products of orogenic metamorphism. They were considered as evidence of geosynclinal cycles occurring in the late Proterozoic and similar to those of the Palaeozoic. Diaschysis and rifting were probably predominant geodynamic regimes in the late Precambrian of the Urals. They resulted in destruction or extensive reworking of the original Archaean and early Proterozoic crust to variable degrees and in the accumulation of volcano-sedimentary and terrigenous rifted-depressional successions. There is no reliable evidence for the existence of an oceanic basin in the late Proterozoic of the Urals.