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The Precambrian in north peri-Atlantic and south Mediterranean mobile zones: general results

1980; Elsevier BV; Volume: 16; Linguagem: Inglês

10.1016/0012-8252(80)90034-3

1872-6828

Georges Choubert, Anne Faure-Muret,

Geological and Geophysical Studies Worldwide

The following regions were studied for Project No. 2 of the I.G.C.P. between 1972 and 1980: The south of the Eastern Desert of Egypt, the Avalon Peninsula of Newfoundland, the Iberian Peninsula, the Precambrian areas of Great Britain, the autochthonous area of southern Norway. Morocco had been studied previously. The majority of these regions are mobile zones which evidence more or less complete, often total, isotopic rejuvenation. A distinction was drawn between the folded, metamorphosed and granitized belts and the apparently quiet zones characterized by regional rejuvenation. At least two great orogenic epochs existed in geological history causing large-scale regional rejuvenation. These are the Eburnean (Birrimian) and the Pan-African (or Pan-Atlantic) epochs, both comprising a great number of orogenic phases which were repeated every 50 or 100 Ma. The first lasted from 2000 Ma (2100 Ma) to 1700 (1650) Ma and the second from 700 Ma (800) to 500 Ma, e.g. from the Vendian to the end of the Middle Cambrian. Where possible the rejuvenated ages have been eliminated to demonstrate the original ages. The following methods were employed: (1) research on relict ages; (2) visible comparison of facies and their stratigraphic succession with equivalent series in stable, non-rejuvenated regions; (3) dating by biostratigraphic means (stromatolites, oncholites, acritarchs) based also on stable regions, e.g. the U.S.S.R. Confusion can arise where the latter method is concerned, particularly acritarchs. It happened that the age of a microphytoplankton was at first based on the rejuvenated age, e.g. Bavlinella Schep. in the Brioverian of Brittany and in the Bohemian Algonkian, etc. These series are directly succeeded by the Cambrian and have thus been attributed to the Vendian. This has been confirmed by rejuvenated Pan-African (Pan-Atlantic) ages. In fact, these Precambrian series belong to the Middle Riphean, which makes equilibration of the local stratigraphy possible and permits precision when specifying the age not only of Bavlinella but also of Charnia and of the Newfoundland metazoans. By using these methods it has been possible to distinguish in all of the seven countries studied the following: Archaean which does not exist throughout: 2700 (2500) Ma. Lower Proterozoic (Birrimian, Eburnean): 2200-1700 Ma. Middle Proterozoic, lower part (= Early Riphean): 1650-1400 Ma. Middle Proterozoic, upper part (= Middle Riphean): 1400-1050 Ma. Late (Vendian) and Upper Proterozoic cover (= Late and Upper Riphean): 1050-600 (570) Ma. Cambrian.