Georges Choubert

Acritarchs of the Precambrian in mobile zones

Abstract This report lists acritarchs found in the various countries studied for I.G.C.P. Project No. 2. The results are provisional as collecting was not always made in accordance with the adopted rules. The research on the acritarchs from different levels in the Precambrian of the countries studied for I.G.C.P. Project No. 2 has given very encouraging results, which have generally confirmed the stratigraphic attribution of successive series based on other criteria. However, the microphytoplankton proved to be very poor in acritarchs, and those of great longevity predominated. This could possibly be due to low temperatures in the seas which surrounded the then northern Atlantic Ocean, as well as to the shallow-water environments and low salinity of this basin.

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

Abstract 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: (1) Archaean which does not exist throughout: 2700 (2500) Ma. (2) Lower Proterozoic (Birrimian, Eburnean): 2200-1700 Ma. (3) Middle Proterozoic, lower part (= Early Riphean): 1650-1400 Ma. (4) Middle Proterozoic, upper part (= Middle Riphean): 1400-1050 Ma. (5) Late (Vendian) and Upper Proterozoic cover (= Late and Upper Riphean): 1050-600 (570) Ma. (6) Cambrian.

4. The armorican massif: a new interpretation

In the Armorican Massif we discovered a stratigraphic and orogenic succession identical to the successions encountered in other countries which we studied for I.G.C.P. Project No. 2. As in Morocco, the regional rejuvenation was most intense during the first Proterozoic orogeny (Lihouides = Sveco-Fenno- Karelides = Hudsonides = Bouazzerides): only the Icartian relicts permit recognition of the importance of the Archaean Basement. Rejuvenation was also very strong during the Pan-Atlantic Thermotectonic Episode. It profoundly modified the ages the two Cadomian belts, which delayed recognition of their true ages and history. Cogne´created the term Pentevrian in 1959 for the ancient basement of the Brioverian. Leutwein (1968) dated it at ca. 1000 Ma, which is in fact the date of its rejuvenation. Its formations have been divided (Roach et al., 1972) between the Archaean (2500–2700 Ma) and the Lihouian (= Karelian 1900–2000 Ma). Thus, it would be the equivalent of the American Grenvillian which also rejuvenated the Archaean-Hudsonian basement. 1000 Ma is not the age of the first Cadomian orogeny which is well-dated by the Coutances diorite (1300 Ma); it is therefore the age of the second Cadomian belt which was entirely rejuvenated by the Pan-Atlantic Episode. It is known that this last great Precambrian Armorican belt was levelled out by the pre-cover surface dating, also ca. 1000 Ma (Normandy). A redefinition of the Pentevrian would be desirable to avoid misunderstanding.