Mémoires du Service Géologique de l'Algérie, n° 8, p. 35-47, 4 fig., 2 tabl., 1 pl., 1996.

Brèche à anorthosite de Teggeur (Dorsale Réguibat, Algérie) : pétrologie, données de la cathodoluminescence, conséquences géologiques

GUILLOT F.*

* Université de Lille 1, Laboratoire de sédimentologie et Géodynamique, S.N. 5,.59655 Villeneuve d'Ascq Cedex, France.

Résumé

Anorthosite breccia of Teggeur (Dorsale Reguibat, Algeria): Petrology, cathodoluminescence data, geological consequences

Extended Abstract

Regional geology - Lower Proterozoic Birrimian (2100-20000 Ma) granites and felsic volcanites (“Aftout”) constitute the most part of the eastern Reguibat Rise (“Dorsale Reguibat”). They separate various remnants of older rocks, the age of which is not precisely known presently. Because of its remoteness, few geological studies have been performed on this part of the West African Craton. The presence/absence of Archean remnants is still a matter of debate (Buffière et al.,1965a,b; Caby, 1965; Cahen et al., 1984; Mahdjoub et al., 1994). The Teggeur breccia could be such a remnant, and its study could provide useful clues about the petrology, origin and metamorphism of the pre-Birrimian mafic rocks.

Outcrop - Close to gabbros recently dated at 2207± 31 Ma (Mahdjoub et al., 1994), the Teggeur breccia is a layered, steeply dipping, coarse-grained to conglomeratic arenite. Its pebbles and smaller grains were issued exclusively from gabbro and anarthosite source rocks. No granitic nor felsic mineral has been found is three samples (EG 93-049, -050, -051). The breccia matrix is often calcite -rich. Among the breccia outcrops, a massive cipolin outcrop (sample EG 93-052) includes mafic pebbles.

Petrology - Magmatic minerals such as olivine, pyroxene and hornblende, have been completely altered into actinolite, chlorite and epidote, because of hydrothermal alteration (presence of prehnite veins), diagenesis (grains have been slightly fattened along the layering), and/or later metamorphism. But these modifications have preserved the calcic feldspar composition and texture, as well as the cumulate texture in the anorthosite pebbles. Both the source rocks and the breccia itself have never endured more than weak, greenschist facies metamorphism conditions. Such alteration processes could explain the presence of the carbonate cement, although the cipolin could result also from contemporaneous carbonate deposition.

Cathodoluminescence (CL) and microprobe data - CL examinations have been coupled with microprobe, major element analyses. The high Fe-content of apatite could explain its anormalously dull, green CL. Conversely the prehnite veins emit a strong, yellow CL, and prehnite has a very low Fe-content. Anorthosite pebbles exhibit CL-blue cores rimmed by CL-green, dull margins. A tentative statistical study of the relationship  between the CL-CoIour and the feldspar major elements microchemistry has been performed.  Fe- and.Mg- contents increase, while K- and Si-contents decrease from the CL-blue core to the CL-green rim.

Conclusions - Recent U-Pb and Sm-Nd datings (Mahdjooub et al., 1994) suggest that if previous datings performed on eastern Reguibat Rise rocks (Rb-Sr and Ar-Ar ages: see Cahen et al., 1984, for a comprehensive review) gave geologically meaningless results. In Buffière et al. pionneer work (1965a,b) about the Algerian part of the Reguibat Rise,  distinctions were drawn between unmetamorphosed Birrimian rocks and highly metamorphic older ones. The migmatisation these authors considered as characterizing pre-Birrimian rocks may be lacking, as shown in the present paper for the Teggeur breccia. Hence some tenets of the eastern Reguibat Rise geology have to be reconsidered.

Key words : Lower Proterozoic - Archean - Anorthosite - Cumulate - Gabbro – Cathodoluminescence - Pagioclase - Prehnite - Iron - West African Craton – Eglab – Aftout - Sahara.