Ouazzani, H. and Pouclet, A. and Badra, L. and Prost, A. (2001) Arc-related volcanism in the west of the Western High-Atlas (Morocco): A witness to the Pan-African ocean western branch convergence Le volcanisme d'arc du massif ancien de l'ouest du Haut-Atlas occidental (Maroc), un témoin de la convergence de la branche occidentale de l'océan panafricain. Bulletin de la Societe Geologique de France, 172 (5). pp. 587-602.

Full text not available from this repository.
Official URL: https://www.scopus.com/inward/record.uri?eid=2-s2....

Abstract

Western High-Atlas is located on the north-west border of the West-African craton (fig. 1). In its western part, numerous volcanic formations have been described and assigned either to Neoproterozoic or Cambrian. But very few petrological and geochemical data are available, precluding any geodynamical interpretation. We performed new structural and petrological analyses in two main areas (fig. 2), the Guedmioua area to the east (four sites), and the High-Seksaoua area to the west (three sites). In other areas of western High-Atlas and Anti-Atlas, different volcanic formations are known and are attributed either to Pan-African orogenic stages or to Cambrian history as basalts of the passive margin extension. The first problem is to determine which of these volcanic events the western High-Atlas activity is related. We subdivided western High-Atlas and Anti-Atlas into four lithostructural domains (fig. 1) : (1) western Anti-Atlas belonging to the margin of the West African craton. (2) Central and eastern-central Anti-Atlas belonging to the Pan-African mobile belt, (3) the eastern part of western High Atlas also related to the Pan-African mobile belt, and (4), the western part of western High-Atlas. The different Palaeoproterozoic, Neoproterozoic and early Cambrian terrains can be correlated in the first three domains, but not in the fourth one. The second problem is to discuss the geodynamical interpretation of the western part of western High-Atlas, regarding the composition of their volcanic formations. The western part of the western High-Atlas formations consists in sedimentary and volcano-sedimentary sequences affected by the Hercynian orogenic event, some being Palaeozoic others being undated. In the studied area, the rocks underwent two main deformations. The first one, D1, caused isoclinal N060°-N080° trending folds and a related S1 schistosity parallel to the statification. This deformation is unknown in the paleontologically dated Palaeozoic formations, the oldest ones being the Atdabanian Archaeocyatha-bearing limestones. The second deformation, D2, produced larger N000°-N020° trending folds, reclining to the east with an axial plane schistosity S2. This deformation is due to the Hercynian orogeny. The Alpine orogeny generated large N070° and N010° cross-cutting faults separating High-Atlas into main blocks. Considering these structural patterns and previous geochemical interpretations, we considered that the studied volcanic and sedimentary formations belong to an active margin context which predated the Atdabanian and may be attributed to the Neoproterozoic Ouazzani et al., 1998 ; Ouazzani, 2000. Mapping and lithological studies have been done in both areas (figs 3 to 7). The broad structural aspect in cross sections is due to the D2 deformation. At the Azegour site, in the Guedmioua area, there is, first, a supply of terrigenous sediments (SI), then, three pulses of explosive and effusive volcanic activity, with andesite flows and dacite protrusions, separated by quiet limestone deposition (VI-3, CI-2), and finally a filling up with etrital sediments (SS). At the Ifri site, in the Seksaoua area, the stratigraphic succession shows a first effusive and explosive andesitic activity (VSI), black shale and dolomite deposition (AD) indicative of a deepening of the basin, a second andesitic activity interrupted by limestone deposition (VSS), and then, a filling up of detrital sediments (SD). At the Ouzaga site, a new volcanic sequence with pillow lavas (VSO) overlies the Ifri detrital deposits and is overlain by limestone layers. At the Amerdoul site, the same sequences as at Ifri are recognized with thicker andesitic flows in the lower sequence, thicker deposition of black shales, and interbedded spilitic flows in the second volcanic sequence. This indicates a deepeer basinal context. The rocks underwent epizonal metaporphism during the D1 deformation. However, the phyric or aphyric microlitic textures of the lavas are recognizable. Magmatic minerals are replaced by secondary minerals of the greenschist facies. Lavas are attributed to andesitic calc-alkaline series and discriminated into three magmatic groups (figs 8 to 10). The first group consists of basalts showing depletion or very weak enrichment in lithophile elements (LaN/YbN 1, 1-2, 6j) and a Nb-negative anomaly (NbN/LaN 0,38-0, 45). Compositions are similar to that of back-arc basin basalts. The second group consists of andesites weakley enriched in incompatible elements (LaN/YbN 2, 2-4, 7) with variable Nb- and Ta-negative anomalies (NbN/LaN 0.27-0,84). Geochemical patterns and sedimentary features suggest an oceanic volcanic arc context or a continentalextensive active margin. The third group consists of enriched andesites (LaN/YbN 6, 1-16, 9) with variable Nb-negative anomalies (NbN/LaN 0,20-0,65) and high field strength element depletion. These features are common to andesites of various andesitic active margins. Thus, the lithological data and magma compositions suggest a context of a convergent margin, at first extensive with a marginal basin and volcanic arc, and then compressive with the filling up of the marginal basin and the building of a second arc. In the neighbouring region of High-Atlas and Anti-Atlas and, farther, in the Mauritanides and Pharusides belts, various Cambrian and Neoproterozoic volcanics can be compared with the studied ones. In High-Atlas, basaltic lavas are interbedded with Archaeocyatha-bearing Cambrian limestones and exhibit a composition of continental intra-plate tholeiites and alkaline basalts Badra et al., 1992 ; Soulïmani et al., 1998. This signature is related to the early Cambrian extension of a passive margin, and is clearly different from that of the studied lavas. In Anti-Atlas, various volcanics affected by the Pan-African orogeny are known in the Neoproterozoic inliers. They belong to the early to Middle Meoproterozoic pre-Pan-African extension, as they show the magmatic signature of continental tholeiites and initial rift tholeiites El Boukhari et al., 1991 ; Leblanc and Moussine-Pouchkine, 1994 ; Fekkak et al., 1999, 2001. Ophiolites of Siroua and Bou Azzer are related to the closing of the Pan-African ocean located to the east of the West African craton Bodinier et al., 1984 ; Hefferan et al., 2000. They were overlain, in the Middle to late Neoproterozoic, by two successive volcanic arc deposits Saquaque et al., 1992 ; Bajja et al., 1998, which belong to the convergent stage of the eastern Pan-African ocean, between West Africa and the Saharian terranes, in the continuation of the Pharusides and Dahomeyides belts. The convergence of the western ocean and the edification of the High-Atlas volcanic arc could be contemporaneous to the Anti-Atlas one. The post-orogenic volcanics of the Ouarzazate formation are mainly of felsic composition and devoid of folding. They seem to be different from the studied western-Atlas volcanics. In the Mauritanides and the Pharusides belts (fig. 11), similar andesitic arcs are known and dated to ca. 730 and 700 Ma Chikhaoui et al., 1980 ; Caby et al., 1981 ; Dupont et al., 1984, 1987; Liégeois et al., 1987. They are interpreted respectively as witnesses of the western and eastern Pan-African ocean convergences. Then, the western High Atlas andesitic arc can be placed in the same convergent context around the West African craton.

Item Type: Article
Uncontrolled Keywords: convergent margin; geochemistry; Pan African orogeny; Proterozoic; tectonic evolution; volcanism, Morocco
Subjects: Earth and Planetary Sciences
Divisions: SCIENTIFIC PRODUCTION > Earth and Planetary Sciences
Depositing User: Administrateur Eprints Administrateur Eprints
Last Modified: 31 Jan 2020 15:46
URI: http://eprints.umi.ac.ma/id/eprint/2971

Actions (login required)

View Item View Item