Geochronological constraints on the polycyclic magmatism in the Bou Azzer-El Graara inlier (Central Anti-Atlas Morocco)

• Published in: Journal of African earth sciences (1994) Vol. 99; pp. 287 - 306
• Main Authors: Blein, O., Baudin, T., Chèvremont, P., Soulaimani, A., Admou, H., Gasquet, P., Cocherie, A., Egal, E., Youbi, N., Razin, P., Bouabdelli, M., Gombert, P.
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.11.2014; Elsevier
• Subjects: Anti-Atlas; Bou Azzer; Cryogenian; Ediacaran; Environmental Sciences; Pan-African; U–Pb geochronology; Anti-Atlas; U–Pb geochronology; Cryogenian; Ediacaran; Bou Azzer; Pan-African
• ISSN: 1464-343X; 1879-1956
• DOI: 10.1016/j.jafrearsci.2014.04.021

•Multiple Cryogenian (lower and upper) volcanic-arc sequences.•At least two distinct accretion of volcanic arc to the WAC margin.•A late Cryogenian Bou Azzer ophiolite.•An Ediacaran active margin of the WAC.•A tectonic deformation with an erosional phase separating lower and upper Ediacaran. New U–Pb SHRIMP zircon ages from the Bou Azzer-El Graara onlier constrains the Neoproterozoic evolution of the Anti-Atlas during Pan-African orogenesis. Within the Central Anti-Atlas, the Bou Azzer-El Graara inlier exposes a dismembered ophiolite, long considered to mark a late Neoproterozoic suture between the West African Craton in the south, and Neoproterozoic arcs to the north. From north to south, this inlier includes four main geological units: a volcanic-arc, an ophiolite, a metamorphic complex and a continental platform. Several plutons intrude the volcanic-arc, the ophiolite, the metamorphic complex, and post-orogenic volcanic and sedimentary deposits unconformably cover these terranes. The age of the volcanic-arc is reported here for the first time. Analyses of zircon of two rhyolites provide ages of 761±7Ma and 767±7Ma. Zircons from two gneisses provide dates of 755±9Ma and 745±5Ma. Both dates are considered best estimates of the crystallization ages of their igneous protoliths. Analyses of zircon from two granitic bodies, which crosscut gneisses, provide younger dates of 702±5Ma and 695±7Ma. The age of an aplitic body of the ophiolite is reported here for the first time, as 658±8Ma (SHRIMP U–Pb on zircons). Theses ages suggest the existence of three distinct orogenic events during Cryogenian times: (i) 770–760Ma Tasriwine-Tichibanine orogeny with rollback of the subducting oceanic plate, leading to the formation of back-arc basins; (ii) 755–695Ma Iriri-n’Bougmmane orogeny; and (iii) the 660–640Ma Bou Azzer orogeny involving the formation and the emplacement of the Bou Azer ophiolite. During Ediacaran times, the Bou Azzer-El Graara inlier is characterized with the development of a continental volcanic arc between 630 and 580Ma (Bou Lbarod Group, 625±8Ma; Bleïda granodiorite, 586±15Ma), and strike-slip pull-apart basins (Tiddiline Group, 606±4Ma and 606±5Ma). These volcanic and sedimentary Lower Ediacaran sequences are deformed before the felsic pyroclastic deposits of the Ouarzazate Group (567±5Ma and 566±4Ma). Finally, the Ouarzazate Group is overlain by early Cambrian volcanic deposits of the Jbel Boho Formation (541±6Ma).