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

Abstract

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).