A newly discovered orogenic event in Morocco: Neoproterozic ages for supposed Eburnean basement of the Bou Azzer inlier, Anti-Atlas Mountains

Abstract

The Bou Azzer inlier within the Anti-Atlas orogenic belt of Morocco exposes a dismembered ophiolite, long considered to mark a Late Neoproterozoic suture between the West African Craton (WAC) in the south, and Neoproterozoic arcs to the north. Southern parts of the Bou Azzer inlier include meta-igneous units, paragneiss, schist and muscovite-bearing granite that have until now been regarded as the c. 2 Ga Eburnean WAC basement onto which the ophiolite was obducted. An example of supposed Paleoproterozoic basement occurs at Tazigzaout. The main units within this complex are metagabbro, augen granite gneiss, finely banded granite gneiss and variably sheared syn-tectonic leucogranite, all of which are intercalated within a complex sub-vertical, upper greenschist facies, ductile dextral shear zone. The meta-igneous rocks are crosscut by a sheeted muscovite-biotite granite pluton. Three concordant U–Pb analyses of zircon from an augen granite gneiss provide a date of 753 +1/−2 Ma. Zircon from a nearby metagabbro provide an indistinguishable date of 752.2 ± 2.4 Ma. Both dates are considered best estimates of the crystallization ages of their igneous protoliths. Analyses of zircon from two crosscutting leucogranite bodies provide younger dates of 705 +2/−3 Ma and 701 +2/−1 Ma. The initial ɛ Nd values of all lithological units in the complex range from +4.9 to +6.0, with depleted mantle model ( T DM) ages of 890–790 Ma. The high, positive ɛ Nd values for all of these igneous units is indicative of an isotopically juvenile source, such as depleted-mantle derived material, and precludes substantial interaction with old, evolved crustal sources. These data demonstrate that the “basement” rocks of the Tazigzaout complex are Neoproterozoic intrusions, not c. 2 Ga Eburnean units, as found in more southerly Anti-Atlas inliers. The structural relationships and U–Pb ages are interpreted as recording dextrally transpressive deformation of c.755 Ma juvenile crust after c. 750 and before 700 Ma. This Neoproterozoic deformation occurred as much as 100 Ma prior to the top-to-the-north thrusting of the meta-igneous units over units of the ophiolite and arc, and provides strong evidence for two discrete ‘Pan African’ events in Morocco.