Structural and geochronological constraints on the evolution of the Bou Azzer Neoproterozoic ophiolite (Anti-Atlas, Morocco)

摘要

The Bou Azzer ophiolite in the Moroccan Anti-Atlas represents the suture of the Neoproterozoic Pan-African orogen. The Bou Azzer inlier includes, from NE to SW, three main geological units: a volcanic-arc, an ophiolite and a continental platform. Several subduction-related plutons intrude the ophiolite and the volcanic-arc, and syn-to-late orogenic deposits unconformably cover the three terranes. The Eburnean (≈2. Ga) continental basement observed in other inliers of the Anti-Atlas appears not to crop out at Bou Azzer, where rocks traditionally attributed to that old basement are in fact Neoproterozoic rocks, either metagranitoids of the continental margin or ophiolitic metagabbros.The age of a metagabbro of the ophiolite is reported here for the first time, as 697 ± 8. Ma (SHRIMP U-Pb on zircons). U-Pb geochronological data are also presented for two syn-kinematic subduction-related plutons which, together with previous data, establish a time-span of ca. 655-635. Ma for these arc-type rocks. Moreover, the Oumlill granite, previously attributed to the Eburnean basement, has been dated as 741 ± 9. Ma.The earliest tectonometamorphic event affecting the ophiolite is recorded as a relic foliation and a garnet-rutile metamorphic assemblage, which might attest to an early peak-pressure exceeding the 5-6. kbar medium pressure reported so far. Thus, we suggest a first stage of ophiolite underthrusting below a volcanic-arc, followed by ophiolite exhumation and the concomitant development of the main low-grade planar-linear tectonic fabric. The ages of the calc-alkaline, syn-kinematic subduction-related plutons (655-635. Ma) might be taken as the end of subduction and the beginning of the ophiolite exhumation-obduction.The author's work in the Bou Azzer region supports the ophiolite-continental platform interpretation of Leblanc (1975) and Leblanc and Moussine-Pouchkine (1994), rather than the tectonic mélange interpretation proposed by Saquaque et al. (1989b) and others. A recumbent anticlinal and the basal thrust of the ophiolitic terrane are the main macrostructures formed during the obduction stage, characterized by top-to-the-W/SW kinematics. This deformational event is also recorded in the rocks of the underlying continental platform. A syn-orogenic unit, namely the Tiddiline Formation, unconformably overlies the arc volcanics, the ophiolite and the continental platform. Further collisional deformation affected the Tiddiline sediments, giving way to ESE-WSW striking upright folds and oblique reverse faults. The latest structures in this Neoproterozoic suture were left-lateral strike-slip faults, pointing to an increase in the left-lateral component of the continental collision.