Episodic magmatism during the growth of a Neoproterozoic oceanic arc (Anti-Atlas, Morocco)

We present an integrated study combining detailed field, geochronological and geochemical data of a Neoproterozoic intra-oceanic arc systems exposed in the Pan-African belt of the Moroccan Anti-Atlas. The arc rock units exposed in Bou Azzer and Sirwa inliers consist of a tectonic patchwork of back-arc ophiolitic sequences to the north thrusted onto accreted arc complexes to the south. Arc complexes are composed of amphibolite, granodioritic and granitic gneisses intruded by various undeformed hydrous ultramafic (hornblendite), mafic (hornblende-gabbro, diorite) and felsic (granodiorite, tonalite, granite) arc lithologies. We show that these complexes are remnants of a long-lived (120 Myr) Neoproterozoic oceanic arc, punctuated by three successive magmatic episodes (760–730 Ma, 710–690 Ma, 660–640 Ma respectively) interspersed with periods of magmatic quiescence. The typical geochemical arc signature and positive ƐNd_t values for the igneous rocks emplaced during each magmatic episode (medians at +7.1, +5.4 and +5.7, from older to younger) attest that their parental magmas derived from a depleted mantle source without substantial assimilation by the WAC older crustal basement. Trace-element geochemistry, i.e. Sr/Y, La/Yb, of intermediate to felsic arc rocks produced during each magmatic pulse suggests that the arc crust was thickened (>30–35 km) over a short time period between the first and second magmatic episodes (730–710 Ma) which coincides with an important regional shortening event. Soft-docking of the oceanic arc on a buoyant transitional margin is invoked to explain tectonic inversion in overriding plate, leading to shortening and related thickening of the arc crust. Concomitant magmatic shutdown resulting from a reorganization of subduction dynamics (i.e. change in slab geometry, flip in subduction polarity). A non-tectonic critical thickening of the arc crust is invoked to explain the second magmatic shutdown (680–660 Ma), by freezing the subarc mantle influx. This lull period is followed by a third magmatic episode which is likely triggered by delamination of the dense lower crust and reactivation of subarc mantle flow. This is supported by the bimodal chemical signature of evolved magmatic products, suggesting two distinct sources partial melts from the foundered lower crust and new magmatic products which differentiated from a post-delamination thinned crust.