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

Abstract

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.

The Moroccan Anti-Atlas ophiolites: Timing and melting processes in an intra-oceanic arc-back-arc environment

“…This ophiolitic sequence is extensively tectonized, marked by tectonic contacts between most of its ophiolitic terms (see Fig. 2 in Triantafyllou et al, 2020). Three major tectonic slices were recognized in the studied area, whose sole appears systematically represented by soft serpentinite bands.…”

“…These reactions induced by contact metamorphism are responsible for the formation of garnet-bearing leucosomes during incipient partial melting of hornblende-rich rocks. As well as for the Iriri-Tachakoucht-Tourtit arc complex of the Sirwa inlier, the geochemical and isotopic studies of D' Lemos et al (2006) on the Tazigzaout complex and of Triantafyllou et al (2018Triantafyllou et al ( , 2020 on the Bougmane complex have shown that all these rocks are the product of a polyphased magmatic activity from a magmatic arc system located in an intra-oceanic domain.…”

“…The leucocratic horizons produced by their localized partial melting induced by the Iriri intrusions are younger, between 651 ± 5 Ma and 641 ± 5 Ma (see zircons' metamorphic overgrowths in, Thomas et al, 2002;Triantafyllou et al, 2016). The Iriri-Tachakoucht-Tourtit complex appears to be the product of several successive magmatic episodes with an intra-oceanic arc geochemical signature (Thomas et al, 2002;Triantafyllou et al, 2016Triantafyllou et al, , 2020.…”

“…These highly metamorphosed complexes are composed of mafic gneisses (amphibolite) as well as granitic orthogneiss and granodioritic augen gneisses (see Triantafyllou et al, 2018). Triantafyllou et al (2018Triantafyllou et al ( , 2020 show that the Bougmane complex consists of 760-740 Ma heterogeneously mingled felsic-mafic orthogneiss intruded by several bodies of 710-690 Ma hornblendites, hornblende gabbros and garnet leucosomes. Garnet reaction zones form at the contact between different hornblende gabbro intrusions as a consequence of dehydration and dehydration melting under high pressure (garnet-present) granulite grade conditions (~800°C, 10 kbar; Triantafyllou et al, 2018).…”

Plombotectonique des gisements du Maroc

“…La genèse des gisements de plomb géants au Protérozoïque a été mise en relation avec l'intense mobilisation crustale lors de la formation du continent Nuna, la stabilisation de la lithosphère et l'ouverture de grands bassins au début du Mésoprotérozoïque (Sawkins, 1989 ;Hawkesworth et al, 2020). La construction continentale sur la marge nord du craton ouest-africain démarre un milliard d'années plus tard avec la formation du Gondwana, un continent à forte épaisseur crustale et largement émergent au-dessus du niveau de la mer (Traintafyllou et al, 2020). Le stock de plomb des formations volcaniques panafricaines devient alors plus facilement accessible et sera essentiel au Maroc.…”