MAGMATIC FLARE-UPS IN A NEOPROTEROZOIC OCEANIC ARC SYSTEM (ANTI-ATLAS, MOROCCO)

Studying accreted oceanic arcs is crucial as they may give access to deep sections of the arc crust and allow constraining igneous and metamorphic processes driving arc growth. Such relics of intra-oceanic arc systems are exposed in the Sirwa and Bou Azzer windows (Anti-Atlas orogen, southern Morocco). Both areas form a tectonic patchwork made of back-arc ophiolitic sequences to the north thrusted onto accreted arc complexes to the south. The oldest meta-igneous unit belonging to the arc complexes (i.e. Tachakoucht, Tazigzaout and Bougmane complexes) are made of granodioritic/andesitic gneisses and amphibolites with typical oceanic arc signature. Their igneous ages (U-Pb zircons) range from 750 to 730 Ma. These magmatic rocks were buried, deformed and metamorphosed under MP-MT conditions in Tachakoucht and Tazigzaout (ca. 700°C - 8 kbar) and HP-MT in Bougmane (ca. 750-800°C - 10 kbar). These events occurred prior to two magmatic pulses dated at 700 and 650 Ma respectively and marked by hydrous mafic to intermediate rocks (i.e. chilled hornblende gabbros, hornblendite cumulates and quartz-diorites) with typical oceanic arc fingerprints. Both 700 and 650 Ma magmatic pulses strongly perturbed the thermal and physical structure of the arc locally leading to the granulitization (and garnetisation) of the middle and lower crust and to the genesis of intermediate to felsic magmas (tonalitic, granodioritic and granitic). The latter formed by dehydration melting of older or sub-contemporaneous hydrous mafic/intermediate rocks. Field, petrological, geochemical and geochronological investigations reveal that oceanic arc magmatism in the Anti-Atlas occurred on a 120 Ma long time span, marked by three successive flare-ups (IGN1-2-3: centred at 750, 700 and 650 Ma respectively). The flare-ups were interspersed with periods of magmatic lull and/or intra-oceanic crustal shortening. The thickening process is here driven by discrete phases of magmatic accretion at different levels of the crust but also by shortening tectonic regime leading to intra-arc stacking deformation (D1). These results suggest that the mechanisms of Neoproterozoic oceanic arc growth may locally differ from those governing the building of Phanerozoic arcs.