200 Ma of magmatism along the northern border of the West African Craton during Pan-African convergence

Convergence zones are marked by a variety of settings that may follow each other in modern-day tectonics, including compressive phases such as subduction, obduction, collision but also extensive ones such as back-arc opening or stress-relaxation during orogenesis. Hence, the protracted evolution leading to a super-continent block amalgamation may be difficult to decipher and so may be the forcings on external enveloppes such as volcanism or erosion caused by the different phases.

This question arises critically at the time of the Pan-African Orogenesis (1-0.5 Ga) assembling Gondwana, a time of supposedly dramatic and diachronical changes for external envelopes: glaciations of debated scales, deposition of various Banded Iron Formations, first (Ediacarian) fauna, replacement by Cambrian faunas. Our goal is to explore in detail the geodynamical succession leading to the amalgamation of blocks along the northern margin of the West African Craton (WAC), outcropping in the Central Anti-Atlas region, Morocco. This region is characterized by the occurrence of extended convergence-related magmatism, ophiolite emplacement and basins fillings (including BIF) during Cryogenian and Ediacaran periods.

Data obtained from compilation of cartographic work, whole-rock geochemistry and datation reveals a polyphased but still poorly constrained evolution through proxies of continentality (εNd) and of crustal thickness (Sr/Y ratio). We present new data spanning metamorphic petrology, basin stratigraphy, coupled datation and trace element analysis in detrital zircons in order to better understand the evolution of the geodynamic, magmatic and drainage systems. We propose a geodynamic scenario based on these data:

• Development of an early oceanic arc (760-720 Ma) with juvenile magmatic signature (3<εNd(t)<7), its accretion on the WAC is followed by an episode of calc-alkaline magmatism (710-700 Ma).

• Second arc development (700-670 Ma) only seen in detrital and inherited zircons, its accretion at 670 Ma is followed by late-orogenic magmatism (660-650 Ma) associated with decreasing crustal thickness (from 70 to 25 km).

• Third arc development on the newly formed continent margin (640-600 Ma) until oceanic closure and collision. Intense late orogenic magmatism occurs (590-570 Ma), coeval with the decreasing crustal thickness (from 100 to 30 km).

• A late phase of calc-alkaline is recorded (570-550 Ma) at constant and regular crustal thickness (25 km). A 550 Ma compressive event is recorded, very few calc-alkaline follows.

• The onset of Cambrian with the development of a large passive margin capping the whole region. This change coincides with disappearance of ice-house climate evidence from the global sedimentological record.