U-Pb geochronology of granitoids from northern Morocco: a two-step tale for the evolution of the Variscan domain in NW Africa

The Variscan belt in NW Africa represents an intraplate orogen, which mainly developed from Late Carboniferous (ca. from the Bashkirian: 323–315 Ma) to the end of Cisuralian (283–273 Ma), in response to the closure of an oceanic domain between NW Gondwana and Laurentia. The orogenic events were accompanied by an important magmatic activity, distributed across the whole northern Morocco, north of the South Meseta Fault Zone which separates the Meseta domain in the north from the Anti-Atlas domain to the south, where no magmatic activity was recorded at that time. Moreover, an older magmatic phase occurred during the Late Devonian-Early Carboniferous however the associated geodynamic context remains under debate.

In northern Morocco, existing geochronological results for these magmatic activities are mainly obtained using Rb-Sr methods (whole rock or on different minerals). However, recent U-Pb datings on different massifs in Morocco have revealed older ages. Therefore, Rb-Sr ages should be considered with caution.

In this contribution, we present new U-Pb geochronological data (LA-ICP MS on zircons) for several plutonic massifs of northern Morocco from the Western (Jebilet) and Eastern Meseta. In the later, we provide new datings on 3 magmatic facies in the High Moulouya massif, but also for the Boudoufoud, Beni Snassen, Tanncherfi, Zekkara and Merguechoum massifs. These new data reinforce the idea that two main magmatic events must be properly separated in northern Morocco. First, a clear Early Carboniferous event, possibly beginning as soon as the end of Late Devonian, is recorded within both Western and Eastern Meseta, and this magmatic event ends at around 330 Ma. Afterwards, from ca. 310–305 Ma to ca.280–275 Ma, the second magmatic event is also expressed across both Western and Eastern Meseta. While this second magmatic event is clearly related to the Variscan orogenic events, the unclear geodynamic context for our new Early Carboniferous ages allows the re-opening of the discussion about the « Eovariscan » phase in NW Africa. Finally, these two magmatic phases are compared to the magmatic records from West Europe Variscan belt.