DETRITAL ZIRCON SOURCE EVOLUTION DURING THE DEVONIAN-CARBONIFEROUS COLLISION IN THE NORTHERN MOROCCAN VARISCIDES

The onset of the Variscan orogeny (Eovariscan phase: lattermost Devonian – earliest Carboniferous) started with the final closure of the Rheic Ocean and the collision between the northern margin of Gondwana and Laurussia. In this study, we report the results of U-Pb dating on detrital zircon grains from 17 syn- to post-orogenic (Famennian – Westphalian) samples from the northern Moroccan Variscides. All the samples analyzed contain significant Cadomian (Ediacaran, ca. 620 Ma) and Eburnean (Rhyacian, ca. 2.1 Ga) detrital zircon populations. These ages suggest that the West African Craton was the primary source of sediments. A Stenian-Tonian (ca. 1.0 Ga) detrital zircon population is locally present, evoking intermittent and distant sources from NE Africa (e.g. Sahara Metacraton). Nevertheless, the syn-orogenic samples from Tiflet and Debdou-Mekkam areas also show a significant Late Devonian population (ca. 370 Ma) probably derived from a magmatic arc formed on the edge of the Laurussian continental margin (Avalonian terrane) during the subduction of the Rheic oceanic lithosphere. This detrital zircon population generally occurs in association with an increasing number of Mesoproterozoic ages, thus reinforcing the Avalonian origin of the sediments. In late-orogenic samples, the Avalonian influence is recorded only in those areas located close to this exotic terrane (e.g. Ben Slimane area). Finally, in the Eastern Moroccan Meseta (Jerada area), Late Carboniferous samples are characterized by a considerable amount of Middle Carboniferous (ca. 330 Ma) detrital zircon grains sourced from Early Variscan granitoids, which, in turn, suggest an important crustal thickening and thermal maturation in this region.