Contribution to a geodynamic reconstruction of the Anti-Atlas (Morocco) during Pan-African times with the emphasis on inversion tectonics and metallogenic activity at the Precambrian-Cambrian transition

New geochronological analyses (U-Pb SIMS zircon ages) have yielded ages of 552 +/- 5 Ma for the Bou Madine rhyolitic dome (Ougnat, eastern Anti-Atlas), 543 +/- 9 Ma for the Tachkakacht rhyolitic dyke (Saghro-Imiter, eastern Anti-Atlas), and 531 +/- 5 Ma for the Aghbar trachytic sill (Bou Azzer, central Anti-Atlas). Inherited zircon cores from the Aghbar trachytic sill and from the Bou Madine rhyolitic dome have been shown to be of Statherian age (ca. 1600-1800 Ma) and Palaeoproterozoic (>2100 Ma) age, respectively, suggesting that a significantly older protolith underlies the Pan-African rocks in the Central and Eastern Anti-Atlas. Granodiorites and rhyolites from the Saghro-Imiter area have similar low Sr-87/Sr-86 (0.702-0.706) and Nd-143/Nd-144 (0.5116-0.5119) initial ratios, suggesting a mixture of mantle and lower crust sources. This can also be inferred from the low Os-187/Os-188 ratios obtained on pyrite crystals from the rhyolites. A recently published lithostratigraphic framework has been combined with these new geochemical and geochronological data, and those from the literature to produce a new reconstruction of the complex orogenic front that developed at the northern edge of the Eburnian West African craton during Pan-African times. Three Neoproterozoic magmatic series can be distinguished in the Anti-Atlas belt, i.e., high-K cale-alkaline granites, high-K calc-alkaline to shoshonitic rhyolites and andesites, and alkaline-shoshonitic trachytes and syenites, which have been dated at 595-570, 570-545 and 530 Ma, respectively. The accretion of the Pan-African Anti-Atlas belt to the West African super continent (WAC) was a four-stage event, involving extension, subduction, moderate collision and extension. The calc-alkaline magmatism of the subduction stage was associated with large-scale base metal and gold mineralisation. Metallogenic activity was greatest during the final extensional stage, at the Precambrian-Cambrian boundary. It is characterised by world-class precious metal deposits, base-metal porphyry and SEDEX-type occurrences. (c) 2005 Elsevier B.V. All rights reserved.