Petrogenesis and geodynamic implications of Ediacaran rocks from the Sirwa massif (Central Anti-Atlas); insights from U-Pb geochronology, whole-rock geochemistry, and Sm-Nd isotopes

Abstract. The geodynamic evolution of the Anti-Atlas belt post-Pan-African orogeny (~650 Ma) remains debated, particularly regarding the basement beneath the Central Anti-Atlas, and the geological processes leading to the formation of the Ediacaran Saghro Group (SG), and Ouarzazate Group (OG). New LA-ICP-MS U-Pb ages of 575 ± 3 Ma and 564 ± 2 Ma were obtained respectively from samples Zg-106, and Zg-119 from the OG. In addition, detrital zircons from SG sediments yield a prominent 2.1 Ga age peak, indicating local recycling of Paleoproterozoic basement material. Geochemically, two magmatic series are identified : (i) a SG mafic-intermediate calc-alkaline series with Nb-Ta and Ti negative anomalies from early back-arc basin setting; and (ii) a felsic-intermediate high-K calc-alkaline to shoshonitic series of the OG, exhibiting continental magmatic arc signatures. Isotope data (εNd (t): +3.2 to +4.5, TDM = 1431 − 1197 Ma for SG; εNd (t): −0.9 to +1.1, TDM = 1526− 1252 Ma for OG), indicates that the SG formed from a dominantly juvenile, mantle-derived source, with limited crustal contribution; while the slightly younger OG involved significant reworking of older, evolved continental crustal material.

These findings sustain a model where Early Ediacaran SG sediments and associated mafic-intermediate volcanics were formed in a back-arc basin. During this basin development, its shoulders were locally formed by the 2.1 Ga Paleoproterozoic basement, supplying Paleoproterozoic zircons to the Saghro host basin. This, further supports the occurrence of the Eburnian basement north of the Anti-Atlas Major Fault (AAMF). Additionally, the younger OG reflects a Late Ediacaran continental crust collapse event involving widespread crustal reworking and the emplacement of a Silicic Large Igneous Province (SLIP).