A REVIEW OF THE PALEOPROTEROZOIC MAGMATIC EVOLUTION OF THE ANTI-ATLAS BELT, MOROCCO

The Anti Atlas belt, in the Northen edge of the West African Craton (WAC), representing the Precambrian nucleus of Morocco, were the Paleoproterozoic rock (~2.2–1.7 Ga) outcrop within several inliers or windows and overlain by Neoproterozoic to Cambrian sequences. Voluminous granitoid complexes that occupy the western Anti-Atlas during the Paleoproterozoic has been hypothesized to represent magmas produced in a subduction-arc setting. Recognizing their importance in understanding early Precambrian crustal evolution, and considering the voluminous literature and arguable interpretations. In this contribution, we present a holistic review from a multidisciplinary study involving field investigations, petrology, zircon U/Pb geochronology, whole rock geochemistry and Nd isotope data. the aim is re-interpreting the available data, re-discussing them, and discussing existing tectonic models and their limitations.

We define five distinct plutonic suites, including: (1) high SiO2, Na2O-rich tonalite-trondhjemite-granodiorite (TTG) granitoids (ca 2106 Ma); (2) TTG transitionnel with high K2O, K2O/Na2O, Rb content and, low REE and Th content; (3) and (4) generation of coeval calc-alkaline, LILE-enriched diorites, sanukitoids (ca. 2059 Ma) and high potassic granite (ca. 2051 Ma) respectively, followed by (5) hybrid granite (ca 2022 Ma) displaying a composition intermediate between the two previous lithologies, and hybrid petrographic features. Trace element patterns of most magmatic suites display negative Nb-Ta, P and Ti anomalies and weakly positive Pb anomalies, suggesting magma genesis in a subduction setting. Yet, in the absence of blueschist facies and high-pressure rocks as well as ophiolites and accretionary prism makes the tectonic regime responsible of formation of this magmatism granitic unclear and the classical model of subduction and collision are not convincing. The identification of such suites in Rhyacian and Orosirian age challenges the popular hypothesis that regards Paleoproterozoic granitoids of the Anti-Atlas as arc magmas. This corroborates the diachronous onset of plate tectonics within different Paleoproterozoic orogens worldwide and raises the possibility that anti-plate tectonic models still existed in the early Paleoproterozoic.