PALEOPROTEROZOIC SANUKITOID-HYBRID GRANITOIDS OF THE BAS DRAA INLIER, ANTI ATLAS (MOROCCO): WINDOW INTO THE TECTONIC REGIME OPERATING DURING EARLY PALEOPROTEROZOIC AT THE NW EDGE OF THE WEST AFRICAN CRATON (WAC)

Voluminous Paleoproterozoic granitoids outcropping in the basement of the of the Anti-Atlas (Morocco) inliers are key for understanding the Precambrian geodynamics of the Western African Craton (WAC). Solely due to their calc-alkaline affinity, these intrusions have previously been interpreted as sensu stricto arc magmas, implying a plate tectonic convergent setting for the WAC during this time. However, independent geological evidence for the existence of this (back-)arc setting (e.g., accretionary prism, ophiolites, and high-pressure metamorphic rocks) have not been reported to date.

Here, we present new whole-rock, major- and trace-element, Sr-Nd isotope compositions, and in-situ zircon U-Pb ages of composite Paleoproterozoic granitoid intrusions from the Bas Draa inlier. U-Pb ages yielded Rhyacian ages for the layered quartz-diorite (~2059 Ma) and leucogranite (~2051 Ma), and Orosirian age for the biotite-bearing granite (~2022 Ma). The layered quartz-diorites display sanukitoid-like features such as elevated concentrations of Mg, V, Ni, and Cr, enrichment in LILEs (Cs, Rb, Ba, Sr) relative to HREE, and absence of Eu anomalies. The leucogranites classify as two-mica granites, and the biotite-bearing granite has hybrid characteristics between the sanukitoids and leucogranite. All lithologies share compositions common to Late-Archean granites world-wide. We argue that the identification of mantle-derived sanukitoid and hybrid granites challenges the classical paradigm that the Paleoproterozoic granitoids of the Anti-Atlas evolved in a modern-style arc setting and advocate that vertical extensional processes, characteristic of the Archean, at least locally prevailed in some cratons through the early Proterozoic times.

Keyword: Anti-Atlas, WAC, Paleoproterozoic, Granitoids, Nd-Sr isotopes, U–Pb zircon geochronology, Sanukitoids, Vertical extension.