Geochronology and petrogenesis of granitoids from the Bas Draa inlier (Western Anti-Atlas, Morocco): Revived debate on the tectonic regime operating during early Paleoproterozoic at the NW edge of the West African Craton

Abstract

Proterozoic terrains of the Anti-Atlas belt (Morocco) represent one of the key areas for studying the Precambrian geodynamics of the northwestern West African craton (WAC). Voluminous Paleoproterozoic granitoids, which outcrop in the basement of the Precambrian inliers of the southern Anti-Atlas, are conventionally regarded as vestiges of a magmatic arc constructed during incipient subduction on Earth. However, except for their calc-alkaline, arc-like signatures, little evidence supports the existence of this arc system and the sources and tectonic context of these granitoids remain elusive. In this study, we present new whole-rock major- and trace-element, and Sr-Nd isotope composition, and in situ zircon U-Pb ages of the composite granitoid intrusions from the Bas Draa inlier. We obtained Rhyacian ages for the quartz-diorite (~2059 ± 6 Ma) and leucogranite (~2051 ± 6 Ma), and Orosirian age for the biotite-bearing granite (~2022 ± 7 Ma). The studied magmatic rocks are classified into three groups according to their petrographic features and chemical composition: a) layered, sanukitoid quartz-diorites, b) biotite-muscovite (i.e., two-mica) leucogranites, and c) biotite-bearing, hybrid granites. All three groups of magmatic rocks share a common geochemical composition with late Archean granites worldwide, including a high Mg# (52.9–64), and Ni (average 40 ppm), Cr (10–250 ppm) and V (139-238 ppm) content. The identification of mantle-derived sanukitoids and hybrid granites, combined with the absence of first-order evidence for subduction, obduction, and collision, as commonly observed in Neoproterozoic and Phanerozoic orogens worldwide (i.e., blueschist facies metamorphics, ophiolites, and accretionary prism units), contests the theory for the onset of plate tectonics during Early Paleoproterozoic in the Anti-Atlas. We instead propose that the available data rather suggests that vertical tectonics, characteristic of the Archean, still prevailed in this domain throughout the Early Proterozoic times.