Genesis and evolution of the Late Mesozoic magmatism of the High Atlas (Morocco)

The Atlas System, developed at the northern margin of the Africa plate (Piqué et al., 2002), represents a well-studied case of tectonic inversion occurred through tectono-sedimentary stages succeeded from a continental rifting to the Europe-Africa plates collision. This articulated dynamic was accompanied by regional magmatic activity related to the evolution of the Central Atlantic Magmatic Province (CAMP). This affected the Moroccan Atlasic domains particularly during the Triassic-Jurassic transition (rift stage) and Middle Jurassic-Early Cretaceous (post-rift stage) with the emplacement of both effusive and sub-intrusive sub-alkaline and alkaline products (Frizon de Lamotte et al., 2008). A well-established interpretation of the Mesozoic magmatic activity refers it to the onset and development of a mantle plume in the frame of the evolution of the North America-Western Africa conjugate margins and interposed Central Atlantic Ocean (ref). Nevertheless, since the Middle Jurassic alkaline-sub-alkaline magmas emplaced within thick poly-deformed continental successions hinting to incipient uplift of the atlasic mountains of Morocco (Choubert & Faure-Muret, 1960-62; Ghorbal et al., 2008; Benvenuti et al., in press). Under this scenario magma emplacement occurred within pulses of crustal shortening predating the Cenozoic tectonic inversion related to the Africa-Europe collision. Studies carried out on the Middle-Jurassic-Early Cretaceous magmatic stage (Hailwood & Mitchell, 1971; Westphal et al., 1979; Harmand & Laville, 1983; Beraˆaouz et al., 1994; Amrhar et al., 1997; Lhachmi et al., 2001; Zayane et al., 2002; Charrière et al., 2005; Bensalah et al…) provided some chronologic calibration and petrographic-geochemical dataset specifically for the basalts occurring on the northern front of the Central High Atlas (CHA). The related findings have been discussed in the frame of the classic hypothesis of a long post-rift phase characterized by tectonic quiescience with plume-related magma migration along the rift-stage fault systems. Detailed field studies suggest that basalts, occurring both as lava flows and hypoabyssal intrusions within the continental deposits, are strictly associated to growth folds recording progressive syn-tectonic deposition in the Middle Jurassic-Cretaceous interval (Benvenuti et al., in press). The present study focuses on the dating, petrographic and geochemical features of two basalts interbedded within “post-rift” continental clastic deposits in a sector of the southern front of the CHA facing the eastern Ouarzazate Basin. These basalts, the only documented in the Late Mesozoic of the southern CHA front, have been previously referred respectively to the Trias-Jurassic transition (CAMP event) and to the Cenomanian basing on stratigraphic criteria. Recent stratigraphic and structural revisions carried out in the area (Benvenuti et al., in press) suggested a litho-and chronostratigraphic coincidence between the two basalts sampled for petrographic and geochemical analyses aimed at verifying this hypothesis. For comparison, further basalt samples were collected in the northern and axial sectors of the CHA and analysed. The results provide the first 40Ar-39Ar ages for the two examined basalts allowing inferences on the relations between magmatic and tectonic processes, which affected the southern portion of the Late Mesozoic Central High Atlas domain.