Jean-Paul Liégeois

The Moroccan Anti-Atlas: the West African craton passive margin with limited Pan-African activity. Implications for the northern limit of the craton

Abstract The Moroccan Anti-Atlas region, located south of the South Atlas Fault, has been viewed traditionally as containing two segments separated by the Anti-Atlas Major Fault. These two segments are said to consist of: (a) 600–700 Ma Pan-African segment located in the northeast; and (b) ∼2 Ga Eburnian segment situated to the southwest. On the basis of observations in the Zenaga and Saghro inliers and of a recent literature review, we suggest that this subdivision is inappropriate in that Eburnian and Pan-African materials occur throughout the Anti-Atlas region: the entire Anti-Atlas is underlain by Eburnian crust, unconformably overlain by a lower Neoproterozoic passive margin; allochthonous Pan-African ocean crustal slices were thrust onto the West African craton (WAC) passive margin sequence ∼685 Ma ago as a result of Pan-African accretion tectonics; high-level high-K calc-alkaline and alkaline granitoids locally intruded the Anti-Atlas sequence as a whole at the end of the Pan-African orogeny at 585–560 Ma; the intervening 100 m.y. interval was marked by quiescence. This succession of events can be related to the behaviour of one single rigid cratonic passive margin during an orogeny and correlated to the Pan-African events that occurred to the east in the Tuareg shield and to the north in Avalonian terranes. This model implies that the actual northern limit of the WAC is located at the South Atlas Fault (SAF) and not at the Anti-Atlas Major Fault (AAMF). We propose that the AAMF corresponds to the southwestern boundary of an aulacogen that formed along the northern margin of the WAC during early Neoproterozoic times. This is consistent with the development of the Gourma aulacogen on the eastern side of the WAC. This model further suggests that the northeastern boundary of the WAC occurs north, and not south, of Ougarta (Algeria). Support for this model is provided by geological and geophysical evidence.

The Boundaries of the West African Craton

The boundaries of rigid cratons can be affected by subsequent orogenic events, leading to ‘metacratonic’ characteristics not often properly recognized and still poorly understood. Major lithospheric thickening is absent and early events such as ophiolites are preserved; however, metacratonic boundaries are affected by major shear zones, abundant magmatism and mineralizations, and local high-pressure metamorphism.nnWest Africa, marked by the large Eburnian ( c. 2 Ga) West African craton, the absence of Mesoproterozoic events, the major Pan-African (0.9–0.55 Ga) mobile belts that generated the Peri-Gondawanan terranes, and the weaker but enlightening Variscan and Alpine orogenies, is an excellent place for tackling this promising concept of metacratonization.nnThe papers in this book consider most of the West African craton boundaries, from the reworking of the Palaeoproterozoic terranes, through the Pan-African encircling terranes, the late Neoproterozoic-early Palaeozoic extension period and the Peri-Gondwanan terranes, the Variscan imprint to the current situation.

The role of fractional crystallization and late-stage peralkaline melt segregation in the mineralogical evolution of Cenozoic nephelinites/phonolites from Saghro (SE Morocco)

Abstract The Saghro Cenozoic lavas form a bimodal suite of nephelinites (with carbonatite xenoliths) and phonolites emplaced in the Anti-Atlas belt of Morocco. Despite the paucity of samples with intermediate composition between the two main types of lava (only one phonotephrite flow is reported in this area), whole-rock major element modelling shows that the two main lithologies can be linked by fractional crystallization. The most primitive modelled cumulates are calcite-bearing olivine clinopyroxenites, whereas the final stages of differentiation are characterized by the formation of nepheline-syenite cumulates. This evolution trend is classically observed in plutonic alkaline massifs associated with carbonatites. Late-stage evolution is responsible for the crystallization of hainite- and delhayelite-bearing microdomains, for the transformation of aegirine-augite into aegirine (or augite into aegirine-augite), and for the crystallization of lorenzenite and a eudialyte-group mineral as replacement products of titanite. These phases were probably formed, either by crystallization from late residual peralkaline melts, or by reaction of pre-existing minerals with such melt, or hydrothermal peralkaline fluid.

Flow of Canary mantle plume material through a subcontinental lithospheric corridor beneath Africa to the Mediterranean: COMMENT

The Atlas mountain [ ARTICLE][1]range in Morocco, northwest Africa, represent an intracontinental belt with an elevation up to 4400 m, marked by the unexpected lack of a thick lithospheric root. This has been explained by the major role of thermal uplift in the Anti-, High- and Middle-Atlas,

The Moroccan Anti-Atlas: the West African craton passive margin with limited Pan-African activity. Implications for the northern limit of the craton: reply to comments by E.H. Bouougri

We agree with El Hafid Bouougri (EHB) that Anti-Atlas geology is complex and that only a multidisciplinary approach can bring new constraints. In our paper (Ennih and Liégeois, 2001), we tried to show that this partly results from particular localization of the Anti-Atlas on the shoulder of a craton, namely the West African craton (WAC). During the Pan-African orogeny, the latter acted as a rigid body but was partly destabilized at its margin, allowing a major but late high-K calc-alkaline plutonism and volcanism to be emplaced (610 /560 Ma; Thomas et al., 2002). Before that event, an oceanic terrane built at around 750 Ma accreted at 685 /665 Ma (Leblanc and Lancelot, 1980; De Wall et al., 2001; Thomas et al., 2002; Admou et al., 2002), generating a major thrust event affecting also the passive margin series. No continental collision affected the Anti-Atlas during the Pan-African orogeny, rendering its geological interpretation useless through usual interpretations and explaining the diversity of geological models proposed for Anti-Atlas, as remarked by EHB. We thank EHB for his comments, this will allow us to specify some points that were misunderstood and to answer well-taken points. This will also allow us to make reference to studies published after the writing of our paper and to include an updated version of the figure summarizing our model.

A complex multi-chamber magmatic system beneath a late Cenozoic volcanic field: evidence from CSDs and thermobarometry of clinopyroxene from a single nephelinite flow (Djbel Saghro, Morocco)

Abstract We used quantitative textural measurement, electron microprobe microanalysis and thermobarometry on clinopyroxene from a Cenozoic pyroxene-nephelinite flow located along the northern boundary of the West African craton to decipher magma differentiation processes in underlying magma chambers. The crystal size distributions of clinopyroxene phenocrysts show straight but also curved and kinked patterns and the clinopyroxene show large compositional variations in a single flow (Mg-number 48–88). These observations are strong evidence for magma mixing between a nephelinite magma and a more differentiated phonolitic melt at depth. Detailed thermobarometry on these clinopyroxene shows that at least three magma chambers are present below Saghro and that they are emplaced at the main physical interface within the lithosphere: (1) at the crust–mantle boundary, where the mantle-derived nephelinite has been mixed with a pre-existing phonolitic magma chamber; (2) at the lower–upper crust boundary; (3) close to the surface in a sub-volcanic magma chamber. Some high-pressure phenocrysts (up to 14 kbar) have also probably crystallized within the upper lithospheric mantle. The high clinopyroxene proportion in samples from the base of the flow is thought to reflect crystal settling during cooling of the nephelinite flow at the surface.