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⁴⁰Ar/³⁹Ar polyorogenic mineral age record within the central Mauritanide orogen, West Africa

¹ Department of Geology, University of Georgia, Athens, Georgia 30602
² U.A. Centre National de la Recherche Scientifique no 132, Faculté des Sciences et Techniques de St Jérme, Université d'Aix-Marseille III, Marseille-Cedex 13, France

The central Mauritanide orogen in the vicinity of Magta Lahjar is characterized by several internally imbricated, polydeformed and variably metamorphosed infrastructural allochthons, which include (1) a calc-alkaline igneous complex (Kelbé intrusive and El Hneïkt extrusive sequences); (2) portions of an ophiolitic sequence (El Aoueïja and Oued Amor Units); and (3) continental-margin rift sequences (Farkka Association), which include amphibolite (low-Ti continental tholeiite dike protoliths), mylonitic quartzite, and kyanite schist. Several variably meta-morphosed, quartzitic supracrustal nappes (Goua and Sangarafa allochthons) structurally overlie the infrastructural units.

⁴⁰Ar/³⁹Ar incremental-release ages have been determined for 10 hornblende and 22 muscovite concentrates prepared from various units of the infrastructural and supra-structural allochthons. Hornblende from Farkka amphibolite is characterized by internally discordant spectra, reflecting wide-spread contamination with extraneous argon. ³⁶Ar/⁴⁰Ar versus ³⁹Ar/⁴⁰Ar isotopic correlations are well defined for several concentrates, and the resultant ages suggest that post-metamorphic cooling occurred following distinct Late Proterozoic tectonothermal events at ca. 700-720 Ma and ca. 640-650 Ma. Muscovite within Farkka lithologies initially cooled through argon closure temperatures between ca. 570 and 595 Ma. Muscovite records similar post-metamorphic cooling ages in the other infrastructural allochthons and in the Goua suprastructural allochthon. These muscovite ages are interpreted to reflect cooling associated with widespread post-orogenic uplift and are consistent with deposition of upper Proterozoic-lower Paleozoic flysch in the proximal foreland (Tichilt el Beïda Group).

The Tichilt el Beïda Group and correlative parautochthonous sequences (Djnba Group) were penetratively deformed prior to the Early Ordovician. ⁴⁰Ar/³⁹Ar incremental-release analyses of 6 very fine-grained white mica concentrates from mylonitic quartzite along thrust faults within the parautochthon indicate that this deformation occurred prior to ca. 540-550 Ma. There is no clear thermal record of this orogenic activity in either infrastructural or suprastructural allochthons, suggesting that it must have occurred at relatively high structural levels and was probably largely of intracontinental character.

All foreland units were affected by post-Emsian folding. Effects of this tectonic activity are widespread throughout the parautochthon and western metamorphic sequences. These effects include emplacement of the suprastructural allochthons into their present structural positions at ca. 300-325 Ma with an associated partial rejuvenation of muscovite argon systems proximal to basal thrust faults. Local reactivation of older thrust faults within the parautochthon and infrastructural allochthons is also indicated by variable 300-325 Ma rejuvenation of muscovite argon systems within some fault zones. Muscovite within westernmost exposures of the infra-structural allochthons was totally rejuvenated during the early Carboniferous and records 300-m.y. plateau ages.

The early tectonic history recorded within the central Mauritanide orogen appears to have been related to restricted rifting of a western crustal block from the West African craton and formation of a limited, intervening oceanic domain. Subsequent closure resulted in compression and imbrication of continental crust with resultant development of an ensialic volcanic arc (calc-alkaline sequences within the infrastructural allochthons) together with widespread metamorphism and deformation between ca. 700 and 650 Ma. Later orogenesis at ca. 550 Ma could have reflected distal effects of collisional events which dominate the Rokelide orogen in Sierra Leon and Guinea. Late Paleozoic tectonotherma activity within the central Mauritanides is interpreted to have developed as a result of a renewed translation between the western continental block and the West African craton during final collision of Laurentia arc Gondwana.

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