Two interconnected stages in the formation of the Tamazeght magmatic complex (Moroccan central High Atlas) determined by apatite, zircon and titanite U-Pb geochronology

Abstract

The intraplate magmatic complex of Tamazeght (or Tamazert), located in the central High Atlas of Morocco, formed during the Eocene in response to N-S oriented compressive shear movements associated with the convergence between Africa and Europe. The ages of emplacement of certain rocks forming this complex have been determined so far using K-Ar radiometric dating on biotite (from carbonatites and monzonites) and K-feldspar (from nephelinite dykes), as well as Rb-Sr geochronology on whole rock (nepheline monzodiorite). Furthermore, U-Pb geochronology has so far only been applied to eudialytes in the syenites and zircon in the monzonite and late-stage vein. However, uncertainties remain regarding the timing of emplacement of basic cumulates, syenites and lamprophyre dykes. In addition, the ages reported in the literature do not allow discussion of the continuous or multistage formation of the Tamazeght complex. In this paper, we use U-Pb geochronology on zircon, apatite and titanite crystals to clarify the sequence of events that formed the Tamazeght complex. Considering previously published and our new data, two interconnected magmatic stages have been identified. The first stage (ca. 47-44 Ma) corresponds to the plutonic rocks of the jacupirangite-ijolite ultrabasic cumulate sequence, whose age was previously undocumented. The second stage (ca. 44-35 Ma) includes more differentiated plutonic units (ca. 44-41 Ma), as defined by two U-Pb ages obtained from syenite zircons that agree with previous ages. A lamprophyre dyke located south of the complex gave a U-Pb age of 42.2 ± 1.9 Ma on titanite and can be attributed to the vein phase (ca. 43-35 Ma) of this second magmatic stage. This phase involves the intrusion and/or extrusion of several carbonatite, lamprophyre, and phonolite dykes. The Tamazeght magmatic activity is characterized by its short duration, from the early Lutetian to the early Priabonian (ca. 10 My), in relation to the regional geodynamic processes of the first Atlasic phase during the Alpine orogeny.