40Ar/39Ar mineral age record of polyphase tectonothermal evolution in the southern Mauritanide orogen, southeastern Senegal
40 Ar/ 39 Ar incremental-release mineral ages have been determined for several tectonostratigraphic units in southernmost portions of the Maiuritanide orogen in Senegal. Here, middle Proterozoic crystalline basement rocks of the West African craton are unconformably overlain by an upper Proterozoic (ca. 1100 Ma) continental sedimentary succession (Madina Kouta Group) transitional into volcanic and intercalated deep-marine sediments (Tennesse Group). Western portions of the volcano-sedimentary unit were variably deformed and metamorphosed prior to deposition of an unconformably overlying, upper Precambrian-Lower Cambrian sedimentary succession (Mali Group). Muscovite within cleavage domains in the low-grade metamorphic terrane records 40 Ar/ 39 Ar plateau dates of ca. 655–665 Ma and are interpreted to closely date the Pan African I phase of orogenic activity. The Mali Group itself was locally folded during a second phase of Pan African tectonothermal activity, which also affected a western, calc-alkaline, volcanic-plutonic sequence (Niokolo-Koba Group). Igneous hornblende from several granites within this sequence displays internally discordant 40 Ar/ 39 Ar release spectra which record variably developed plateaus of ca. 640–660 Ma. These are interpreted to approximately date initial postmagmatic cooling through ∼500 °C and are consistent with a previously published Rb-Sr whole-rock iso-chron crystallization age of 683 ± 18 Ma for one of the plutons. Associated calc-alkaline volcanic and volcaniclastic rocks are more penetratively deformed. Muscovite that is aligned along a regional cleavage records 40 Ar/ 39 Ar plateau ages of ca. 550–560 Ma, which are interpreted to closely date the Pan African II orogenesis. Widespread deposition of a molassic sequence (Youkoukoun Formation) occurred in the Middle Cambrian-Early Ordovician. This was disconformably overlain by Upper Ordovician-Upper Devonian sedimentary rocks (Bove basin succession), which are locally folded. More significant late Paleozoic tectonothermal activity is recorded in north-westernmost portions of the study area where the calc-alkaline, upper Proterozoic granites locally display penetrative mylonitic fabrics. Development of these fabrics is interpreted to be closely dated by the 40 Ar/ 39 Ar 270- to 280-Ma plateau ages recorded by dynamically recrystallized muscovite developed along fluxion foliations. These results allow demarcation of a late Paleozoic tectonic “front” and permit correlations with genetically related events in southeastern portions of the Appalachian orogen.