U-Pb Zircon geochronological and petrologic constraints on the post-collisional Variscan volcanism of the Tiddas-Souk Es-Sebt des Aït Ikko (Tsesdai) basin (Western Meseta, Morocco)

Abstract

The NE-SW Tsesdai basin (110 km to the SE of Rabat) is the third largest Late Palaeozoic continental trough in the northern Central Moroccan Meseta. It is <20 km long and ~2-3 km wide, comprising mixed volcano-sedimentary reddish-purple Permian rocks that rest on Visean deep marine siliciclastic sediments, overlain by the Triassic and Cenozoic formations. The lower and upper boundaries correspond to angular unconformities. The Permian age of these formations was obtained with the help of plant remains and vertebrate ichnofossils (e.g., Voigt et al., 2011, etc.). The occurrence of volcanics in this basin was first reported by Termier (1936) who considered them (as well as those in Khenifra) as forming the “Central Moroccan volcanic axis” stretching from Khenifra to Tiddas and displaying felsic and mafic volcanics emplaced in the Neogene/Quaternary. This “axial-model” has been accepted by several authors (e.g. Michard, 1976) until the 1980s. Later contributions largely improved the geological knowledge of this basin (e.g., Gonord et al.,1980; Cailleux et al., 1983; Zouine, 1986; etc.). The conclusions of these authors are:

(i) the volcanic complex is Permian s.l. (post-Carboniferous and pre-Triassic, presumably Stephanian/Permian) a percept based on (a) the intrusion of rhyolites into the Visean deep marine siliciclastics of the basement (shark-fin rhyolitic dome of Ari el Mahsar extrusive and its satellite dykes); (b) the angular unconformity between the red detritic sediments and the Visean siliciclastics folded during the late Westphalian A; (c) the presence of reworked rhyolitic/andesitic pebbles in the red sediments of the Tsesdai basin and their absence in the detrital sediments of the Westphalian C/ D preserved nearby in the Sidi Kacem graben; (d) the angular unconformity between the Triassic and the red detritals; and (d) strike-slip fault that postdates the basin sedimentary/volcanic formations and predates the Triassic/Cenozoic. (ii) the distinction of 3 major volcanic pulses: the first one (mainly andesitic) predates the Permian series; witnesses of this pulse can still be observed in Tiddas, but most material is reworked in the conglomerates; the second pulse is synsedimentary, displays different petrographic characteristics, and it is remarkable for its andesitic lava flows interbedded with the detritals. The third pulse of intrusive rhyolitic dykes precedes the Ari el Mahsar rhyolitic dome emplacement and the dacitic flows. (iii) identification of three intra-permian tectonic events. The close relationship between volcanism and fracturation is obvious: dykes and effusive centers are located in relation with N40-N70 faults, mostly strike-slip. We present new insights on the petrology and geochronology of the Tsesdai Permian volcanism affecting the northwestern Gondwanaland margin. The studied volcanic has typical orogenic calc-alkaline signatures. For the last volcanic pulse we determined U-Pb ages of zircon by Sensitive High-Resolution Ion Microprobe (SHRIMP). The shark fin rhyolitic extrusive dome of Ari el Mahsar (sample ARM15/GPS: 33°39'51.48"N, 6° 4'2.06"W) yielded two predominant ages (603 ± 25 Ma and 286.4 ± 4.7 Ma): the young age is more concordant, being interpreted as the crystallization age of rhyolites, while