Field, geochemistry and Sr-Nd isotopes of the Pan-African granitoids from the Tifnoute Valley (Sirwa, Anti-Atlas, Morocco): a post-collisional event in a metacratonic setting

Abstract

International audienceIn the Tifnoute Valley, three plutonic units have been defined: the Askaoun intrusion, the Imourkhssen intrusion and the Ougougane group of small intrusions. They are made of quartz diorite, granodiorite and granite and all contain abundant mafic microgranular enclaves (MME). The Askaoun granodiorite and the Imourkhssen granite have been dated by LA-ICP-MS on zircon at 558 ± 2 Ma and 561 ± 3 Ma, respectively. These granitic intrusions are subcontemporaneous to the widespread volcanic and volcano-detrital rocks from the Ouarzazate Group (580-545 Ma), marking the post-collisional transtensional period in the Anti-Atlas and which evolved towards alkaline and tholeiitic lavas in minor volume at the beginning of the Cambrian anorogenic intraplate extensional period. Geochemically, the Tifnoute Valley granitoids belong to an alkali-calcic series (high-K calc-alkaline) with typical Nb-Ta negative anomalies and no alkaline affinities. Granitoids and enclaves display positive εNd-560Ma (+0.8 to +3.5) with young Nd-TDM between 800 and 1200 Ma and relatively low 87Sr/86Sr initial ratios (Sri: 0.7034 and 0.7065). These values indicate a mainly juvenile source corresponding to a Pan-African metasomatized lithospheric mantle partly mixed with an old crustal component from the underlying West African Craton (WAC). Preservation in the Anti-Atlas of pre-Pan-African lithologies (c. 2.03 Ga basement, c. 800 Ma passive margin greenschist-facies sediments, allochthonous 750-700 Ma ophiolitic sequences) indicates that the Anti-Atlas lithosphere has not been thickened and was never an active margin during the Neoproterozoic. After a transpressive period, the late Ediacaran period (580-545 Ma) is marked by movement on near vertical transtensional faults, synchronous with the emplacement of the huge Ouarzazate Group and the Tifnoute Valley granitoids. We propose here a geodynamical model where the Tifnoute Valley granitoids as well as the Ouarzazate Group were generated during the post-collisional metacratonic evolution of the northern boundary of the West African craton. The convergence with the peri-Gondwanan active margin produced brittle fracturing of the cratonic boundary without thickening, allowing rising of magmas. The Tifnoute Valley granitoids display a metasomatized lithospheric mantle source mixed with a minor ancient (2 Ga) continental crust component from the underlying WAC

Mantle Dynamics of the Central Atlantic Magmatic Province (CAMP): Constraints from Platinum Group, Gold and Lithophile Elements in Flood Basalts of Morocco

“…10a show the calculated changes in a magma (average Upper unit magma) that has assimilated 10% of (i) Eburnean granitoid (average from Ennih & Liegeois, 2008), (ii) Pan-African granitoid (average from Toummite et al, 2013), and (iii) average continental crust of Albarède (2003). While the average continental crust represents bulk assimilation, the Eburnean granitoids are interpreted as crustal melts (Ennih & Liegeois, 2008) and the Pan-African granitoids as a mix of mantle and crustal melts (Toummite et al, 2013). We therefore consider them representative for the likely range of crustal components that may have been assimilated by the CAMP magmas.…”

“…11a showing magmas assimilating up to 30% crust. The Pan-African (Toummite et al, 2013) and Eburnean (Ennih & Liegeois, 2008) granitoids plot very similarly in this diagram and we have used the average composition of the Pan-African rocks in the AFC calculations. The AFC curves using N-MORB and E-MORB as starting magmas clearly trend obliquely not only to the mantle trend as argued by Pearce (2008), but also to the trend of the four lava units in Morocco.…”

Geochemical Evidence for Large‐Scale Drainage Reorganization in Northwest Africa During the Cretaceous

“…To characterize the sedimentary sources of the Cretaceous sediments deposited in the eastern Central Atlantic Ocean, we use a compilation of 794 published Sr and Nd isotope values measured on sedimentary and magmatic rocks of the WAC (supporting information Table S1, Ajaji et al, ; Ali et al, ; Allègre et al, ; Asiedu et al, ; Azzouni‐Sekkal et al, ; Bea et al, ; Blanc et al, ; Boher et al, ; Deckart et al, ; Dia et al, ; D'Lemos et al, ; Dupuy et al, ; El Baghdadi et al, ; Ennih & Liégeois, ; Errami et al, ; Essaifi et al, ; Fullgraf et al, ; Gasquet et al, ; Jakubowicz et al, ; Key et al, ; Kouamelan et al, ; Le Goff et al, ; Meyer et al, ; Montero et al, ; Othman et al, ; Pawlig et al, ; Peucat et al, ; Potrel et al, ; Roddaz et al, ; Samson et al, ; Schaltegger et al, ; Schoepfer et al, ; Soumaila et al, ; Mohammed et al, 2015; Tahiri et al, ; Taylor et al, ; Thomas et al, ; Toummite et al, ; Verati et al, ). This database allows a comprehensive picture of differences in the isotopic composition of the main geological units present in the WAC (Figure c).…”

Revised stratigraphic framework for the lower Anti-Atlas Supergroup based on U–Pb geochronology of magmatic and detrital zircons (Zenaga and Bou Azzer-El Graara inliers, Anti-Atlas Belt, Morocco)

“…These basins comprise mostly Mesoproterozoic-Neoproterozoic to Devonian-Carboniferous sediments. The WAC is surrounded by Pan-African belts, including the Trans-Saharan and Anti-Atlas belts (Ennih and Liégeois, 2008;Liégeois et al, 2013;Toummite et al, 2013;Brahimi et al, 2018).…”