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Jean-Paul Liégeois
Earth Sciences
Geodynamics and mineral resources
Geodynamics and mineral resources
Publication details
Toummite, A., Liégeois, J.P., Gasquet, D., Bruguier, O., Beraaouz, E.H. & Ikenne, M. 2013. ‘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’. Mineralogy and Petrology 107: 739-763. Wien : Springer Verlag. DOI: DOI 10.1007/s00710-012-0245-3. I.F. 1.278.
Article in a scientific Journal / Article in a Journal
In 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-ICPMS
on zircon at 558±2Ma and 561±3Ma, 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 1200Ma and relatively low87Sr/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–
545Ma) 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 theWest 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.