Defense of the Doctoral Thesis of Muhammad Ouabid

On July 7 the defense of the doctoral thesis of Muhammad Ouabid took place at the Faculty of Sciences of the University Moulay Ismail with the title:

Precambrian–Lower Paleozoic Boundary: Insights from Fieldwork, Petrography, Geochronology and Geochemistry. Example of Goaïda inlier (Moroccan Central Meseta)

The jury was formed by:

Mr. A. MOKHTARI, PES, Faculté des Sciences, Meknés (President)

Mr. J.L. BODINIER, Université Mohammed VI Polytechnique, Benguerir

Mr. A. MAHMOUDI, Faculté des Sciences, Meknés

Mme. A. WAFIK, Faculté des Sciences Semlalia, Marrakech

Mr. C. J. GARRIDO, IACT-CSIC, University of Granada (Research Director)

Mr. A. NTARMOUCHANT, Faculté des Sciences Dhar El Mahraz, Fés

Mr. O. SADDIQI, Faculté des Sciences Ain Chock, Casablanca

Mr. H. OUALI, Faculté des Sciences, Meknés (Director of Thesis)

 

Abstract

This Ph.D thesis has two parts. The first is a detailed study of granitic plutons in the Goaïda region (Moroccan Western Meseta); while, the second, it relates to the rhyolitic intrusions crosscut the basement and the sedimentary series of the Cambrian platform of the same region.

Field observations and U–Pb LA–ICP–MS zircon ages of granitoid outcrops in the Goaïda area, provide firm evidence of Neoproterozoic silicic magmatism in the basement of the Moroccan Variscan Meseta belt. This magmatic activity produced three distinct plutons —granodiorites, pink and white granites— associated with volcanic tuffs, which form a Neoproterozoic basement strongly reworked in the Hercynian orogeny and intruded by gabbro and dolerite bodies. The Goaïda granodiorites are syeno–granodiorites, whereas the pink and white granites are granites sensu stricto. Concordant U–Pb zircon data yield a Lowermost Ediacaran mean age for white granites (625±9 Ma) and Upper Ediacaran mean ages for granodiorites (600±10 Ma) and pink granites (552±10 Ma). The old white granites and granodiorites have I-type, high-K and calc-alkaline affinities formed in subduction-related to post-collisional regimes. Whereas, the young pink granites have high-K calc-alkaline I-type to sub-alkaline/alkaline A-type signatures formed in the post-collisional environment. White granites record old high-K calc-alkaline magmatism that started in the Anti-Atlas belt at c. 620 Ma, coinciding with the end of the second period of the Pan-African orogenesis. On the other hand, the granodiorites and pink granites formed in subduction-related to post-collisional regimes during the final stage of the Pan-African orogenesis, namely the WACadomian orogeny. Sr–Nd–Pb isotope systematics report the Goaïda granitoids could be derived from the lithospheric mantle or a crust recently derived from the mantle or mixing of both, perhaps suggesting an active continental margin environment.

We combine new field, geochronological and geochemical data with results from the literature to propose a new lithostratigraphic reconstruction of the Precambrian to Early Cambrian times in the Moroccan Meseta front at the NW margin of the West African Craton. The presence of Precambrian outcrops —including Paleoproterozoic and Neoproterozoic rocks— in the Moroccan Variscan belt demonstrates the continuity of the Panafrican basement at least from the eastern Anti-Atlas to the Central Meseta domain. Along with the lack of Paleozoic ophiolitic suture and subduction-related metamorphic rocks, this continuity supports the hypothesis that the Moroccan Variscan Mountains were part of the NW margin of Gondwana in the Paleozoic.

The Goaïda rhyolitic intrusions have been ascribed first to the Hercynian orogenesis. Here, we provide new LA–ICP–MS U–Pb zircon dates—ranging from 513 to 480 Ma— of this felsic volcanism that unveils, for the first time in the Moroccan Meseta, the existence of Upper Cambrian–Early Ordovician volcanism. Inherited zircons in these volcanic rocks yield 1.2–1.0 Ga to 652–530 Ma dates, pointing to their provenance from Stenian–Tonian to Cryogenian–early Cambrian crustal sources beneath the exposed Paleozoic Moroccan Meseta. The affinity of the precursor magmatism is mainly peraluminous, high-K, calc-alkaline, in good agreement with their Sr–Nd–Pb isotopic data, and evidence of the inherited zircons indicate the presence of multicycle Precambrian crust derived from the Panafrican belt of NW margin of the Western African Craton. Because its Cambro–Ordovician age and calc-alkaline affinity, the Moroccan Meseta Goaïda felsic volcanism is akin to the well-known Cambrian–Ordovician volcanism in the Spanish Iberian and French Armorican Massifs. Therefore, Goaïda felsic volcanism constitutes a southernmost record of the Cambro–Ordovician rift along the NW Gondwana margin, providing evidence of a continuous Variscan belt from the Moroccan Meseta to Western Europe.

Keywords: Goaïda, Meseta, Anti-Atlas, West African craton, NW margin of Gondwana, Granitoids, Panafrican basement, rhyolitic intrusions, Cambro–Ordovician rift, Weste