November 2024 LIP of the Month

Evidence of a large igneous province at ca. 347–330 Ma along the northern Gondwana margin linked to the assembly of Pangea: Insights from U–Pb zircon geochronology and geochemistry of the South-Western Branch of the Variscan Belt (Morocco)

Oussama Moutbir¹, El Mostafa Aarab¹, Nasrrddine Youbi^1,2,3, Abdelhak Ait Lahna¹, Colombo Celso Gaeta Tassinari⁴, João Mata², Ross N. Mitchell^5,6, Andreas Gärtner⁷, Alvar Soesoo^8,9, Mohamed Khalil Bensalah^1,2, Abderrahmane Soulaimani¹, Moulay Ahmed Boumehdi^1,2, and Ulf Linnemann¹⁰

¹Department of Geology., Faculty of Sciences-Semlalia, Cadi Ayyad University, Prince Moulay Abdellah Boulevard, P.O. Box 2390, Marrakech, Morocco. Email: youbi@uca.ac.ma

²Instituto Dom Luiz, Faculdade de Ciências, Universidade de Lisboa, 1749-016 Lisboa, Portugal. Email: jmata@ciencias.ulisboa.pt

³Faculty of Geology and Geography, Tomsk State University, 36 Lenin Ave, Tomsk 634050, Russia

⁴Centro de Pesquisas Geocronológicas (CPGeo), Instituto de Geociências (IG), Universidade de São Paulo- USP, Caixa Postal 11348, CEP 05422-970, São Paulo (SP), Brazil. Email: ccgtassi@usp.br

⁵State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China; Email: ross.mitchell@mail.iggcas.ac.cn

⁶College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing 100049, China

⁷Museum für Mineralogie und Geologie, Sektion Mineralogie / Isotope Forensics, Senckenberg Naturhistorische Sammlungen Dresden, Königsbrücker Landstraße 159, 01109 Dresden, Germany; Email: Andreas.Gaertner@senckenberg.de

⁸University of Tartu, Institute of Ecology and Earth Sciences, Department of Geology, Estonia

⁹Geological Survey of Estonia. Email: alvar.soesoo@egt.ee

¹⁰Museum für Mineralogie und Geologie, Sektion Geochronologie, GeoPlasma Lab, Senckenberg Naturhistorische Sammlungen Dresden, Königsbrücker Landstraße 159, 01109 Dresden, Germany; Email: Ulf.Linnemann@senckenberg.de

Extracted and modified from: Moutbir, O., Aarab, E., Youbi, N., Ait Lahna, A., Tassinari, C.C.G, Mata, J., Mitchell, R.N., Gärtner, A., Soesso, A, Bensalah, M.K., Soulaimani, A., Boumehdi M.A., Linnemann, U. (2024)Evidence of a large igneous province at ca. 347–330 Ma along the northern Gondwana margin linked to the assembly of Pangea: Insights from U–Pb zircon geochronology and geochemistry of the South-Western Branch of the Variscan Belt (Morocco). Earth-Science Reviews, 258, 104905. For full details, see this publication.

1. Introduction

The Moroccan Meseta, divided into the Western and Eastern domains by the Middle Meseta Fault Zone (MMFZ), contains significant Variscan magmatic activity, particularly in the Western Meseta's Jebilet, Rehamna, and Moroccan Central Massifs. The Jebilet Massif is notable for its well-preserved igneous rocks, making it ideal for studying Variscan magmatism. It records multiple magmatic episodes from Carboniferous until Permian (ca. 347–255 Ma) (e.g., Mrini et al., 1992; Youbi et al., 2001; Essaifi et al., 2003; Ait lahna et al., 2018; Delchini et al., 2018; Chopin et al., 2023; Michard et al., 2023).Early Carboniferous rocks here include basaltic lavas, mafic intrusions, and silicic rocks, though their tectonic setting and petrogenesis remain unclear (e.g., Mrini et al., 1992; Youbi et al., 2001; Delchini et al., 2018).

This study examines early Carboniferous igneous rocks from the Central Jebilet Massif using zircon U–Pb ages and geochemical data to explore their origins and geodynamic significance. Magmatism dated to ca. 347–330 Ma, linked to a mantle plume beneath Avalonia, likely represents remnants of a large igneous province (LIP) which we name here the North Gondwana–Avalonia LIP. The plume’s activity influenced the Western and Eastern Meseta, generating varied magmatism due to lithospheric differences and contributing to terrane export along the Gondwana margin and Pangea assembly (Baes et al., 2021; Murphy et al., 2024).

2. Geological background

The South-Western Branch of the Variscan Belt in Morocco includes the Souttoufide Belt (Moroccan Sahara), the Anti-Atlas, and the Moroccan MesetaDomain, with the Western and Eastern Meseta subdomains. The Moroccan Meseta, part of Gondwana's northwestern margin, features a nearly complete Paleozoic sedimentary sequence intruded by Variscan magmatic bodies, with significant folding, metamorphism, and pre-, syn- to late orogenic magmatism (e.g., Michard et al., 2010; 2023; Leprêtre et al., 2024).

The Jebilet Massif, in the southern Western Meseta, spans about 170 km east–west and 7–40 km north–south. It contains folded Paleozoic rocks overlying Pan-African basement including Paleoproterozoic Eburnean components. This observation, initially proposed by Huvelin (1977) and Bernardin et al. (1988), is further supported by U–Pb zircon dating of crustal xenoliths revealing ages of approximately 2000 Ma, 700 Ma, 615–540 Ma, and 328–280 Ma (Dostal et al., 2005). The massif is divided into Western, Central, and Eastern domains, separated by shear zones such as the Marrakech Shear Zone (Lagarde and Choukroune, 1982;Essaifi et al., 2001) and the Western Meseta Shear Zone (Piqué et al., 1980;Le Corre and Bouloton, 1987; Mayol, 1987).

The Central Jebilet represents the western segment of a Devonian–early Carboniferous basin (Fig. 1) (Huvelin, 1977; Bordonaro et al., 1979; Delchini et al., 2018). Its connection to the eastern Jebilet is evidenced by the late Visean–Namurian Teksim Formation, which overlies the Sarhlef series, equivalent to the Kharrouba Formation (Bordonaro et al., 1979; Gaillet, 1980; Gaillet and Bordonaro, 1981). The Sarhlef series, dated to the late Visean–Namurian, consists of shales with bioclastic limestone lenses containing middle-upper Visean microfossils (Hollard et al., 1977). Palynostratigraphic data suggest a late Visean (Asbian) age (Moreno et al., 2008; Playford et al., 2008), though older ages (Frasnian–Famennian) based on metamorphosed conodonts have been proposed (Lazreq et al., 2021; 2023). These limestones likely represent klippe or olistoliths rather than interbedded layers (Huvelin, 1977).

The main magmatic bodies in the Jebilet Massif include: (i) Bimodal tholeiitic magmatism, represented by felsic and mafic–ultramafic intrusions (e.g., Huvelin, 1977; Aarab, 1995; Essaifi et al., 2014). This magmatism has been linked to either pre-orogenic extensional or synorogenic transpressional settings (Lagarde and Choukroune, 1982; Essaifi et al., 2003). (ii) Calc-alkaline granitoids, such as the Oulad Ouaslam, Bamega–Tabouchent, and Bramram plutons, are composed of biotite granodiorite, leucogranite, and tonalite (Rosé, 1987; Mrini et al., 1992). (iii) Late dykes, consisting of calc-alkaline to alkaline lamprophyres, microdiorites, and microgranites. Xenoliths in these dykes reveal Eburnean and Pan-African basement beneath the Meseta (Dostal et al., 2005).

3. U-Pb geochronology of bimodal magmatism from the Central Jebilet Massif.

Four zircon U–Pb ages were obtained using a sensitive high-resolution ion microprobe (SHRIMP) from key outcrops in the Central Jebilet (Fig. 1). These results are presented on the concordia diagram in Fig. 2. Concordant ages were determined for the samples, yielding 343.6 ± 2.9 Ma for a gabbro dyke and 345.6 ± 2.8 Ma for a trondhjemite dyke at Koudiat Kettara, 335.8 ± 3.3 Ma at Koudiat Arkhil, and 347.1 ± 2.9 Ma for the Gour Essafra Massifs. These results demonstrate that the bimodal magmatic activity in the Central Jebilet was emplaced between 347.1 ± 2.9 Ma and 335.8 ± 3.3 Ma, predating the shortening episodes.

4. Geochemistry and magma source characteristics

Geochemistry data indicate that the early Carboniferous magmatism of the Central Jebilet Massif is tholeiitic (Fig. 3a), intraplate with consistent geochemical indicators pointing to the role of multiple melting events and fractional crystallization on the variability of chemical compositions. In addition, the source is inferred to be the asthenospheric mantle, with some magmas experiencing lithospheric mantle contamination (Fig. 3b).On the diagram Zr/Y vs. Nb/Y Condie (2005), the rocks plot above the ΔNb line in the plume-derived basalt field as defined by Fitton et al. (1997, 2003) (Fig. 3c).The samples cluster near the primitive mantle (PM) composition within the oceanic plateau basalt field, with some plotting near the shallow depleted mantle (DM) component. This distribution aligns with major, trace element, and isotopic evidence supporting a plume-origin hypothesis for the Central Jebilet magmas. Further, using the LIP printing diagram (Pearce et al., 2021) the observed crustal signatures in the studied samples can be attributed to lithospheric mantle that had been metasomatized in an earlier subduction event (Fig. 4a, b). Supported by this geochemical evidence, we propose that the early Carboniferous magmatism of the Jebilet Massif originated from a shallow mantle metasomatized by an earlier subduction event and subsequently underwent magmatic differentiation within magma chambers.

6. Evidence for a ca. 347–330 Ma LIP on along northern Gondwana margin

6.1. Early Carboniferous Moroccan Meseta “fragment” LIP

Magmatism in Morocco during the early Carboniferous (ca. 347–330 Ma) was extensive, with volcanic and plutonic activity occurring in two main zones (e.g., Michard et al., 2008; 2010; 2023; Chopin et al., 2023; Leprêtre et al., 2024): the Western Meseta, where Tournaisian–Visean magmatism (ca. 347–330 Ma) predates major tectonic events, and the Eastern Meseta, where Visean–lower Namurian volcanism (ca. 347–323 Ma) is intracontinental and late- to post-orogenic. In the Variscan Jebilet Massif, southwest of the Meseta, there is evidence for a large early Carboniferous igneous province, including basaltic lavas, mafic sills, dykes, and gabbro intrusions, together with subordinate layered ultramafic intrusions and silicic intrusive and volcanic rocks. Similar magmatic rocks are found in other basins of the Western Meseta (Rehamna and Moroccan Central Massif), with zircon U–Pb ages obtained from the literature and data presented here (Fig. 5) indicating that magmatic activity occurred between ca. 347 and ca. 330 Ma, coeval with the Eastern Meseta volcanism and plutonism in northeastern Morocco.

6.2. The 347–330 Ma LIP in other blocks

The early Carboniferous magmatic activity in Morocco coincides remarkably with magmatism observed in other crustal blocks (Fig. 6), including: (i) the Iberian Pyrite Belt (IPB), where late Devonian–Carboniferous bimodal volcanism, driven by mantle plumes and crustal processes, migrated eastward from ca. 371 to 346 Ma during intracontinental extension (e. g., Paslawski et al., 2021; Inverno et al., 2015); (ii) the ca. 353–346 Ma St. Jean du Doigt bimodal magmatism, formed during extensional tectonics in a pull-apart basin, with magma emplaced in multiple pulses over 7 Myr (e.g., Barboni et al., 2013); (iii) the Southern Vosges magmatism (345–340 Ma), emplaced during Visean transtensional tectonics, with S-type granitoids linked to orogenic processes (e.g., Schaltegger, 2000); and (iv) the Maritimes Basin multiphase magmatism (ca. 380–330 Ma), involving mantle plume activity and crustal extension, producing tholeiitic basalts, granites, and alkalic basalts along fault systems (e.g., Dessureau et al., 2000;Murphy and Keppie, 2005) (Fig. 6 ).

7. Geodynamic model for early Carboniferous magmatism of Morocco

Carboniferous and Permian magmatism in Morocco was extensive, with both plutonic and volcanic rocks (e.g., Michard et al., 2008; 2010; 2023; Chopin et al., 2023; Leprêtre et al., 2024). Plutonic rocks are represented by meta-aluminous and peraluminous calc-alkaline granitoids, with rare alkaline varieties. Volcanic rocks include meta-aluminous high-K calc-alkaline, peraluminous, MORB, transitional, and alkali rocks (e.g., Kharbouch et al., 1985; Chalot-Prat et al., 1991; 1995; Aarab, 1995;Roddaz et al., 2002; Essaifi et al., 2014;). The mantle source of much of this magmatism is debated, with two main hypotheses: subduction (Michard et al., 2010; 2023) and mantle plume (Youbi, 1998). The mantle plume that developed under Avalonia during Devonian–Carboniferous times (Murphy et al., 1999; Murphy and Keppie, 2005; Dessureau et la., 2000; Jutras and Dostal, 2019; Dostal et al., 2024) is the most plausible geodynamic scenario to explain the huge volume of mafic intrusions preserved in the present-day Central Atlantic region. The large-scale sublithospheric plume material, flowing northeastward from the Avalonia plume site, channeled towards the large thin-spot in the Western Meseta before reaching the Eastern Meseta (Fig. 7). This process leads to widespread tholeiitic/alkaline magmatism in the thinner lithosphere of the Western Meseta and calc-alkaline-type magmatism in the thickened lithosphere of the Eastern Meseta (Fig. 7). The mantle plume likely has been most active between ca. 390–330 Ma (Maritimes or Magdalen Event), ca. 370–338 Ma (Iberia Event), ca. 347–330 Ma (Meseta Event), and the multipulsed ca. 300 Ma, 290–275 Ma, and 250 Ma European North West African Magmatic Province (EUNWA or EUNWAMP), which were the times when most of the Variscan mafic rocks were produced in these areas (Doblas et al., 1998; Dostal et al., 2019). These igneous complexes have been attributed to the arrival of a mantle plume that preferentially emanates from the edges of deep-mantle upwellings (LLSVPs) (e.g., Steinberger and Torsvik, 2010). This process is thought to have played a key role in the amalgamation of the supercontinent. The early Carboniferous North Gondwana–Avalonia LIP and the late Carboniferous–Permian European North-West African Magmatic Province (EUNWA or EUNWAMP) are examples LIPs during supercontinent assembly (i.e., assembly of Pangea).

References