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)

Abstract

The migration and composition of magmatism over time can provide important insights into the tectonic evolution of an orogen like the Variscan Belt. To identify Large Igneous Provinces (LIPs), key criteria include large magmatic volume, intraplate-origin volcanic geochemistry, and significant plumbing systems. Based on such criteria, we present evidence of ca. 347–330 Ma LIP “fragments” in the South-Western Branch of the Variscan Belt (Morocco), exemplified by the Variscan Central Jebilet Massif. The interpretations are based on four new zircon U–Pb ages obtained by sensitive high-resolution ion microprobe (SHRIMP), a geochemical database of Carboniferous mafic sills, dykes, and gabbroic intrusions together, with subordinate layered ultramafic intrusions, silicic intrusive and volcanic rocks of Central Jebilet Massif, combined with previously published and unpublished data including Sr–Nd isotope analyses. Geochemistry data indicate that the early Carboniferous magmatism of the Jebilet Massif is plume-related. Furthermore, primary magmas of the mafic rocks were generated in an intraplate setting and derived by partial melting of complex sources involving asthenosphere, lithospheric mantle, and subducting slab components (dead subduction slabs), and were modified by crustal contamination during ascent. Magmatic rocks in the same stratigraphic position also occur in other Carboniferous basins including Western Meseta (Rehamna and Moroccan Central Massif). The newly obtained and compiled zircon U–Pb ages from Western Meseta rocks, encompassing an area of ∼400,000 km², indicate that magmatism occurred between ca. 347–330 Ma, coeval with volcanic activity in the Eastern Meseta in northeastern Morocco. The similar emplacement ages, in combination with the tectonic reconstruction of northwestern Gondwana at ca. 330 Ma, suggest that the igneous subprovinces of the Jebilet, Rehamna, and Moroccan Central Massif in Western Meseta, along with Tazekka, Debdou, and Mekkam in Eastern Meseta, the igneous rocks of the Maritimes (Magdalen) Basin, the St. Jean du Doigt bimodal layered intrusion (Brittany, France), and other equivalents such as the Iberian Pyrite Belt and the Southern Vosges magmatism, may represent the eroded and/or deformed remnants of a Large Igneous Province (LIP), which we name here the North Gondwana–Avalonia (NGA) LIP. We argue that this newly identified LIP was formed by a mantle plume that may have played a role in the breakup along the northwestern margin of the precursor megacontinent Gondwana and the assembly of Pangea. The plume was likely centered under the thick lithosphere of Avalonia. The large-scale sublithospheric plume-flow channeling from the plume head led to the development of widespread tholeiitic/alkaline magmatism in the thinned lithosphere of Western Meseta, interpreted here as a large thin-spot domain, and calc-alkaline/alkaline magmatism in the thickened lithosphere of the Eastern Meseta. The mantle plume may have been most active during the periods of ca. 390–330 Ma (Maritimes 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 periods when most of the Variscan mafic rocks were produced in these areas.