Syenitic Magmatism and Crust–Mantle Interaction during Late Pan-African Tectonism in Southwestern Nigeria

Abstract

Syeinite intrusions within the Igarra Schist Belt of southwestern Nigeria constitute an important yet understudied component of late Pan-African magmatism in the Nigerian Basement Complex. This study integrates petrographic observations with whole-rock major, trace, and rare earth element (REE) geochemistry to constrain the petrogenesis and tectonic significance of these syenites. Petrographic data indicate crystallization from an alkali-rich, moderately evolved magma dominated by alkali feldspar, with subordinate plagioclase and mafic phases, consistent with metaluminous, high-K compositions. Major-element geochemistry supports emplacement at shallow to mid-crustal levels during the waning stages of regional deformation. Trace-element characteristics and tectonic discrimination diagrams indicate a transitional syn- to post-collisional magmatic affinity. Chondrite-normalized REE patterns are strongly fractionated, showing pronounced light REE enrichment relative to heavy REE and coherent inter-sample trends. The subdued heavy REE segment and absence of a significant Eu anomaly suggest partial melting of an enriched lithospheric mantle source at relatively shallow depths, with limited residual garnet and minimal pre-emplacement feldspar fractionation. These features point to magma generation during lithospheric relaxation and crust–mantle interaction following continental collision. The results indicate that the Igarra syenites record late-orogenic syenitic magmatism linked to Pan-African tectonic reorganization in southwestern Nigeria. This study highlights the role of enriched mantle inputs in generating syenitic magmas during orogenic collapse and contributes to a broader understanding of crust–mantle processes associated with the terminal stages of the Pan-African orogeny.