Mixing, mingling and metasomatism: Magmatic and hydro-carbothermal processes in an Angolan subvolcanic alkaline silicate complex

Abstract

The Angola Alkaline Province hosts 35 alkaline igneous complexes that offer valuable insights into rift-related alkaline‑carbonatite magmatism and their potential for strategic critical metal resources such as rare earth elements (REE) and niobium (Nb). This study presents the first detailed investigation of the Nejoio Alkaline Complex, including field observations, petrography, whole rock and mineral chemistry, and U-Pb age data. Nejoio forms a subvolcanic ring complex composed mainly of pyroxene-biotite ijolite and feldspathoidal syenite. Its circular inner zone features heterogeneous ijolite-syenite hybrids rich in pyroxene, biotite, amphibole, apatite and titanite, formed from melanocratic ijolite melts mixed with meso- to leucocratic syenitic crystal mushes. These hybrids exhibit large phenocrysts of alkali feldspar and aegirine-augite with diopside-hedenbergite cores, set in a fine-grained matrix dominated by aegirine, biotite, titanite, apatite, and minor amphibole and fluorite. Interstitial melt pockets in the ijolite contain zircon and Na-Zr-Ti-Nb silicates such as wöhlerite, hiortdahlite, rosenbuschite, and catapleiite, recording a late-magmatic transition from miaskitic to agpaitic mineral assemblages. The inner zone is surrounded by an outer ring of feldspathoidal syenites with abundant alkali feldspar, nepheline, sodalite and cancrinite. These contain accessory magnetite, zircon, and pyrochlore and host late-stage veins with needles of rosenbuschite. Hydro- and carbothermal fluids overprinted both units, replacing primary Mg-Ca rich aegirine-augite with aegirine, and secondary assemblages with dolomite, dawsonite, calcite, cancrinite, pyrite, magnetite and REE-Sr‑carbonates, as well as analcime, natrolite, fluorite, cryolite, paragonite-pyrophyllite and catapleiite. U-Pb dating of primary titanite and apatite indicates that Nejoio was emplaced during the late Cretaceous (107–101 Ma), well after the Atlantic Ocean's opening, reflecting continued magmatism in a failed rift arm following Gondwana's breakup. Nejoio serves as a representative analogue for understanding alkaline magmatism across the Angolan province, emphasizing its economic and geotectonic significance.