Petrogenesis and tectonic implications of the Late Palaeozoic Koga alkaline–carbonatite complex, NW Pakistan: insights from U–Pb geochronology, geochemistry, and Sr–Nd–C–O isotopes

The Koga Alkaline Carbonatite Complex (KACC) in NW Pakistan contains quartz syenite, nepheline syenite, and carbonatite intrusions, yet its origin and tectonomagmatic evolution remain poorly constrained. This study integrates LA–ICP–MS titanite U–Pb geochronology, whole-rock and mineral geochemistry, and Sr–Nd–C–O isotopic data to investigate the origin, magmatic processes, and tectonic significance of the complex. The quartz syenite exhibits depletion in HFSE, Sr, and P, enrichment in LREEs, K, Rb, and Ba, a positive Eu anomaly and 87Sr/86Sr(i) and ƐNd(t) values of 0.7066–0.7082 and 0.73–3.21, respectively, suggesting derivation by partial melting of juvenile crust followed by K-feldspar accumulation. In contrast, the nepheline syenite is enriched in LILE, LREE, and HFSE, with negative P and Ti anomalies, indicating derivation from partial melting of pargasite- and garnet-bearing lithospheric mantle. The carbonatite exhibits enrichment in LREE, Sr, and P, and depletion in Rb, K, and HFSE relative to the nepheline syenite, consistent with silicate–carbonate liquid immiscibility. The nepheline syenite and carbonatite exhibit overlapping titanite U–Pb ages (294 ± 16 Ma and 295 ± 15Ma), initial 87Sr/86Sr ratios (0.7026–0.7043), and Nd(t) values (0.53–4.75), indicating derivation from a common, relatively depleted mantle source, with limited crustal assimilation. Similar C–O isotopic compositions of calcite and mineral compositional similarities support a genetic relationship between the carbonatite and the nepheline syenite through liquid immiscibility of an evolved carbonated silicate magma following extensive silicate crystallization. U–Pb titanite ages constrain the emplacement of the complex to the Late Carboniferous–Early Permian in a rift-related setting during the break-up of the Cimmerian ribbon continents from Gondwana.