Compositional and textural variations of multiple generations of silicate minerals in peralkaline rocks from the Kohistan Island Arc, North Pakistan: Implications for magma evolution and REE mineralization

• Chemistry of ferromagnesium minerals and titanites records magmas’ evolution. • Magmatic processes have a dominant role in REE mineralization in alkaline rocks. • Rare earth elements mineralization in the pegmatites evolved from syenite. Peralkaline rocks serve as a major repository of rare earth elements (REEs), high field strength elements (HFSE), and other critical elements. However, uncertainties persist regarding their geodynamic settings, magmatic-hydrothermal processes, and the factors governing the enrichment of these metals to ore-grade concentrations. This study investigates magma evolution and REE enrichment mechanisms in newly identified syenite and pegmatite within the Kohistan Batholith, focusing on compositional and textural variations across multiple generations of silicate minerals (pyroxene, biotite, and titanite). U–Pb isotopic dating of titanite from the syenite yields an emplacement age of 30.4 ± 1.5 Ma, which closely aligns with the ages of titanite from the pegmatite (31.7 ± 1.4 Ma), indicating a coeval origin. The rock suite comprises three generations of magmatic pyroxene (augite-aegirine), two generations of biotite (biotite–phlogopite), and titanite, distinguishable by their textures and compositions. Compositional trends in pyroxene and biotite from the syenite to the pegmatite show increasing F, SiO 2 , MgO, XMg, Na 2 O, and FeO t , alongside decreasing TiO 2 , Al 2 O 3 , CaO, and MnO, reflecting the evolution of a parent peralkaline magma toward more fractionated compositions. Titanite grains from the syenite to the pegmatite also exhibit consistent evolutionary patterns in major and trace element compositions, characterized by elevated REEs, Sr, Ga, Eu*, and Fe 2 O 3 /Al 2 O 3 ratios , while CaO, Ce*, and Zr decrease. Collectively, these chemical variations suggest that pegmatite magma evolved through prolonged fractional crystallization of the syenitic magma, resulting in increased volatile contents, higher alkalinity, and lower temperatures. These conditions facilitated the enrichment and concentration of REEs in the late-stage pegmatite. The presence of magmatic textures in multiple generations of silicate minerals, their primary chemical signatures, and the absence of significant alteration in both rocks suggest that magmatic processes, rather than late-stage hydrothermal activity, were primarily responsible for enhancing REE concentrations to ore-grade levels in this peralkaline system. Our findings emphasize that magmatic silicate minerals, including pyroxene, biotite, and titanite, are effective tracers for understanding magma evolution and REE enrichment mechanisms in peralkaline systems.