fenitizing

Fenitizing is a metasomatic alteration process in which alkaline fluids derived from an intrusive alkaline igneous body modify the surrounding country rocks, producing fenite. The altered rocks are referred to as fenitized rocks or fenites, and they typically exhibit a distinctive alkali-rich mineral assemblage compared with the unaltered host.

Geologic setting and features

Fenitization commonly occurs around carbonatite complexes and other alkaline intrusions, such as nephelinite or syenitic bodies. Fluids derived from the intrusion percolate into adjacent rock, driving chemical replacement and mineralogical change. The resulting fenite commonly shows enrichment in alkalis (potassium and sodium) and movement of trace elements, with the original plagioclase often altered or replaced by alkali feldspars. Mineral assemblages characteristic of fenite frequently include albite or microcline, nepheline and other feldspathoids, and alkaline minerals such as arfvedsonite or aegirine. Fluorine-bearing minerals and minor quartz, calcite, and other accessory phases may occur depending on the local chemistry. The texture of fenite reflects metasomatic overprinting, often preserving evidence of the original rock structure while displaying pervasive chemical alteration.

Formation mechanisms and effects

Fenitization is driven by alkali-rich, silica-undersaturated fluids associated with the intrusive event. Temperatures are typically low to moderate relative to the crystallizing magma, allowing selective mineral replacement and new mineral growth without wholesale melting. The process can create a zoned alteration halo around the intrusion, varying in intensity with distance from the source.

Significance

Fenitization helps illuminate the interaction between intrusions and country rocks, affects local geochemistry, and bears on mineral exploration in regions with alkalic complexes. It is distinct from high-temperature contact metamorphism in its emphasis on metasomatic exchange and alkali mineralogy.