The Alkaline Complex of Iivaara (Finland): Shallow Intrusive Setting and Fluid Separation

This study presents the petrogenesis of intrusive rocks of the Iivaara alkaline complex (Finland) and associated enrichment of elements, which are mostly carried by titanite and apatite. Comprehensive mineralogical and textural observations were made and mineral chemical analyses were performed on apatite, titanite, clinopyroxene, and feldspathoid group minerals using EPMA and LA-ICP-MS.

Intrusive rocks exposed in the Iivaara alkaline complex are mainly composed of ijolite group rocks, nepheline syenite, and pyroxenites. They generally show low mineralogical variability but large textural variety. Several textures indicate magmatic formation conditions, including special textures of mingling, schlieren, and Unidirectional Solidification Texture (UST). Some others show fluid-derived formation conditions like replacement texture of primary minerals, breccia as well as veins.

The marked textural heterogeneity present at the margin of the magmatic intrusion and the innermost zone of the fenite aureole, reveals that the current erosion level of the Iivaara exposes the roof zone of an alkaline silicate intrusion. It is characterized by fast cooling, steep temperature gradients, association of small magma batches crystallizing at different conditions, and by expulsion of fluids.

The high abundance of secondary feldspathoid group minerals, apatite, and titanite attests to the significant enrichment of halogens (F, Cl), CO2, SO4, P, and Ti in these rocks crystallized from melts or fluids, which in part have agpaitic character.

Mineral chemical data reflect two main stages of fluid expulsion, a syn-magmatic Ca-dominated fluid causing marked fenitization of the wall rocks and a syn- to post- magmatic fluid forming feldspathoid minerals.