Magmatic vs. metasomatic ijolite petrogenesis – constraints from the type locality Iivaara, Finland

Complex field relationships and association to alkaline ultramafic rocks, textural diversity and locally marked isotopic heterogeneity of ijolites lead to contrasting petrogenetic concepts for these rocks. They may either be formed by magmatic differentiation from nephelinitic magmas or by assimilation of silicate wall rocks in a carbonatite melt formed after metasomatic reaction during its’ ascent from mantle to crustal levels. We show new petrographic and mineral chemical results from the ijolite type locality, Iivaara. It is the aim of this presentation to set constraints to ijolite formation in the Iivaara alkaline complex and to discuss applicability of contrasting petrogenetic concepts. Pegmatitic growth, comb layering or brecciation indicate ijolite crystallization from a low viscosity and volatile rich melt. Small scale textural heterogeneity reveals locally highly variable crystallization conditions characterized by steep temperature gradients and sudden pressure drop. Irregular clinopyroxene zonation points to repeated disturbance of magmatic crystallization at depth prior to emplacement at the actual erosion level. Veins and matrices of breccia are dominated by clinopyroxene next to titanite and apatite attesting to the high Ca-concentration of a fluid that is violently expelled from the magma. The transitional zone between wall rocks and ijolite, too, is dominated by pyroxenitic compositions. Lack of a clearly defined contact between ijolite and wall rock is in line with the observation of migmatitic textures in high grade fenites, which probably obtained plastic behavior with increasing degree of fenitization. Such fenites with syenitic composition or partial melts of them may have been assimilated by ijolite.