Fenitization at the Alnö carbonatite complex, Sweden; distribution, mineralogy and genesis

Abstract

Re-mapping of the Alnö complex has radically reduced the area identified as fenite, in comparison with the classic work of Eckermann (1948). A marginal fenite zone, generally 500–600 m wide, is present around the complex, and the petrography and mineralogy of six selected key areas have been investigated in detail. Fenitization of the country rock migmatitic gneiss led to replacement of quartz, feldspars, biotite and chlorite by alkali pyroxene and amphibole, new generations of feldspars, calcite, titanite, fluorite and apatite. In some areas, however, a distinctive narrow band of fenitization, referred to as ‘contact fenite’, adjacent to large sövite dykes, contains mineral assemblages that include phlogopite, nepheline, melanite and wollastonite. Amphiboles in the fenites are richterite, katophorite, arfvedsonite and eckermannite. There is a very wide variation in the composition of pyroxenes which vary between diopside, aergirine-augite and aergirine. Although trends from aergirine to aegirine-augite and aegirine-augite to diopside have been defined, which are similar to those of other fenite localities, distinctive trends for the eastern part of the aureole have been identified that converge on aegirine, and approximate trends in some series of alkaline igneous rocks. Analyses of mica, garnet, wollastonite and feldspar are also presented and discussed. The mineralogical data are used to estimate the conditions of temperature, oxidation state, and activity of CO₂, H₂O and silica pertaining during the fenitization process. The fluids with which the fenites equilibrated were apparently different in composition in different parts of the aureole, and varied with time, implying more than one magmatic source. The various evolutionary trends identified in the pyroxenes and amphiboles, in particular, are explained in terms of two main fluid types, which emanated from ijolitic and carbonatitic magma sources.