Fe-Ti deposits in Rogaland anorthosites (South Norway): geochemical characteristics and problems of interpretation

Abstract

The Rogaland anorthosite province (S. Norway) contains numerous Fe-Ti oxide deposits, including the second most important ilmenite deposit in the world, the Tellnes deposit. The largest deposits are located in the Åna-Sira anorthosite massif. Others occur in the Håland-Helleren anorthosite massif, particularly along the deformed contact with the Egersund-Ogna massif, where they were previously considered formed by metasomatic processes. All deposits are now regarded as magmatic. The structure, mineralogy and geochemistry of 11 selected Fe-Ti deposits (Tellnes, Storgangen, Blåfjell, Laksedal, Kydlandsvatn, Kagnuden, Rødemyr, Hestnes, Eigerøy, Svånes, and Jerneld) are discussed in light of recent models proposed for the origin of Rogaland anorthosites and related rocks. Massif-type anorthosites result from the diapiric uprise of a plagioclase crystal mush which crystallized along a large P–T interval. Except for Tellnes, which is related to a post-deformation dyke, the Fe-Ti deposits in anorthosite massifs have been deformed by this movement during and after their crystallization. The differentiation process of the jotunitic parental magma has built up cumulates in the Bjerkreim-Sokndal layered intrusion and liquids in the Tellnes dyke and other jotunitic intrusions. Ilmenite is a liquidus mineral immediately after plagioclase in the sequence of crystallization of these jotunites, its interstitial character in the rocks resulting from subsolidus recrystallization. Ilmenite can thus accumulate early in the evolution of jotunitic magmas. This feature, together with high contents in Cr, V, Mg and Ni, links the Jerneld, Blåfjell and Svånes deposits (type␣1) to the early evolution of a jotunitic magma. In the Bjerkreim-Sokndal intrusion, magnetite can appear with ilmenite at the very beginning of the sequence of crystallization, but normally crystallizes after orthopyroxene and before clinopyroxene and apatite. The early appearance of magnetite is a characteristic feature of type 2 deposits (Tellnes, Storgangen, Kydlandsvatn, Rødemyr I) and suggests conditions similar to the early magnetite cumulates in the Bjerkreim-Sokndal intrusion. Evidence of layering further favours gravity-controlled sorting processes to concentrate the oxides. Large-scale subsolidus segregation of the oxides due to high-temperature deformation can further concentrate these minerals in silicate-absent meter-sized masses. Type 3 deposits (Rødemyr II, Kagnuden, Hestnes and Eigerøy) could be derived from the more evolved stages of differentiation, as indicated by high REE in apatite, high Ti and Zn in magnetite and relatively low Cr, V, Mg, Ni contents in both oxides. The Cr content in both oxide minerals is however higher than in the equivalent cumulates of the Bjerkreim-Sokndal intrusion. Although immiscibility as the mechanism of enrichment leading to silicate-absent oxide-apatite veins, as in Hestnes and Eigerøy, cannot be precluded, there is no direct evidence in the veins, nor has any structural or geochemical evidence of immiscibility ever been found in jotunite dykes and Fe-Ti-P-rich rocks. Further investigations on the influence of subsolidus exchange of elements between the two oxides are needed to improve the use of trace elements as differentiation indexes.