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

The Rogaland anorthosite province (S. Norway) contains numerous Fe-Ti oxide deposits, including the second most important ilmenite deposit in the worl d, the Tellnes deposit. The largest deposits are located in the (A) over ci rcle na-Sira anorthosite massif. Others occur in the H (a) over circle and- Helleren anorthosite massif, particularly along the deformed contact with t he Egersund-Ogna massif, where they were previously considered formed by me tasomatic processes. All deposits are now regarded as magmatic. The structu re, mineralogy and geochemistry of II selected Fe-Ti deposits (Tellnes, Sto rgangen, Bl (a) over circle fjell, Laksedal, Kydlandsvatn, Kagnuden, Rodemy r, Hestnes. Eigeroy, Sv (a) over circle nes, 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 jotunit ic parental magma has built up cumulates in the Bjerkreim-Sokndal layered i ntrusion and liquids in the Tellnes dyke and other jotunitic intrusions. Il menite is a liquidus mineral immediately after plagioclase in the sequence of crystallization of these jotunites, its interstitial character in the ro cks resulting from subsolidus recrystallization. Ilmenite can thus accumula te early in the evolution of jotunitic magmas. This feature. together with high contents in Cr, V, Mg and Ni. links the Jerneld, Bl (a) over circle fj ell and Sv (a) over circle nes 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, bu t normally crystallizes after orthopyroxene and before clinopyroxene and ap atite. The early appearance of magnetite is a characteristic feature of typ e 2 deposits (Tellnes, Storgangen, Kydlandsvatn, Rerdemyr I) and suggests c onditions 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 the se minerals in silicate-absent meter-sized masses. Type 3 deposits (Rodemyr II, Kagnuden, Hestnes and Eigeroy) 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 equival ent cumulates of the Bjerkreim-Sokndal intrusion. Although immiscibility as the mechanism of enrichment leading to silicate-absent oxide-apatite veins , as in Hestnes and Eigeroy, cannot be precluded, there is no direct eviden ce in the veins, nor has any structural or geochemical evidence of immiscib ility ever been found in jotunite dykes and Fe-Ti-P-rich rocks. Further inv estigations on the influence of subsolidus exchange of elements between the two oxides are needed to improve the use of trace elements as differentiat ion indexes.