Iron-Titanium Oxide Minerals in the Bjerkrem-Sogndal Massif, South-western Norway

Abstract

Ilmenite and magnetite are investigated from the point of view of their distribution, microtexture, and chemical composition (major and minor elements) in the Bjerkrem-Sogndal massif (Egersund area, South-Rogaland, SW. Norway). This massif is an igneous layered synkinematic lopolith made up of cumulates of the anorthosite-mangerite suite. The lower part of the massif presents a rhythmic structure.The microtextures of ilmenite result from simple exsolution of ilmenite-hematite solid solutions. Magnetite contains intergrowths of ilmenite formed by oxidation-exsolution of ulvospinel-magnetite solid-solutions.In the stratigraphic sequence, on a large scale, ilmenite appears first alone, and is then accompanied by magnetite; its hematite content decreases towards the top of the massif, while the titanium content of the magnetite increases. On the scale of the rhythms, similar trends but of lesser amplitude are also observed.Evidence of deuteric readjustment of the orthomagmatic composition of the two oxides is provided (1) by the observation of microtextures at the contact between grains (zoning of primary ilmenite and rim of secondary ilmenite) (2) by the existence of differences in chemical composition between isolated grains and grains in contact, and (3) by the determination of the equilibrium temperature by means of the Buddington and Lindsley geothermometer.Reconstitution of the T-fo, orthomagmatic conditions in two particular levels of the massif shows that the reducing character of the magma increases during differentiation. The sudden changes in the oxide assemblage at the base of the rhythms reflect a sudden increase in the/o, of the magma. These increases, as shown by variation in Cr, Ni, and Co, are due to recurrences of the basic character of the magma.The variations of the minor elements Mn, V, Ga, and Zn are interpreted in terms of the influence of the deuteric readjustment. It follows that the ratios Mn/Fe a+ , Ga/Fe 34 ", and Zn/ Fe 1+ increase and that the ratio V/Fe 3+ decreases in the magma in the course of differentiation. The distribution of Mn between ilmenite and magnetite is discussed.Intermittent supplies of undifferentiated magma are proposed as the geological mechanism controlling the chemical recurrences associated with the rhythmic structure.

Trace elements and anorthosite genesis

“…The transition elements in Fe-Ti oxides are interesting tracers for magmatic differentiation as was demonstrated in Bjerkrem-Sogndal [29]. The ranges of concentrations are reported in Table 5.…”

“…The increase in the Eu 2+ proportion is simultaneous with a decrease in the fugacity of oxygen, which has been demonstrated in the course of crystallization of the massif [29]. The Eu 3+ /Eu ratio in the magma can be calculated using the method suggested by Philpotts [54] and Jensen [55] : it varies from 80% at the beginning to 60% at the stage 3, ratios which, according to Jensen [55], fall well within terrestrial values.…”

“…Rocks are characterized by macro-and micro-textures typical of igneous layered cumulates (lamination, small-scale rhythmic layering, banding, etc.). Taking into account recurrences due to successive influxes of fresh magma, the evolution of the mineralogy throughout the whole sequence of rocks [28][29][30] is as follows: in the anorthosites, the leuconorites and the lower part of the norites, an unzoned plagioclase (An 50 to An 43 ) is associated with an orthopyroxene and oxides; in the norites upwards in the sequence, clinopyroxene and apatite appear as additional cumulate minerals and plagioclase is more albitic (An 45 to An 40 ). Still higher in the norites, potassium feldspar appears -first as antiperthite, then progressively as a cumulate mineral -and the rocks grade into jotunitic (hypersthene monzodioritic) cumulates which constitute a rapid transition towards the mangeritic phase: mangerites (hypersthene, mesoperthite-monzonite) and, on top of the massif, quartz mangerites.…”

Magma influx and mixing in the Bjerkreim-Sokndal layered intrusion, South Norway: evidence from the boundary between two megacyclic units at Storeknuten

“…Thin exsolution lamellae of ilmenite can be observed in some grains. A rim of spinelliferous ilmenite is always developed at the contact between the cumulus magnetite and the cumulus ilmenite, indicating subsolidus readjustment of chemical composition as described by Duchesne (1972). The second type of magnetite occurs in the Ca-poor pyroxene-Fe-Ti oxide intergrowths and is usually devoid of spinel or ilmenite lamellae.…”

“…Michot (1960Michot ( , 1965 divided the northwestern part of the BKSK (the Bjerkreim lobe) into five major layered units, which he called rhythmic units. These rhythmic units have been interpreted by Duchesne (1972) as reflecting separate influxes of magma into the chamber. Lateral variations related to the boundary between two of these rhythmic units (III and IV) have recently been described by Nielsen and Wilson (1991).…”