Magma chamber processes in the Tellnes ilmenite deposit (Rogaland Anorthosite Province, SW Norway) and the formation of Fe–Ti ores in massif-type anorthosites

Abstract

The origin of igneous Fe–Ti oxide ores associated with massif-type anorthosites is investigated through a detailed study of the world-class Tellnes ilmenite deposit, part of the late-Proterozoic (930–920 Ma) AMC series of the Rogaland Anorthosite Province (SW Norway). More than 100 samples from drill cores reveal significant petrographical and compositional variations within the ore body. Four zones are defined, based on variations in modal proportions and cumulus mineral assemblages: the Lower and Upper Central Zones and the Lower and Upper Marginal Zones. Plagioclase and whole-rock compositions discriminate the zones and display patterns interpreted as a result of mixing of either plagioclase–ilmenite or plagioclase–ilmenite–orthopyroxene–olivine cumulates with a melt of ferrodioritic (jotunitic) composition with a content decreasing from 80 to 20% from the margins to the central part of the ore body. Phase diagrams for a jotunitic parental magma reproduce the crystallization sequence at 5 kb. The orthopyroxene–olivine liquidus boundary is a peritectic in the Bjerkreim-Sokndal layered intrusion and a cotectic in Tellnes and this explains the differences in the sequence of crystallization of the two intrusions. The high concentration of ilmenite, well above cotectic proportions, resulted from gravity-sorting in the Tellnes ore body, which represents the lower part of a larger magma chamber. Uniform Sr isotope ratios do not support magma mixing. The cryptic layering of the ore body precludes injection as a crystal mush but favours in situ crystallization from an evolving magma in a sill-like magma chamber. The present trough-shape and mineral orientations result from deformation during gravity-induced subsidence and by up-doming of the anorthosite. Fractional crystallization of a TiO 2-rich magma with ilmenite as an early liquidus mineral and plagioclase buoyancy are the principal mechanisms responsible for the formation of Fe–Ti deposits in Proterozoic massif-type anorthosites.