Trace elements in ilmenite, titanomagnetite and apatite unravel the petrogenesis of Fe-Ti-P-(+/-Zr) rich rocks and associated nelsonite from the Raftsund intrusion, Vesterålen-Lofoten AMCG suite, Northern Norway

Abstract

The formation of Fe-Ti-P- (+/− Zr)-rich rocks and nelsonite (Fe-Ti oxide-apatite +/− zircon-rich rocks mostly devoid of silicates) remains poorly understood. Accumulation of mafic minerals during the evolution of magma by fractional crystallization and silicate-liquid immiscibility are the two main processes considered. However, the latter process is difficult to identify in intrusive rocks as melt compositions are rarely preserved. In this study, we analyzed the trace element content of ilmenite, titanomagnetite, and apatite from the Raftsund intrusion, a 1800 Ma monzonitic to syenitic batholith belonging to the Lofoten-Vesterålen anorthosite-mangerite-charnockite-granite suite in order to test whether liquid-immiscibility was responsible for the formation of Fe-Ti-P-rich rocks. In addition, we extend the study to a new nelsonite occurrence. Detailed microtextural characterization of apatite in the monzonite shows evidence of metasomatic alteration in the form of thin rims of allanite, whereas apatite in the nelsonite contains small monazite inclusion along fractures. Ilmenite and titanomagnetite, not affected by later events, record enrichments in elements that should have behaved compatibly (Mg, Sc, Co, Cu, V) and incompatibly (Zr, Mn, Zn) in a fractionating system where zircon and sphalerite are absent from the mineral assemblage. This observation supports the liquid immiscibility hypothesis. Similar enrichments are observed in other parts of the system, including the nelsonite. However, the high field strength elements are affected by the stability and crystallization of abundant zircon and ilmenite, resulting in lower concentration of these element in the Fe-Ti oxides. The Fe-Ti oxides in the nelsonite record enrichments that are more pronounced than the Fe-Ti oxides in the surrounding rocks, which indicate that not only liquid immiscibility is responsible for the genesis of the nelsonite, but that the Fe-rich melt from which the Fe-Ti oxides crystallized was more enriched in trace elements than the Fe-Ti-P-rich rocks in the rest of the intrusion. Enrichment in transition metals (Co, V, Cu) is related to mingling with a less evolved monzodioritic magma, which most likely triggered the liquid immiscibility that formed the nelsonite. Fractionation and segregation of Fe-rich silicates, such as augite and fayalite, are necessary to explain the formation of the nelsonite.