Trace element concentrations in apatites from the Sept-Îles Intrusive Suite, Canada — Implications for the genesis of nelsonites

Abstract

Apatite-oxide-rich rocks (e.g. nelsonite) occur in a wide variety of rock types. Nelsonites occur in felsic and calco-alkaline granitic rocks but are more common in mafic systems, layered intrusions and massif type anorthosites. Models for their formation are currently highly debated. They could form by crystallization of apatite and oxide from a fractionated magma, and accumulation on the crystal pile. Alternatively, an Fe–Ti–P-rich liquid may segregate from the fractionated magma and the nelsonite could crystallize from this liquid. In order to provide further constraints on the formation processes of nelsonites in layered intrusions, we have studied the petrography, the major and trace element contents of apatite and whole rock in 14 apatite-oxide-rich rocks from the Sept-Îles Intrusive Suite (Canada). The mineralogical association and mineral compositions (REE, Cl content in apatite and MgO content in ilmenite) are consistent with the crystal accumulation model. Combining the in situ LA-ICP-MS analyses of the apatite and the major element content of the Fe–Ti oxides, the trace element modeled concentrations in the liquid from which the nelsonites could have formed are similar to the dykes at the margins of the Sept-Îles Intrusive Suite. The PELE software was used to estimate the composition of the magma derived from the dykes at the time of apatite crystallization. Our results support the crystal accumulation hypothesis for the formation of the nelsonites and associated rocks.