Chilled margin and marginal reversal formation in the Koillismaa Deep Intrusion: implications for parental magma compositions in the 2.44 Ga Tornio–Näränkävaara Belt

Abstract Chilled margin compositions are commonly used to estimate the parental magmas of mafic–ultramafic layered intrusions. Chilling along intrusion margins is associated with supercooling, which typically results in the development of a marginal reversal—a zone characterised by chemical trends that become more primitive from the margin towards the centre, accompanied by a reversed crystallisation sequence. However, marginal reversals may also form through alternative geological processes, complicating the interpretation of chilled margins as true proxies for parental magma compositions. In this study, we use Fe–Mg equilibria and thermobarometric calibrations between the chilled margin and adjacent cumulus phases of the Koillismaa Deep Intrusion to demonstrate that both the chilled margin and the associated marginal reversal formed through magma supercooling. Our age determination confirms that the intrusion belongs to the 2.44 Ga Tornio–Näränkävaara Belt, which is of considerable economic interest, with a continuous history of PGE–Cu–Ni, V–Ti–Fe, and Cr exploration and mining from the 20th century to the present. Our results indicate that the magma which formed the Koillismaa Deep Intrusion resembles the siliceous high-Mg basalt composition proposed for the 2.44 Ga diabase dykes of Fennoscandian Shield. Validating chilled margin compositions is crucial for the Tornio–Näränkävaara Belt and other 2.51–2.43 Ga Fennoscandian mafic–ultramafic layered intrusions due to their significant economic potential. However, the literature-sourced data used for comparison have not been adequately validated, and multiple sources of error may affect their reliability.