The origin of nelsonite and high-Zr ferrodiorite associated with Proterozoic anorthosite

A petrological and geochemical (major and trace elements, Sr-Nd isotopes) study of two nelsonite occurrences (the Kydlandsvatn and Hestnes deposits) in the Rogaland anorthosite province (Southern Norway) reveals that both deposits are cumulates but were generated from strikingly different magmas and processes. The Kydlandsvatn deposit was formed from a primitive jotunite similar to the Bjerkreim-Sokndal parental magma by crystal accumulation. By contrast, the Hestnes cumulate, with high REE and Zr contents, negative Eu anomaly, Cr-rich magnetite, Nb-rich ilmenite and high Nb/Ta and Zr/Hf whole-rock ratios appears to result from the crystallization of an immiscible Fe-rich and Si-poor melt under highly oxidized conditions. Other jotunites/ferrodiorites with high Zr-contents are typically associated with anorthosites in the Rogaland, Laramie and Adirondacks anorthosite complexes. In Rogaland, they also have an identical Sr-Nd isotope signature as the Hestnes nelsonite. These high-Zr jotunites cannot have been produced through extreme differentiation of ordinary jotunite magmas. Their formation from direct melting of a crustal source or assimilation of a nelsonite cumulate is highly unlikely. Occurrence of a high-Zr jotunite at the margin of a quartz mangerite dyke is evidence that immiscibility takes place, though furtively, along the jotunite liquid line of descent. It is suggested here that high-Zr jotunites in dyke rocks or parental to high Zr-nelsonites resulted from fractional crystallization of a Fe-rich immiscible melt formed at higher pressures in the polybaric evolution of anorthosites, conditions that are not yet explored experimentally.