Fractionation of geochemical twins (Zr/Hf, Nb/Ta and Y/Ho) and HREE-enrichment during magmatic and metamorphic processes in peralkaline nepheline syenites from Norra Kärr (Sweden)

Abstract

The 1.49 Ga Norra Kärr complex (Sweden) consists of deformed and metamorphosed peralkaline nepheline syenites that contain eudialyte-group minerals as the major host of high field strength elements and rare earth elements. Earlier studies revealed the presence of distinct generations of eudialyte-group minerals and clinopyroxene of magmatic and metamorphic origin. Here, we present the trace element characteristics of the different generations of rock-forming minerals.The trace element chemistry of eudialyte-group minerals mimic whole-rock compositions and display well-developed negative Eu-anomalies and strong Sr- and Ba-depletions in chondrite-normalized diagrams. They imply that the Norra Kärr rocks developed by intense fractional crystallization from an alkali basaltic parental magma. Our data further illustrate that eudialyte-group minerals do not strongly fractionate the geochemical twins Zr/Hf, Y/Ho and Nb/Ta during magmatic processes. The transition from a magmatic to metamorphic environment is marked in EGM by a drop in Y/Ho and Nb/Ta. The exceptional enrichment of heavy rare earth elements in late metamorphic eudialyte may result from residual enrichment: light rare earth elements (and Nb) were preferentially mobilized to form local secondary light rare earth-rich rinkite-group mineral assemblages, while the heavy REE stayed behind and formed heavy REE-rich eudialyte.Magmatic clinopyroxene shows a clear preference for Hf over Zr, while on a subunit scale a negative correlation of the total REE in cpx and in EGM is noticed. Both indicate that early cpx fractionation would not only increase Zr/Hf of the evolving melt, but also affect the REE deportment in major HFSE-hosts.