Distinguishing magmatic and metamorphic processes in peralkaline rocks of the Norra Kärr complex (Southern Sweden) using textural and compositional variations of clinopyroxene and eudialyte-group minerals.

The 1.49 Ga old Norra Kärr complex in Southern Sweden contains rocks characterized by a very high ratio of (Na+K)/Al ≥ 1.2 and a complex and highly unusual mineralogy, including rock-forming catapleiite, eudialyte-group minerals as well as minor rinkite- and britholite-group minerals. In contrast to other well-studied examples of agpaitic rocks, the Norra Kärr rocks have been deformed and partially metamorphosed during the Sveconorvegian/Grenvillian orogeny, and are now preserved in a westward dipping synform.
Magmatic and metamorphic processes at the Norra Kärr complex are distinguished by combining rock fabrics of clinopyroxene and eudialyte-group minerals. Both mineral groups are stable over a large P-T range, which makes them excellent monitors of the geochemical evolution of such systems and enables the reconstruction of magmatic and subsequent metamorphic conditions.
The magmatic mineral assemblage crystallized from a subsolvus syenite at continuously decreasing temperatures (700 - 450ºC) and silica activity (0.6 - 0.3). Due to initially relatively low peralkalinity and reducing conditions, Zr was first incorporated in Zr-aegirine. Subsequent destabilization of the latter indicates increasing peralkalinity, oxygen fugacity and water activity, which resulted in the crystallization of early magmatic catapleiite. Crystallization of presumably later magmatic Mn- and REE-poor eudialyte-group minerals, happened as soon as sufficient Cl, REE and HFSE were enriched in the residual melt.
Metamorphic conditions during the Sveconorvegian/Grenvillian orogeny are constrained to T between 400 - 550ºC and aSiO2 range of 0.25 - 0.4. Due to deformation and interaction with fluids, post-magmatic Al-rich aegirine as well as post-magmatic eudialyte-group minerals enriched in REE, Y and Mn formed. Subsequently, the eudialyte-group minerals were destabilized and decomposed to post-magmatic catapleiite and secondary REE-bearing minerals. During the whole history of the complex, aSiO2 remains very similar, indicating very little interaction with the surrounding granitic rocks.
Regardless of the intense deformation due to folding of the Norra Kärr body during the Sveconorvegian/Grenvillian orogeny, indications for primary magmatic layering of the intrusion are retained on the deposit scale. In addition, the compositional changes of magmatic eudialyte-group minerals from the outer to the inner subunit indicate a primary geochemical evolution feature due to fractional crystallization.