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
Journal of Petrology
The 1Á49 Ga Norra K€ arr complex in Southern Sweden contains rocks characterized by a very high ratio of (Na þ K)/Al ! 1Á2 and a complex and highly unusual mineralogy, including rock-forming catapleiite and eudialyte-group minerals, as well as minor rinkite-and britholite-group minerals. In contrast to other well-studied examples of agpaitic rocks, the Norra K€ arr rocks have been deformed and partially metamorphosed during the Sveconorwegian-Grenvillian orogeny, and are now preserved in a
... preserved in a westward-dipping synform. Magmatic and metamorphic processes at the Norra K€ arr 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 allows the reconstruction of magmatic and subsequent metamorphic conditions. The magmatic mineral assemblage crystallized from a subsolvus syenite at continuously decreasing temperatures (700-450 C) and silica activity (0Á6-0Á3). Owing to initially relatively low peralkalinity and reducing conditions, Zr was first incorporated into 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 rare earth element (REE)-poor eudialytegroup minerals happened as soon as sufficient Cl, REE and high field strength elements were enriched in the residual melt. Metamorphic conditions during the Sveconorwegian-Grenvillian orogeny are constrained to T between 400 and 550 C and an a SiO2 range of 0Á25-0Á4. As a result of 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 REEbearing minerals. During the whole history of the complex, a SiO2 remains very similar, indicating very little interaction with the surrounding granitic rocks. Regardless of the intense deformation owing to folding of the Norra K€ arr body during the Sveconorwegian-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 owing to fractional crystallization.