科研之友
微信
新浪微博
Facebook
分享链接
摘要
The 1.49 Ga Norra Karr 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 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 Karr rocks have been deformed and partially metamorphosed during the Sveconorwegian-Grenvillian orogeny, and are now preserved in a westward-dipping synform. Magmatic and metamorphic processes at the Norra Karr 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 degrees C) and silica activity (0.6-0.3). 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 eudialyte-group 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 degrees C and an a(SiO2) range of 0.25-0.4. 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 REE-bearing 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 Karr 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.
- 出版日期2017-2
