Geochemistry and geodynamic implications of the anitápolis and lages alkaline-carbonatite complexes, Santa Catarina state, Brazil

Abstract

The Early Cretaceous (~132 Ma) Anitapolis stock is made up of cumulate pyroxenites, minor nepheline syenites and ijolites and scarce dykes of nephelinitic composition. Carbonatites form a small body, but are also widespread as dykes and veins. The complex is entirely surrounded by Late Proterozoic granitic-gneissic rocks (570–630 Ma). The Late Cretaceous (70–77 Ma) Lages complex constitutes a proeminent dome structure underlined by a concentric arrangement of Permian to Triassic sediments. It consists mainly of peralkaline phonolites and nepheline syenites with subordinate ultramafic rocks (melilitite, olivine nephelinite, basanite and tephrite). Other important rock-types are kimberlitic breccias and carbonatites as a minor intrusion. Chemical data suggest that the Anitapolis carbonatites are essentially Ca-carbonatites and the Lages ones Fecarbonatites. Normalized IE and REE diagrams for both carbonatites display different patterns. d 18 O and d 13 C isotopes for the Anitapolis carbonatites plot near the primary carbonatite box, whereas the Lages rocks point to a large spreading of d 18 O values. Radiogenic isotope data indicate that the carbonatites and the associated silicate rocks present similar Sr i and Nd i values, spanning from time integrated depleted to enriched types. Nd-model ages for the Early (Anitapolis) and Late Cretaceous (Lages) carbonatites are 1.3 ± 0.1 Ga and 1.2 ± 0.2 Ga, respectively, but the Lages rocks seem to have been affected by two different Proterozoic mantle metasomatic events. Considerations on the geodynamic implications of the alkaline and alkaline-carbonatitic magmatism are made on the basis of models other than mantle plume.