Constraints on the genesis and evolution of alkaline and carbonatitic rocks

Abstract

Alkaline complexes comprise a broad spectrum of mantle-derived silicate rocks which are of considerable scientific and economic interest. They reflect some of the most evolved igneous rocks on Earth, and may contain attractive levels of Rare Earth Elements (REE) and other High Field Strength Elements (HFSE) such as Zr, Hf, Nb, Ta. Some of these complexes are additionally associated with carbonatites. However, many questions regarding the crystallization conditions and processes of such peculiar rock types are still unanswered. This thesis focuses on two mineralogically contrasting alkaline complexes (and nearby localities), namely the Tarosero Volcanic Complex in northernTanzania, and the Kaiserstuhl Volcanic Complex in Southwest Germany. The Tarosero volcano is one of the very few complexes that comprises extrusive agpaitic rocks. Early crystallization of OH-bearing amphibole prevented the exsolution of aqueous fluids, thus enabling the enrichment of halogens, REE/HFSE, and the late-stage formation of eudialyte. The variable evolved rocks formed under low redox conditions (ΔFMQ≤0) and derived from a basaltic magma. Moreover, in the nearby Ogol basalts, multistage-magma mixing was demonstrated by various xenocryst phases. The rocks of the Kaiserstuhl Volcanic Complex formed under high redox conditions (ΔFMQ=+1 to +2) and derived from two parental magmas. Fractional crystallization of a basanitic magma produced the tephritic to phonolitic rock series, while fractional crystallization of an olivine melilititic magma with variable CO2 concentrations resulted in the formation of the nephelinitic to limburgitic, or the melilititic to haüynitic rock series and the carbonatites. The magmatic to hydrothermal evolution of the carbonatites can be deciphered by textural and compositional variations in pyrochlore. Furthermore, unusual high amounts of mica in one carbonatite body (Badberg) indicate a steady and continuous supply of elements due to the interaction with the wall rock. The decomposition of silicate xenoliths raised the silica activity of the carbonatite magma, which additionally enabled the crystallization of clinopyroxene, and enhanced the incorporation of REE in apatite. However, despite the high geochemical contrast between primitive dyke magmas and potash salt rocks (at Buggingen), there is no evidence for magmatic interaction processes, presumably due to the relatively dry orthomagmatic melt conditions and the fast cooling of the dyke magma. In addition, based on a worldwide compilation, combined with new results from seven carbonatite complexes (Kaiserstuhl, Sokli, Kovdor, Palabora, Oka, Magnet Cove, Jacupiranga), it was demonstrated that carbonatites and their associated silicate rocks indicate significantly higher magmatic redox conditions than alkaline rocks without association to carbonatites. This presumably is attributed to the prerequisite of a carbonate-bearing, and hence relatively oxidized mantle source for the genesis of carbonatites.