Many alkaline magmatic complexes are composed of nepheline- and alkaline-syenites, ijolites to melteigites as well as melilite-bearing rocks. Carbonatites can be present as well as ultramafic rocks. Furthermore, fenitization in the marginal parts is a characteristic of alkaline complexes. Chemically converging trends of fenites and alkaline silicate rocks in addition to a spatial proximity and local interference of these lihologic units lead to the conclusion of a metasomatic origin of syenites, nepheline-syenites and ijolitic rocks. Magmatic features given by textures and contact relations may be explained by rheomorphism or remobilization of metasomatic alkaline rocks.
Despite of the fact that numerous petrologists working on alkaline complexes in various provinces (Monteregian Hills, Kola-Peninsula, Alnö, Zimbabwe, East-Africa) have briefly mentioned this item, it has never been subject to a systematic study since von Eckermann (1948). And with the exception of a few unquestionable cases, such as alkaline glasses in xenoliths of fenites in alkaline volcanics of the carbonatite volcano Oldoinyo Lengai in Tanzania (Morogan and Martin, 1985), rheomorphism has never been accepted as an explanation of the genesis of larger amounts of alkaline silicate rocks.
In this poster the result of field work as well as geochemical and isotopic data from the alkaline complexes of Iivaara (Finland), Kovdor (Russia, Kola-Peninsula) and from Lueshe (Zaire) are presented. Different initial isotopic ratios of various magmatic rocks in any single complex disprove direct magmatic relationships, such as crystallization sequences from one magma in any of these complexes. Elevated initial 87Sr/86Sr ratios (> 0.7060) and lower initial 143Nd/144Nd ratios (< 0.5120) in syenitic to nepheline-syenitic and ijolitic magmatic rocks point to an interaction with crustal material of the wall rocks. Furthermore isotopic disequilibrium between mineral fractions as well as whole rock portions within single samples of alkaline silicate magmatic rocks which cannot be explained by any kind of assimilation, in combination with fractional crystallization or not, lead to the assumption that many silicate alkaline rocks in the complexes studied here are highly affected by metasomatism. Melting (rheomorphism) of metasomatic rocks, such as fenites, may lead to the formation of alkaline silicate rocks which show major and trace element compositions as well as isotopic signatures typical of high grade fenites but have textures typical of magmatic rocks.
In addition, thermal calculations are presented in order to set some constraints to the possibility of rheomorphism taking place in metasomatic aureoles of the alkaline complexes investigated here.
Morogan, V. & Martin, R.F., Am. Miner. 70, 1114-1126, (1985).
von Eckermann, H., Sveriges. Geol. Unders. Ca36, 176pp. (1948).