Druck- und Temperaturbedingungen für sekundäre Schmelzbildungen im Rahmen alkalisilikatischer und karbonatitischer Intrusionen

摘要

Nepheline syenites and/or ijolithes are frequent constituents of carbonatite and alkaline silicate complexes. Their genesis were since then controversially discussed. Compositional and morphological characteristics of fenite aureoles lead to the assumption that nephelin syenites or ijolithes may be formed from the palingenesis of high-grade fenites. Thus high-grade fenites can be remeltet by progression of fluidflow and steady heating (v. Eckermann, 1948; Currie, 1970; Sindern, 1998; Körner, 2003).A high-grade nepheline bearing alkali-feldspar-pyroxene fenite reacted together with selective additive components (NaF+H2O, NaCl+ H2O, H2O, NaCl und NaF) in autoclaves. In this way the process of fenitisation was continued in the laboratory.Thirty-eight hydrothermal experiments were conducted in four experimental series at 1 kbar and different temperatures (750°C-350°C). The first three was started to determine the composition and quantity of the additive, which causes, by temperature lowering the highest possible melt rate. After the calculation of quantitative proportions the fourth experimental series was started. All samples reacted with 5 % NaF+H2O by lowering of temperature. The reaction duration attempts 118 h. The formation of melt respectively after a rapid quenching, of glass can be observed in all the treated samples.Chemical investigations were carried out by electron microprobe analysis. In comparison to the alkali feldspar and clinopyroxene of the starting material the feldspar and clinopyroxene of the treated samples don’t show any chemical change. The influence of the additive is limited to the lowering of melting point and lead to the participation of feldspar and clinopyroxene into the melt. The melting of clinopyroxene leads by 750°C and 650°C in some samples to the formation of hematite.In contrast to alkali feldspar and clinopyroxene the chemical composition of nepheline shows changes in comparison to the nepheline crystals in the starting material. The additives lead, apart from the lowering of the melting point of the nepheline also to an increase of Na and Si content in the crystals. Moreover nepheline reacts with NaCl and forms at 750°C sodalite. The participation of alkali feldspar and nepheline into the melt lead in combination with NaF to the formation of cryolite. Chemical investigations have been also carried out by the accessories ilmenite and pyrochlore. Ilmenite, the only titanium-rich mineral, doesn’t show changes in his chemical composition. The addition of NaF+H2O and/or NaF makes pyrochlore instable. At high temperatures the zoned crystals show increased contents of Na in the cores. Also rounded crystal boundaries can be observed. This is an evidence of pyrochlore melting.New formed glasses can be observed in all treated samples at the rim and in the core. Some samples melted nearly completely, others show melt inclusions in nepheline crystals and bizoned glass formations. The different glasses vary in their chemical composition. This is an indication of the not reached equilibrium and show the different influence of added components on the samples. Furthermore the properties of the samples play an important role for the reaction with the additive, because cracks and cleavages in the sample and in the individual minerals can function as ways for the additive.Up to 625C° a participation of the three main phases to the melting process can be observed. The Fe, Ti and Ca values in the melt originate only from the melting of clinopyroxene. Si and Na values can be derived from the three main phases and the added additive. The values of K and Al come from alkali feldspar and nepheline. Below this temperature clinopyroxene doesn’t take part to the melt formation. The added additive continues, in comparison to the clinopyroxene, the lowering of the melting temperature of feldspar and nepheline. Therefore the binary system alkalifeldspar -nepheline has a lower eutectic temperature (350°C) as the ternary system alkalifeldspar –nepheline-clinopyroxen (625°C).The chemical composition of glass from samples which were treated in a temperature range from 750° to 625°C is Ä13-17Ne29-25Alk58-54 and is similar to the ternary minimum of the nephelin-syenites. The results show that the melting temperature of high-grade fenite was lowered by the addition of additive components, especially 5 % NaF+H2O, from 950°C ± 25°C (Evdokimov, 1982) to 625°C ± 25°C. The experimentally determined temperatures represent a temperature minimum, which the Carbonatite (carbonatitic melt) should possess in order to function as heat source for the palingenetic process. Therefore Carbonatites like the Kalkfeld carbonatite complex, which crystallise within these temperatures, can be a cause for palingene melting of fenite rocks. The combination of heat source, properties of the samples and fluid composition causes a metasomatic palinegenetic process, which has as consequence the formation of metasomatites.