Alkali Basaltic Melts Research Articles

Der petrogenetische Werdegang der Klinopyroxene in den tertiären Vulkaniten der Hocheifel

The course of crystallization of basalt forming clinopyroxenes in the olivine-basalt-hawaiite-mugearite-trachyte-and basanitoid (= basanite)-series from the Hocheifel area (Western Germany) has been presented by the author in two earlier papers (Hucken-holz, 1965 a, 1965 b). The present paper deals with the evolution of clinopyroxenes from the ankaramites in the same area. The ankaramites are richer in olivines and clinopyroxenes than the normal olivine basalts and may be a product of accumulation of olivines and clinopyroxenes in an olivine basalt magma. The first clinopyroxene formed in the ankaramites is a greenish chromian salite Ca47.1 Mg40.9 Fe12.0 which is preserved in the cores of the phenocrysts. Strongly zoned brown titansalite Ca45 Mg42Fe13 surrounds the chromian salite (HF 5) or appears in independent microphenocrysts (HF53). The groundmass clinopyroxene is titansalite Ca45Mg37Fe18 with a small optic axial angle. Phenocrysts of nickel-rich olivine Fe12–14 and chrome spinell were formed together with chromium clinopyroxene. The same minerals together with orthopyroxenes have been observed in the olivine basalts, hawaiites and basanitoids (= basanites). This paragenesis indicates high pressure and high temperature in the alkali basalts of the Hocheifel during an initial stage of magmatic evolution. With the decrease of pressure, mineral reaction occurred with the alkali basalt melt. The high pressure clinopyroxenes of the ankaramites and basanitoids (= basanites) were changed to a larger extent than the clinopyroxenes of the olivine basalts and hawaiites or were completely destroyed (HF 53) because they are in contact with the hot magma for a longer period of time. This favours the reaction between the solids and the melt, and the high pressure garnet and/or enstatite components in the clinopyroxenes are substituted by Ca-Tschermak’s and titanaugite molecules.

Volcanoes of central madagascar : A. Lacroix. ( Compt. Rend., cliv, 476)

The almost unknown Ambohimirahavavy ring complex in the Cenozoic alkaline province of northwestern Madagascar has recently attracted considerable interest because of the discovery of important rare-metal mineralization. The complex consists of arc-shaped bodies made up of silica-under- and -oversaturated syenites and extremely evolved peralkaline granitic dykes, as well as several mafic to felsic volcanic units, including basalt, phonolite and trachyte, all of which have an alkaline affinity. Uranium–lead zircon ages of 24.2 ± 0.6 Ma and 23.5 ± 6.8 Ma have been obtained for nepheline syenites and peralkaline granitic dykes, respectively, which, together with field data and ages of neighboring complexes, support emplacement controlled by regional lithospheric structures, rather than an evolving hot spot.Whole-rock major and trace-element and Sr–Nd isotopic data for the mafic suite suggest that the parental melt of this complex was generated by low degrees of melting of a metasomatized mantle source with residual amphibole. Fractional crystallization of this alkali basaltic melt likely produced the silica-undersaturated suite. We propose that the silica-oversaturated suite evolved from the undersaturated melt after contamination of the latter by crustal material. Further evolution to peralkaline compositions in both suites is attributed mainly to plagioclase and alkali feldspar segregation.Nepheline and feldspar compositions, as well as considerations of mineral equilibria among mafic silicates and Fe–Ti oxide minerals indicate crystallization temperatures of 1000 to 700 °C and an oxygen fugacity of 0.4 to 0.8 log units below the fayalite–magnetite–quartz (FMQ) buffer at 1 kbar for the silica-undersaturated melt, and temperatures of 860 to 570 °C and an oxygen fugacity of 1.5 to 3.8 log units below FMQ for the oversaturated syenitic melt. The undersaturated melt evolved towards a more peralkaline composition. Crystallization of arfvedsonite plus aegirine further reduced the melt the evolution of which ended with fluid exsolution. At late stages of crystallization, the oversaturated melt departed from the reducing trend of the undersaturated melt, evolving towards high oxygen fugacity. Very late exsolution of the fluid permitted concentration of the HFSE in the last stages of magmatic evolution with local production of low-temperature pegmatitic phases extremely enriched in these elements.