Analcime is common in magmatic rocks of the alkaline basalt-phonotephrite-phonolite suite of the Mecsek Mts., Hungary. Besides the occurrence of xenomorphic groundmass analcime, wedge-shaped crystals between feldspars, and products of feldspar and nepheline alteration, analcime also occurs in calcite ocelli formed in basaltic dikes. Microscopic textures of these ocelli are characteristic of rapidly crystallized carbonate melt, suggesting that the ocelli are droplets of carbonate-rich melt separated from the silicate magma by liquid immiscibility. This carbonate would have crystallized below magmatic temperatures, and above hydrothermal ones. Thus, the analcime might be regarded as transitional between primary magmatic (P type) and hydrothermal (H type) analcimes. SEM studies reveal that the analcimes of the ocelli form euhedral crystals whose surfaces are smooth without signs of the porous texture characteristic for products of leucite alteration. Chemical compositions determined by electron microprobe are close to the theoretical NaAlSi2O6·H2O formula, with minor Ca substitution. XSi values (0.642 to 0.686) fall between the ranges of analcimes considered to be of primary magmatic and hydrothermal origins, whereas the low Fe contents indicate relationships with H type ones. Oxygen isotope compositions of silicate minerals in lavas and dikes and calcite ocelli in dikes have been determined in order to investigate the preservation of magmatic compositions, and the effect of low-temperature isotope exchange. The most positive δ18O values among the studied mineral separates were found in the analcimes (17.5 to 19.0‰). Based on comparisons with oxygen isotope compositions of calcites of the ocelli (13.0 to 13.3‰) and amphiboles (7.4‰) of their host rock, this 18O-enrichment could be a result of retrograde oxygen isotope exchange with magmatic fluids at decreasing temperatures. Effects of low-temperature isotope exchange appear also in the amphiboles, biotites and feldspars of the Mecsek series, resulting in increasingly more positive δ18O ranges (5.1 to 7.4‰, 7.2 to 7.4‰, and 7.6 to 15.0‰, respectively) as a function of sensitivity to retrograde isotope exchange. Primary magmatic compositions have been preserved in pyroxenes (6.0 to 6.5‰), indicating generation of basaltic melt by low degree partial melting of mantle peridotite.
ASJC Scopus subject areas
- Geochemistry and Petrology