Summary A swarm of subparallel steeply dipping carbonate dikes is exposed on numerous small islands in the central part of Paint Lake in the Superior Boundary Zone in central Manitoba. The swarm has been traced over a distance of 21 km and is generally conformable to the regional tectonic structure and gneissosity. The principal constituent of all dikes is calcite enriched in Sr, Y and rare-earth elements (REE) and showing evidence of plastic deformation and cataclasis, but the modal composition and texture of individual bodies vary from anchimonomineralic zones of coarse-grained calcite to fine-grained saccharoidal rocks with phlogopite-rich stringers to inequigranular foliated varieties containing a large proportion of calcic amphiboles, apatite, diopside, scapolite and xenocrysts. Regardless of these textural variations, the rocks are consistently enriched in Sr, light REE and show 18 OSMOW, 13 CPDB, Y/Ho, Zr/Hf, Th/U and Nb/Ta ratios similar to the primitive-mantle values. The contents of chalcophile and high-field-strength elements are systematically low. On the basis of the available structural, petrographic and geochemical data, the examined rocks are interpreted as calcite carbonatites of postorogenic affinity. The Paint Lake carbonatites host a variety of REE minerals, including (in order of decreasing abundance): allanite, titanite, monazite and bastnasite.
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.
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