Original paper

Stable isotope and chemical compositions of carbonate ocelli and veins in Mesozoic lamprophyres of Hungary

Demény, Attila; Fórizs, István; Molnár, Ferenc

European Journal of Mineralogy Volume 6 Number 5 (1994), p. 679 - 690

45 references

published: Sep 28, 1994
manuscript accepted: May 19, 1994
manuscript received: Apr 27, 1993

DOI: 10.1127/ejm/6/5/0679

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Abstract

Abstract Lamprophyres and carbonatites of late Cretaceous age occurring in Palaeozoic to Mesozoic granites and sedimentary rocks contain carbonate ocelli and veins which show alteration resulting in the formation of disseminated carbonate and clay minerals. Those bodies intruding granites contain calcite and dolomite whose carbon isotopic compositions (δ13 C ≈ 4.5 %o) are within the primary mantle ranges, whereas the oxygen isotopic compositions (δ18O ≈ 14 %c) are well outside the ranges of primary carbonatites. Carbonate ocelli of the lamprophyres were formed by interactions of igneous fluids with xenoliths of sedimentary origin. Interactions with the magma have also changed the chemical composition of limestone xenoliths enclosed in the lamprophyres. Carbon isotopic compositions and calcite-dolomite solvus temperatures measured on calcite ocelli with dolomite rims point to both chemical and isotopic disequilibrium between calcite and dolomite despite apparent complete recrystallization of the original sedimentary material. The isotopic compositions of calcite veins may be attributed to the effects of igneous fluids, exchange of fluids with sedimentary host rocks, oxidation of organic matter or possible degassing of magma and/or magmatic fluids. The former two processes produce calcites with sedimentary δ13C values and δ18O values down to 15.0 %o, whereas the latter three factors may be responsible for a trend of δ13C shifts down to - 9.0 %o, and δ18O values up to 24.2 %o

Keywords

stable isotopescarbonateslamprophyresthermometry