STABLE-ISOTOPE AND CHEMICAL-COMPOSITIONS OF CARBONATE OCELLI AND VEINS IN MESOZOIC LAMPROPHYRES OF HUNGARY

Lamprophyres and carbonatites of late Cretaceous age occurring in Pala eozoic to Mesozoic granites and sedimentary rocks contain carbonate oc elli and veins which show alteration resulting in the formation of dis seminated carbonate and clay minerals. Those bodies intruding granites contain calcite and dolomite whose carbon isotopic compositions (delt a(13)C approximate to -4.5 parts per thousand) are within the primary mantle ranges, whereas the oxygen isotopic compositions (delta(18)O = 14 parts per thousand) are well outside the ranges of primary carbonat ites. 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 wi th dolomite rims point to both chemical and isotopic disequilibrium be tween calcite and dolomite despite apparent complete recrystallization of the original sedimentary material. The isotopic compositions of ca lcite veins may be attributed to the effects of igneous fluids, exchan ge 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 delta(13)C values and delt a(18)O values down to 15.0 parts per thousand, whereas the latter thre e factors may be responsible for a trend of delta(13)C shifts down to -9.0 parts per thousand, and delta(18)O values up to 24.2 parts per th ousand.