STABLE-ISOTOPE STUDIES AND PROCESSES OF CARBONATE FORMATION IN HUNGARIAN ALKALI BASALTS AND LAMPROPHYRES - EVOLUTION OF MAGMATIC FLUIDS ANDMAGMA-SEDIMENT INTERACTIONS

Processes of carbonate formation have been related to C and O isotopic compositions in the Mesozoic alkali basalt (Mecsek Mts.) and lamproph yre (Transdanubian Range) suites of Hungary. In the studied magmatic r ocks, carbonates are present as ocelli, amygdales, xenoliths, veins an d groundmass carbonate, C and O isotope studies of these types of carb onate have yielded information on the origin of the carbonates and ind icated the following processes of formation that determined the delta( 13)C and delta(18)O values of the carbonates:(1) Crystallization of ma gmatic carbonate. Textural characteristics and delta(13)C values sugge st formation of magmatic carbonate in alkali basalt and lamprophyre di kes, whereas the delta(18)O compositions of these carbonates indicate low temperature oxygen isotope exchange with magmatic fluids.(2) Assim ilation of sedimentary carbonate by silicate magmas. Even completely r ecrystallized amygdales and ocelli of basalts and lamprophyres have pr eserved their sedimentary delta(13)C values. In contrast, Variations i n the extent of mobilization and isotope exchange with magmatic fluids are reflected in differences in the ranges of the delta(18)O values o f amygdales, ocelli and veins, and can be attributed to different amou nts of fluids involved in the magmatic events.(3) Low temperature alte ration of magmatic rocks caused only O-18-enrichment in the carbonate amygdales of basalts and the groundmass carbonates of lamprophyres, in dicating that no externally-derived CO2 was present in the alteration fluids.(4) Degassing of magma and magmatic fluid. Correlations between delta(13)C and delta(18)O data, magma crystallization depths and amyg dale sizes in the alkali basalts suggest that CO2 degassing has been r esponsible for the negative delta(13)C and positive delta(18)O shifts observed. A similar trend was found in the lamprophyres, but the exten t of the delta(18)O shift indicates that in these rocks H2O degassing also played an important role.