CARBONATITE METASOMATISM IN THE UPPER MANTLE BENEATH THE BAKONY-BALATON HIGHLAND AND LITTLE HUNGARIAN PLAIN VOLCANIC FIELDS, WESTERN HUNGARY: EVIDENCE FROM UPPER MANTLE XENOLITHS

Abstract

Lithospheric upper mantle xenoliths hosted in Neogene alkaline basalts from the Bakony-Balaton Highland and Little Hungarian Plain Volcanic Fields of the Carpathian-Pannonian region have been studied extensively petrologically, geochemically and isotopically (e.g., Embey-Isztin et al., 1989; Downes et al., 1992; Szabo et al., 1995). Based on these studies, remarkable incompatible trace element enrichment and variable eNd and eSr values of the deformed Type I. xenoliths have been recognized and interpreted by Downes et al. (1992) as being due to the host alkaline magmas and subduction-related calc-alkaline magmas or fluids. However, apparent evidence for mantle metasomatism such as presence of amphiboles and phlogopites, melt pockets, veins and fluid and/or silicate melt inclusions occurring in these xenoliths have been reported very rarely (Embey- Isztin, 1976;. Embey-Isztin et al., 1989; Szabo et al., 1995). Here we present the results of detailed petrographic and major element analyses of melt pockets and veins found in six selected upper mantle samples (four peridotites and two pyroxenites) collected at the best known xenolith locations (Szentbekkalla and Gerce) of the studied volcanic fields. The goal of this work is to characterize the mantle melts and/or fluids formed and migrated in the mantle. According to the petrography, the melt pockets (up to 4.5 mm in diameter) occur as partially crystallized multicomponent aggregates replacing partially or totally melted Cr-diopsidic clinopyroxenes or rarely pargasitic amphiboles. The melt pockets are composed of silicate glass, newly formed clinopyroxene, olivine, spinel, carbonates, ± vesicles ± sulfide blebs. The melt veins (up to 1.5 mm in thickness) occur frequently as parallel cracks crosscutting the whole xenolith. They consist mostly of silicate glass and carbonates. Newly formed clinopyroxenes, olivines and spinels are very rare. Based on electron microprobe analyses, the composition of carbonates is calcite and Mg-calcite with small amounts of FeO (up to 0.54 wt%) and MnO (up to 1.5 wt%). Compositions of silicate glasses show relatively wide silica range (50 to 59 wt%), high alumina (18 to 25 wt%) and high total alkalis (5.5 to 7.8 wt%), relatively low MgO (1.8 to 4.5 wt%) and FeO (2.4 to 6.8 wt%) content. To estimate the bulk compositions of the melt pockets and veins, we have performed mass balance calculation, based on the modes and chemistry of the melt pockets and veins. The bulk compositions of the melt pockets are basaltic and trachybasaltic somehow resembling the composition of the host basalt reported by Embey-Isztin et al. (1993) and Harangi et al. (1995). The bulk compositions of the veins fall in fields of andesite and basaltic trachyandesite close to the large area of “andesitic” silicate melt inclusions occurring in Nograd-Gomor xenoliths studied by Szabo et al. (1996).