Almandine garnet in calc-alkaline volcanic rocks of the northern PannonianBasin (eastern-central Europe): Geochemistry, petrogenesis and geodynamic implications

Almandine garnet-bearing andesites and dacites occur frequently in the Neog ene calc-alkaline volcanic series of the northern Pannonian Basin (Hungary and Slovakia). They were erupted during the early stage of volcanism and oc cur along major tectonic lineaments. On the basis of petrographic and geoch emical characteristics, garnets from these rock types are classified into ( 1) primary phases, (2) composite minerals containing xenocrystic cores and magmatic overgrowths and (3) garnets derived from metamorphic crustal xenol iths. Coexisting phenocrysts of primary garnets include Carich plagioclase, hornblende (magnesiohastingsite to tschemakite) and/or biotite. The primag garnets have high CaO (>4 wt %) and low MnO contents (<3 wt %). They have strongly light rare earth element depleted patterns and are enriched in hea vy rare earth elements. Negative Eu anomalies occur only in garnets in the more silicic host rocks. <delta>O-18 values for primary garnets are 6.1-7.3 parts per thousand, whereas composite garnets have elevated delta O-18 val ues (>8 parts per thousand). Chemical compositions of the primary garnets a nd coexisting minerals suggest that they crystallized at high pressures (7- 12 kbar) and temperatures (800-940 degreesC) from mantle-derived magmas. Sr -Nd isotopic compositions of their host rocks and O isotopic values of the garnets are consistent with two-component mixing between mantle-derived mag ma and lower-crustal metasedimentary material. The garnet-bearing silicic m agmas were erupted during extension of the Pannonian Basin and the tensiona l stress field may have enhanced their fast ascent from lower-crustal depth s, allowing preservation of early-formed almandine phenocrysts.