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

Almandine garnet-bearing andesites and dacites occur frequently in the Neogene calc-alkaline volcanic series of the northern Pannonian Basin (Hungary and Slovakia). They were erupted during the early stage of volcanism and occur along major tectonic lineaments. On the basis of petrographic and geochemical 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 xenoliths. 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 heavy rare earth elements. Negative Eu anomalies occur only in garnets in the more silicic host rocks. O-18 values for primary garnets are 6.1-7.3 parts per thousand, whereas composite garnets have elevated delta O-18 values (>8 parts per thousand). Chemical compositions of the primary garnets and 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 magma and lower-crustal metasedimentary material. The garnet-bearing silicic magmas were erupted during extension of the Pannonian Basin and the tensional stress field may have enhanced their fast ascent from lower-crustal depths, allowing preservation of early-formed almandine phenocrysts.