GEOCHEMISTRY AND PETROGENESIS OF THE TERTIARY ALKALINE VOLCANIC SUITEOF THE LABE TECTONO-VOLCANIC ZONE, CZECH REPUBLIC

The Tertiary alkaline, nepheline-normative olivine melanephelinite-bas anite-phonolite suite in the Labe tectono-volcanic zone, Czech Republi c, displays mineralogical and geochemical features indicative of the p resence of products of more than one fractionation series and of more than one parental magma. The Mg- and Ni-rich olivine melanephelinites (ankaratrites), which contain disintegrated lherzolite xenoliths, repr esent low degree (c. 4%) partial melts of a highly enriched mantle sou rce. Many of the abundant basanitic rocks also represent primary or ne ar-primary basaltic melts subject to a small but variable degree of ol ivine fractionation. Other basanites with lower mg-numbers and Mg and Ni contents had a crystallization history involving hornblende and phl ogopite. Melaphonolites (tephriphonolites) provide a link between basa ltic rocks and phonolites with plagioclase fractionation progressively playing a more significant role. The phonolites represent products of crystallization of highly evolved melts but poor correlation between SiO2, MgO, CaO, Na2O and degree of undersaturation as well as between major and trace element contents indicate that a single line of descen t cannot have been in operation: the products of different parental ma gmas showing a wide compositional range that evolved under different c onditions of fractionation are represented. A very advanced stage of m agma evolution, characterized by the dominance of alkali feldspar frac tionation is represented by phonolites that contain the highest Hf, Nb , Rb, Ta, Th, Y, Zr and REE in the whole Tertiary volcanic suite.