CRETACEOUS ALKALINE LAMPROPHYRES FROM NORTHEASTERN CZECH-REPUBLIC - GEOCHEMISTRY AND PETROGENESIS

The northeast part of the Czech Republic (Moravia) and the adjoining p art of Poland host a 100-km-long and 15- to 25-km-wide belt containing numerous isolated bodies (mainly sills) of lamprophyre of Lower Creta ceous age. The lamprophyres range from mafic (melanocratic) to evolved , feldspar-rich (mesocratic) variants. Mineralogically, they are chara cterized by compositionally zoned kaersutite phenocrysts, biotite and high Al-Ti salitic to diopsidic clinopyroxene. The lamprophyres are ty pically alkaline as shown by high contents of P2O5, TiO2, alkalies and incompatible trace elements such as light REE, Zi;Nb, Y, Pa and Sr, a nd by high Ti/V (>50) and chondrite-normalized (La/Yb), (8-25) ratios. They resemble alkali basalts, basanites and nephelinites. Major eleme nt composition and trace element patterns and Nd-Sr isotopic values (e psilon(Nd) Ca +5.5 to +6.6 and epsilon(Sr) ca. -9.5 to -24.0) indicate that the lamprophyric magma was derived from a mantle source that was compositionally similar to the source of ocean island basalts with HI MU affinities and some continental extension-related alkali basaltic s uites. The lamprophyres do not show any subduction imprint. They were generated in the garnet stability field by a variable degree of meltin g. Evolved lamprophyres were formed by clinopyroxene-dominated fractio nal crystallization of mafic lamprophyric magma. The lamprophyres are interpreted to have been emplaced along conduits formed during the for mation of a basin/graben structure in the Early Cretaceous.