Cretaceous alkaline lamprophyres from northeastern Czech Republic: geochemistry and petrogenesis

Abstract

The northeast part of the Czech Republic (Moravia) and the adjoining part of Poland host a 100-km-long and 15- to 25-km-wide belt containing numerous isolated bodies (mainly sills) of lamprophyre of Lower Cretaceous age. The lamprophyres range from mafic (melanocratic) to evolved, feldspar-rich (mesocratic) variants. Mineralogically, they are characterized by compositionally zoned kaersutite phenocrysts, biotite and high Al–Ti salitic to diopsidic clinopyroxene. The lamprophyres are typically alkaline as shown by high contents of P₂O₅, TiO₂, alkalies and incompatible trace elements such as light REE, Zr, Nb, Y, Ba and Sr, and by high Ti/V (>50) and chondrite-normalized (La/Yb)_n (8–25) ratios. They resemble alkali basalts, basanites and nephelinites. Major element composition and trace element patterns and Nd–Sr isotopic values (εNd ca. +5.5 to +6.6 and εSr 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 HIMU affinities and some continental extension-related alkali basaltic suites. The lamprophyres do not show any subduction imprint. They were generated in the garnet stability field by a variable degree of melting. Evolved lamprophyres were formed by clinopyroxene-dominated fractional crystallization of mafic lamprophyric magma. The lamprophyres are interpreted to have been emplaced along conduits formed during the formation of a basin/graben structure in the Early Cretaceous.