Original paper

Petrogenesis of alkali pyroxenite and ijolite xenoliths from the Tertiary LoucnaOberwiesenthal Volcanic Centre, Bohemian Massif in the light of new mineralogical, geochemical, and isotopic data

Ulrych, J.; Lloyd, F.E.; Balogh, K.; Hegner, E.; Langrová, A.; Lang, M.; Novák, J.K.; Řanda, Z.

Abstract

Alkali pyroxenite and ijolite xenoliths occur in ca. 37–30 Ma old nephelinite of the Loučná–Oberwiesenthal Volcanic Centre. The latter is located on the uplifted shoulder of the Ohře/Eger Rift within the Variscan basement of central Europe. The alkali pyroxenites and transitional xenoliths are abundant whereas ijolite xenoliths are rare. The host nephelinite is chemically evolved (Mg# 47–46), and the entrained alkali clinopyroxenite xenoliths (Mg# 64–47) and transitional to ijolite xenoliths (Mg# 69–25) show a range of compositions from little to highly evolved. The alkali pyroxenite xenoliths probably represent fragments of an intracrustal, possibly layered alkaline complex, overprinted by a late-magmatic pegmatoid phase of ijolite composition. This late stage metasomatic process may account for a spectrum of rocks ranging from alkali pyroxenite to ijolite.Initial εNd values of +2.3 in clinopyroxene samples from the host nephelinite and +3.1 to +3.0 in clinopyroxene from the xenoliths indicate similar, yet different sources. The initial 87Sr/86Sr ratios of 0.70361 to 0.70365 and inital εNd values of +3.0 and +3.1 for the alkali pyroxenite xenoliths are consistent with mantle sources of HIMU-affinity. A similar source may be inferred from the isotopic composition of the host nephelinite yielding an initial 87Sr/86Sr of 0.70368 and initial εNd value of +2.3.

Keywords

tertiary european volcanismohre/eger riftxenolithsalkali pyroxeniteijolitekrusne hory/erzgebirge mtsbohemian massif