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

摘要

Alkali pyroxenite and ijolite xenoliths occur in ca. 37-30 Ma old nephelinite of the Loucna-Oberwiesenthal Volcanic Centre. The latter is located on the uplifted shoulder of the Ohre/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 epsilon_(Nd) 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 ~(87)Sr/~(86)Sr ratios of 0.70361 to 0.70365 and inital epsilon_(Nd) 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 ~(87)Sr/~(86)Sr of 0.70368 and initial epsilon_(Nd) value of +2.3.