Geochemical characteristics of the high- and low-Ti basaltic rocks from the uplifted shoulder of the Ohre (Eger) Rift, Western Bohemia

摘要

High-Ti melanephelinite (3.8-5.9 wt% TiO2), medium-Ti (phono)tephrite (2.7-3.1 wt% TiO2), and low-Ti olivine melanephelinite/basanite (1.9-2.3 wt.% TiO2) are subordinate rock types in the central European Cenozoic Volcanic Province. A contrasting melanephelinite to (phono)tephrite series occurs in the Loucna-Oberwiesenthal Volcanic Centre (37-28 Ma) and also as satellite volcanic bodies (26-12 Ma) together with olivine melanephelinite/basanite (17-20 Ma) on the southwestern periphery of the Krusne hory mountains (Erzgebirge). The volcanic rocks intrude the Variscan basement of the uplifted shoulder of the Ohre/Eger Rift in the Krusne hory mountains of the Bohemian Massif. Low Mg# (44-59) and Cr, Ni contents and enrichment of LILE, Zr, Hf, Nb, Ta, U, Th and LREE in the high-Ti melanephelinites contrast with the composition of primitive low-Ti olivine melanephelinites/basanites displaying high Mg# (63-74) and high contents of compatible elements. The high-Ti melanephelinites reveal a wide range in initial ~(87)Sr/~(86)Sr of ca. 0.7034-0.7038 and ε_(Nd) of 2.4-4.9. The low-Ti melanephelinites show an overlapping range of initial ~(87)Sr/~(86)Sr of ca. 0.7035-0.7036 and ε_(Nd) of 4.3-5.5. The large variation in initial ~(87)Sr/~(86)Sr ratios at similar ε_(Nd) values in those rock types is interpreted as evidence for melting of metasomatized lithospheric mantle sources comprising K-bearing phases with radiogenic Sr. Modification of the olivine-free alkali basaltic magmas by differentiation or crustal contamination could give rise to the medium-Ti (phono) tephrites. The initial isotope ratios of all samples are consistent with HIMU-mantle sources and contributions from lithospheric mantle. The olivine-free melanephelinitic rocks often contain alkali pyroxenite-ijolite xenoliths with initial ~(87)Sr/~(86)Sr ratios of ca. 0.7036 and ε_(Nd) of 3.0. We interpret these xenoliths as samples of an intra-crustal alkali complex derived from similar mantle sources as those for the basaltic volcanic rocks.