AGE-RELATED CONTRASTING ALKALINE VOLCANIC SERIES IN NORTH BOHEMIA

The Late Cretaceous-Tertiary volcanism of the north-eastern arm of the Ohre (Eger) Rift in North Bohemia displays characteristic features of two intraplate magmatic series. An older (79-50 Ma) unimodal ultramaf ic ultra-alkaline (melilititic) series, representing the precursor of rifting, occurs exclusively in external blocks of the Ohre Rift. A you nger (40-18 Ma) bimodal (basanite-phonolite) rock series of alkaline c haracter predominates in the internal blocks of the rift. The later se ries is characteristic of highly-volcanic active rifts. In addition th e youngest picrobasaltic intrusions (9 Ma) occur there in association with regional faults. A similar, however, mildly alkaline bimodal alka li basalt/basanite-trachyte association prevails in the central part o f the Ohre Rift, in the Ceske stredohori Mts. Enrichment of the ultram afic rock series in incompatible elements (Rb, Cs, REE, Y, U, Th, Zr, Hf, Nb, Ta, P) can be explained by low-degree partial melting of a met asomatically enriched mantle source and/or steady zone refining proces s of magma generation. Further enrichment during the postmagmatic proc ess from the high level crustal reservoir source is likely. The Sr-87/ Sr-86 (0.70327-0.70366) and Nd-143/Nd-144 (0.51267-0.51287) ratios of the (ultra)mafic rocks of both magmatic series are similar and close t o those of HIMU OIB. The majority of phonolites (PH1-type) was derived from basanitic to olivine-poor nephelinitic magma and only a minority of the more evolved phonolites (PH2-type) probably represent extreme differentiates of the first mentioned phonolites. The addition of lith ophile elements in post-magmatic process to evolved and anomalous phon olites is highly probable. Phonolites that originated by low-pressure crustal fractionation were affected by some crustal contamination (Sr- 87/Sr-86 = 0.70407-0.70534, Nd-144/Sm-143 = 0.51280).