Original paper

Petrology and geochemistry and KAr ages for Cenozoic tinguaites from the Ohre/Eger Rift (NW Bohemia)

Ulrych, J.; Nová k, J.K.; Lang, M.; Balogh, K.; Hegner, E.; Ř anda, Z.

Abstract

Dikes of tinguaite-textured (acicular clinopyroxene arranged in radial or criss-cross patterns interstitially in a mosaic of alkali feldspar and foids) phonolites, hereafter referred to as tinguaites, represent the most evolved rocks in the alkaline (camptonite/monchiquite - phonolite/tinguaite) dike suite of the Cenozoic Roztoky Intrusive Centre (RIC), České středohoří Mountains. Petrographic and field evidence reveals different textural varieties of dikes ranging from tinguaites, porphyritic tinguaites to mixed-textured phonolite - tinguaites. The development of a tinguaitic texture depends on the rate of magma cooling as can be concluded from the observation that the thin phonolitic dikes exhibit tinguaitic texture whereas thick phonolitic dikes mostly show a trachytic texture. Some thick dikes show also glassy tinguaitic margins and trachytic-textured phonolitic central parts. Rare perrierite and a Zr-silicate mineral occur among the accessory minerals. The tinguaite samples are rich in alkali and other incompatible elements and have high concentrations of Cl, SO3, CO2, and H2O. The alkalinity index (A. I.) is high and ranges from 0.79 to 0.96. Initial 87Sr/86Sr ratios range from 0.70335 to 0.70419 and initial εNd values of +2.1 to +4.3 indicating a moderately depleted mantle source. The chemically evolved composition of the tinguaites was attained via melt differentiation at shallow crustal levels with limited assimilation of crust. K-Ar ages of 31-28.5 Ma for five samples suggest that the tinguaitic dike rocks are coeval with the subvolcanic members of the RIC at 33 to 24 Ma. But, since field evidence requires a younger crystallization age for the dikes than the RIC, we suggest that excess Ar may be responsible for some of the older ages obtained in this study.

Keywords

ceske stredohori mtsohre/eger riftroztoky intrusive centertinguaite dikescenozoic volcanism