Rhyolites from the Roztoky Intrusive Centre, Ceske stredohori Mts.: Xenoliths or dyke differentiates?

Dykes or xenoliths of rhyolite were found recently within a bostonite dyke in a borehole at Male Brezno. They represent the first occurrence of the qu artz bearing volcanic rock in the Ceske stredohori Mts. Geochemical and iso tope data at the time of intrusion (Sr-87/Sr-86 = 0.70822, Nd-143/Nd-144 = 0.512259, epsilon (t)(Nd) = -6.3) Of the rhyolites reflect substantial crus tal contamination of the magma in comparison with data on spatially associa ted camptonites (0.70405-0.70433; 0.512627, epsilon (t)(Nd) = +0.6), gautei tes (0.70446; 0.512735, epsilon (t)(Nd) = +2.5), monzodiorites (0.70431-0.7 0139; 0.512703, epsilon (t)(Nd) = +2.0) or phonolite (0.70416; 0.512752, ep silon (t)(Nd) = +2.9). Unusual rhyolite dykes or xenoliths were generated e ither by AFC process or by partial melting and transformation of an older f elsic rock possibly originating from the Saxothuringian basement. The rhyol ites occur in a spatial association with the Roztoky Intrusive Centre (RIC) . The centre is formed by a crater vent filled by trachytic breccia and hyp abyssal intrusions, together with more than 1000 almost radial arranged dyk es. Weakly alkaline hypabyssal rock series of essexite - monzodiorite - sod alite syenite (D.I. = 46-64) and linked coexisting dykes of weakly alkaline series (D.I. = 39-81) (trachybasalt) - gauteite/camptonite - bostonite/sod alite syenite porphyry - (trachyte) - (rhyolite?) and strongly alkaline ser ies (D.I. = 39-87) (tephrite/basanite) - camptonite/monchiquite - tephripho nolite - phonolite/tinguaite/nepheline syenite porphyry were recognized. Ge ochemical characteristics of differentiation suite of the RIC prove its ori gin by fractional crystallization of mantle derived magma. Layering and alm ost single step emptying of magma chamber is a presupposed mechanism of its genesis. The Sr-87/Sr-86 ratios of the RIC rock suite, calculated back to the time of intrusion, yielding values ranging from 0.70363 (hornblendite c umulate) to 0.70446 (gauteite) confirm its primary mantle origin without si gnificant crustal contamination.