Roztoky Intrusive Centre in the Ceske stredohori Mts.: Differentiation, emplacement, distribution, orientation and age of dyke series

The Roztoky Intrusive Centre (RIC) is formed by a trachytic crater vent, hy pabyssal intrusions, together with more than 1000 almost radially arranged dykes and more rare cone sheets. Hypabyssal Weakly Alkaline Series of essex ite (33-31 Ma)-monzodiorite (33-30 Ma)-sodalite syenite (30-28 Ma) acid two coexisting weakly [camptonite (31 Ma)/gauteite I?-sodalite syenite porphyr y-gauteite II? (24 Ma)/trachyte?] and strongly alkaline dyke series [campto nite (28 Ma)/monchiquite (26 Ma)-phonolite/tinguaite (26 Ma)/nepheline syen ite: porphyry cent: sheet (30 Mall wt-re recognized. Four principal dyke gr oups were distinguished: I - lamprophyres (58 ro) dominated over II semilam prophyres (28 %), minor III - basaltic rocks (6 %) and IV - felsic derivati ves (9 %). Both radial steeply dipping dykes of lamprophyres, semilamprophy res, basaltic and rare phonolitic rocks and flat dipping cone sheets of tra chyte and phonolite/nepheline syenite porphyry are present. Dykes of (semi) lamprophyres, and basaltic rocks show similar preferred strikes of 90 degre es and 0 degrees; felsic derivatives of 330 degrees and 0 degrees. Majority (91-98 %) of dykes (100 % of felsic dykes) art: present within a distance of 7 km from the Roztoky main centre. Joint and dyke patterns are controlle d by the regional paleostress field existing in the upper crust during magm a ascent, by orientations of pre-existing fracture sets in the region and b y the superimposed local stress field exerted by the rising intrusion. For the interval of 31-26 Ma in the RIG, the analysis of dyke geometries indica tes the dominance of regional stress characterized by N-S tension (lamproph yres approximate to semilamprophyres > basaltic rocks > felsic derivatives) .