VARISCAN VS CADOMIAN TECTONOTHERMAL ACTIVITY IN NORTHWESTERN SECTORS OF THE TEPLA-BARRANDIAN ZONE, CZECH-REPUBLIC - CONSTRAINTS FROM AR-40 AR-39 AGES/

Five muscovite concentrates from high-grade, pelitic metasedimentary b asement rocks exposed in northwestern sectors of the Tepla-Barrandian zone (Czech Republic) record Ar-40/Ar-39 mineral plateau ages which ra nge between ca. 376 and 362 Ma. Hornblende concentrates from metagabbr o (Marianske Lanze complex) and fine-grained basement amphibolite disp lay plateaux which define Ar-36/Ar-40 VS Ar-39/Ar-40 isotope-correlati on ages of ca. 370 and ca. 375 Ma. The mineral ages are interpreted to date relatively rapid cooling through appropriate argon retention tem peratures following early phases of Variscan (Early Devonian) regional metamorphism. A slate/phyllite basement sample collected within lower -grade metasedimentary rocks in southeastern portions of the Tepla-Bar randian zone is characterized by an internally discordant Ar-40/Ar-39 whole-rock age spectrum which suggests partial Variscan rejuvenation o f intracrystalline argon systems which had cooled through appropriate argon retention temperatures following an initial regional metamorphis m at or prior to ca. 500 Ma (Cadomian). Hornblende from undeformed dio rite of the Kdynz complex records a well-defined Ar-40/Ar-39 age plate au which corresponds to an isotope-correlation age of ca. 516 Ma. This is interpreted to date post-magmatic cooling following emplacement.