The geological significance of the Rb-Sr whole-rock isochron of hornfelsed schists from the Izerskie Garby Zone, Karkonosze-Izera block, southern west Poland

Abstract

Preliminary Rb-Sr isotope data for hornfelsed schists from the "Stanislaw" quarry in the Izerskie Garby Zone are reported. An isochron based on three points representing three whole-rock samples yields an age of 333 [plus-minus] 4 Ma (Visean) and an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.709567. The Rb-Sr age of 333 [plus-minus] 4 Ma, indicating the date of the Rb-Sr system closure, can be interpreted as the cessation of contact metamorphism in the Izerskie Garby Zone. The age of 333 [plus-minus] 4 Ma is close to the peak of contact metamorphism in this zone. Contact metamorphism and silification in the Izerskie Garby Zone were genetically related to the activity of the Variscan Karkonosze pluton. Contact metamorphism was older than the biotite cooling at ca. 320 Ma.