GRANITOID ROCKS OF THE VELKA-FATRA MTS - RB SR ISOTOPE GEOCHRONOLOGY (WESTERN CARPATHIANS, SLOVAKIA)/

Isotope study of granitoid rocks of the L'ubochna Massif in the Velka Fatra Mts. has been carried out on whole-rock (WR) samples and separat ed mineral phases (muscovite, biotite) using the Rb/Sr method. It conf irmed the long duration of the granitoid magmatism with ''re-cycled'' crustal-mantle source material (I-Sr = 0.7057) During the main Meso-He rcynian event (350-340 Ma) the crust tectonically thickened. As a resu lt of the collision of the upper overheated unit with the lower, coole r one, the anatectic L'ubochna leucogranites (LLG) in the middle crust were melted. In the subsequent, extension phase, these leucogranites were emplaced into the upper cover unit, into the block of the ''relat ively older crystalline complex'', composed of the Smrekovica tonalite s (ST), Kornietov granodiorites (KGD) and Lipova granites (LG), which were also partly melted and re-magmatised, however, preserving their o riginal composition and zoning. This event is documented by the WR, Bi , Mu isochron (340 Ma) for all rock types. The age of the pre-existing granitoid body (ST, KGD and LG), showing an ''inherited isochron'' wi th an apparent age of 420 Ma is problematic. More than 340 Ma ago, thi s zoned Velka Fatra pluton, like the Eo-Hercynian crystalline block, w as probably already isotopically homogenized and solidified. The prese nted study confirmed that during the Meso-Hercynian stage there was no isotopic homogenization affecting the whole Velka Fatra pluton. Only local Sr-87/Sr-86 isotopic homogenization has been recorded.