Cretaceous evolution of a metamorphic core complex, the Veporic unit, Western Carpathians (Slovakia): P-T conditions and in situ Ar-40/Ar-39 UV laserprobe dating of metapelites

Alpine metamorphism, related to the development of a metamorphic core compl ex during Cretaceous orogenic events, has been recognized in the Veporic un it, Western Carpathians (Slovakia). Three metamorphic zones have been disti nguished in the metapelites: 1, chloritoid + chlorite + garnet; 2, garnet staurolite + chlorite; 3, staurolite + biotite + kyanite. The isograds sep arating the metamorphic zones have been modelled by discontinuous reactions in the system K2O-FeO-MgO-Al2O3-SiO2-H2O (KFMASH). The isograds are roughl y parallel to the north-east-dipping foliation related to extensional updom ing along low-angle normal faults. Thermobarometric data document increasin g P-T conditions from c. 500 degreesC and 7-8 kbar to c. 620 degreesC and 9 -10 kbar, reflecting a coherent metamorphic held gradient from greenschist to middle amphibolite facies. Ar-40/Ar-39 data obtained by high spatial res olution in sill ultraviolet (UV) laser ablation of white micas from the roc k slabs constrain the timing of cooling and exhumation in the Late Cretaceo us. Mean dates are between 77 and 72 Ma; however, individual white mica gra ins record a range of apparent 40Ar/39Ar ages indicating that cooling below the blocking temperature for argon diffusion was not instantaneous. The re constructed metamorphic P-T-t path is 'clockwise', reflecting post-burial d ecompression and cooling during a single Alpine orogenic cycle. The present ed data suggest that the Veporic unit evolved as a metamorphic core complex during the Cretaceous growth of the Western Carpathian orogenic wedge. Met amorphism was related to collisional crustal shortening and stacking, follo wing closure of the Meliata Ocean. Exhumation was accomplished by synorogen ic (orogen-parallel) extension and unroofing in an overall compressive regi me.