Cretaceous evolution of a metamorphic core complex, the Veporic unit, Western Carpathians (Slovakia): PT conditions and in situ40Ar/39Ar UV laser probe dating of metapelites

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Abstract:

Abstract

Alpine metamorphism, related to the development of a metamorphic core complex during Cretaceous orogenic events, has been recognized in the Veporic unit, Western Carpathians (Slovakia). Three metamorphic zones have been distinguished in the metapelites: 1, chloritoid + chlorite + garnet; 2, garnet + staurolite + chlorite; 3, staurolite + biotite + kyanite. The isograds separating the metamorphic zones have been modelled by discontinuous reactions in the system K2O–FeO–MgO–Al2O3–SiO2–H2O (KFMASH). The isograds are roughly parallel to the north-east-dipping foliation related to extensional updoming along low-angle normal faults. Thermobarometric data document increasing PT conditions from c. 500 °C and 7–8 kbar to c. 620 °C and 9–10 kbar, reflecting a coherent metamorphic field gradient from greenschist to middle amphibolite facies. 40Ar/39Ar data obtained by high spatial resolution in situ ultraviolet (UV) laser ablation of white micas from the rock slabs constrain the timing of cooling and exhumation in the Late Cretaceous. Mean dates are between 77 and 72 Ma; however, individual white mica grains record a range of apparent 40Ar/39Ar ages indicating that cooling below the blocking temperature for argon diffusion was not instantaneous. The reconstructed metamorphic PTt path is ‘clockwise’, reflecting post-burial decompression and cooling during a single Alpine orogenic cycle. The presented data suggest that the Veporic unit evolved as a metamorphic core complex during the Cretaceous growth of the Western Carpathian orogenic wedge. Metamorphism was related to collisional crustal shortening and stacking, following closure of the Meliata Ocean. Exhumation was accomplished by synorogenic (orogen-parallel) extension and unroofing in an overall compressive regime.

Keywords: 39Ar UV laser probe dating; 40Ar; Cretaceous orogeny; Western Carpathians; extension; metamorphic core complex

Document Type: Research Article

Affiliations: 1: Geological Institute, Slovak Academy of Sciences, Dúbravská 9, 842 26 Bratislava, Slovak Republic (, Email: geolmjan@savba.sk 2: Mineralogisch-Petrographisches Institut, Universität Basel, Bernoullistrasse 30, CH-4056 Basel, Switzerland 3: Institut de Minéralogie, Université de Lausanne, BFSH2, CH-1015 Lausanne, Switzerland 4: Department of Mineralogy and Petrology, Comenius University, Mlynská dolina G, 842 15 Bratislava, Slovak Republic 5: Department of Geochemistry, Comenius University, Mlynská dolina G, 842 15 Bratislava, Slovak Republic

Publication date: February 1, 2001

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