Alpine (c. 96 Ma) metamorphism revealed by monazite dating in the Veporic unit, Western Carpathians

 

The Central Western Carpathians represent a tectonic system that extends eastward from the Alps and may be well correlated with the Austroalpine units of the Alps. The pre-Tertiary complexes of the Central Western Carpathians originated during the Cretaceous collisional events following the closure of the Meliata ocean by Late Jurassic times. Alpine metamorphism, related to the development of a metamorphic core complex during Cretaceous orogenic events, has been recognised in the Veporic unit (Janák et al. 2001). Increasing P-T conditions from greenschist to middle amphibolite facies reflect a coherent metamorphic field gradient. The isograds are roughly parallel to the north-east dipping foliation related to extensional updoming along low-angle normal faults, ⁴⁰Ar/³⁹Ar data constrain the timing of cooling and exhumation in the Late Cretacous. To constrain the timing of prograde metamorphism we dated monazite using electron microprobe. One reason for that is a shortage of geochronological data on Alpine metamorphism from the polymetamorphosed rocks which include pre-Alpine, mostly Variscan relicts. The dated monazite occurs in semi-pelitic schists of the highest grade Alpine metamorphic zone, exposed from the deepest levels of the Veporic dome. The peak metamorphic assemblage consists of garnet + biotite + muscovite + paragonite + rutile + quartz. Garnet occurs as clusters involving the fragments of fractured (pre -Alpine) garnet, and newly-formed, idioblastic Alpine garnet. Alpine garnet is zoned with decreasing Ca (X_Grs = 0.23-0.16) and increasing Mg (X_Prp = 0.05-0.12) from the core to the rim. Thermodynamic modelling indicates that Alpine garnet was growing during the burial (up to 1.2-1.4 GPa; 580-600°C). The metamorphic P-T-t path is “clockwise”, reflecting post-burial decompression (down to 0.8-0.1; 600-610°C) and cooling during Alpine orogenic cycle. Monazite (30-50 µm in size) occurs in the matrix occasionally associated with allanite (REE-epidote) and xenotime, suggesting monazite formation via allanite breakdown during the prograde P-T path. Chemical Th-U-Pb dating yielded Cretaceous age of 95.8 ± 2.7 Ma (MSWD = 1.13) with point ages ranging from 75 to 124 Ma. Similar age of 97 Ma was reported from the ICP-MS dating of monazite in a lower grade chloritoid-kyanite schists. This timing is in  excellent agreement with metamorphism between c. 100 and 90 Ma in the  Eo-Alpine HP/UHP belt in the Austroalpine Nappes of the Eastern Alps (Miladinova et al. 2022). We argue that the Veporic Unit represents direct continuation of the Eo-Alpine high-pressure belt into the West Carpathians.

 

Janák, M., Plašienka, D., Frey, M., Cosca, M., Schmidt, S. T., Lupták, B. & Méres, Š. 2001. Cretaceous evolution of a metamorphic core complex, the Veporic unit, Western Carpathians (Slovakia): P-T conditions and in situ ⁴⁰Ar/³⁹Ar UV laser probe dating of metapelites. Journal of Metamorphic Geology, 19, 197-216.

 

Miladinova, I., Froitzheim, N., Nagel, Th., Janák, M., Fonseca, R.O.C., Sprung, P. & Münker, C. 2022. Constraining the process of intracontinental subduction in the Austroalpine Nappes: Implications from petrology and Lu-Hf geochronology of eclogites. Journal of Metamorphic Geology 40, 423-456.