Heterogeneous extrusion and exhumation of deep-crustal Variscan assembly: Geochronology of the Western Tatra Mountains, northern Slovakia

Abstract

The nature and style of mid-crustal assembly and exhumation during continental collision has been investigated in the Tatra Mountains of the Western Carpathians. The pre-Alpine basement of the Western Carpathians represents the easternmost exposure of the Variscan orogen in Europe, which marks the collision of Laurasia with Gondwanian-affiliated terranes during the Palaeozoic. The Tatric crystalline unit of the Western Tatra in northern Slovakia displays an inverted metamorphic sequence where a high-grade unit comprising migmatites with relicts of eclogite has been thrust over a lower-grade mica schist unit. New geochronological and thermochronological data together with published thermobarometry illuminate the metamorphic history of the Western Tatra. The Upper Unit eclogites with occasionally preserved omphacite record near isothermal decompression from 1.6GPa to 1.0–1.2GPa at 750–800°C which lead to intensive re-equilibration at high-pressure granulite facies conditions, comparable to the peak metamorphic conditions of the host migmatite. Both eclogite and migmatite shared a retrograde P–T path following the insertion of the eclogite assemblage into the migmatites. The metamorphic evolution of the Lower Unit mica schist is constrained to peak P–T conditions of 0.6–0.8GPa and 640 and 660°C followed by retrogression. This suggests that different rock types of the Western Tatra metamorphic core shared only their exhumation path from mid-crustal levels. ID-TIMS Sm–Nd dating of garnet from eclogite yields a whole rock-garnet isochron age of 337±10Ma, with an initial εNd isotopic composition of +8.3. In situ U–Pb dating of monazite from a migmatite surrounding the eclogite shows one age population of c. 380Ma whereas monazite from a migmatite away from the eclogite preserves a robust 340±11Ma age which is indistinguishable from Sm–Nd garnet age and U–Pb age of zircons in the anatectic leucosome of the migmatite (347±7Ma). A younger monazite age population from the migmatite of 300±16Ma is consistent with 40Ar/39Ar mica ages of c. 310Ma. This argues for a contemporaneous, and likely shared, exhumation path of the assemblage pair. In situ monazite total-Pb analyses from the Lower Unit mica schists yields xenocrystic and c. 370Ma ages, but no geochronologic evidence for peak Variscan tectonism. Exhumation of the deep crustal root occurred most probably in a two-stage process. The timing of the high-pressure, eclogite facies metamorphism before the onset of exhumation into the mid crust, was likely between c. 380Ma and 360Ma. Subsequent exhumation into the middle crust was coeval with migmatite generation at c. 340Ma and garnet diffusion modeling suggest ~30°C/Ma cooling rates. The exhumation was likely tectonically forced by the action of a rigid indentor, which prompted the weak lower crust to be heterogeneously extruded to mid-crustal levels at a time coeval with anatexis and subsequently extruded with mid-crustal material to the upper crust.