Polymetamorphic evolution of pelitic schists and evidence for Permian low‐pressure metamorphism in the Vepor Unit, West Carpathians

Abstract

Phase equilibrium modelling and monazite microprobe dating were used to characterize the polymetamorphic evolution of metapelites from the northern part of the Vepor Unit, West Carpathians. Three generations of garnet and associated metamorphic assemblages found in these rocks correspond to three distinct metamorphic events related to the Variscan orogeny, a Permian phase of crustal extension and the Alpine orogeny. Variscan staurolite-bearing and Alpine chloritoid-bearing assemblages record medium-temperature and medium-pressure regional metamorphisms reaching 540-570°C ⁄ 5-7.5 kbar and 530-550°C ⁄ 5-6.5 kbar respectively. The Permian metamorphic assemblage involves garnet, andalusite, sillimanite, biotite, muscovite, plagioclase and corundum and locally forms silica-undersaturated andalusite-biotite-spinel coronas around older staurolite. The transition from andalusite to sillimanite indicates a prograde low-pressure and medium-temperature metamorphism characterized by temperature increase from 500 to 650°C at 3 kbar. As accessory monazite is abundant in the rocks, an attempt was made to derive its age of formation by means of electron microprobe-based Th-U-Pb chemical dating. Despite the polymetamorphic nature of the metapelites, the monazite yielded uniform Permian ages. Microstructures confirm that monazite was formed in relation to the low-pressure and medium-temperature paragenesis, and the weighted average ages obtained for two different samples are 278 ± 5 and 275 ± 12 Ma respectively. The virtual lack of Variscan and Alpine monazite populations points to interesting aspects concerning the growth systematics of monazite in metamorphic rocks.

Geological evolution of the southwestern part of the Veporic Unit (Western Carpathians): based on fission track and morphotectonic data

“…In these early convergent stages, the Veporic Unit suffered an internal shortening and thickening documented by upright folding. These early structures were mostly obliterated by the subsequent major deformation associated with the development of subhorizontal mylonitic foliation and the W-E orogen-parallel extension (Hók et al 1993;Plašienka 1993;Janák et al 2001;Jeřábek et al 2007Jeřábek et al , 2008a. In the study area, the Alpine metamorphism shows the greenschist facies conditions of 450 °C at 400-450 MPa (Jeřábek et al 2008a).…”

“…Lupták et al 2003;Jeřabek et al 2008a). However, the south-eastern portion of the crystalline basement suffered 450-500 °C (Jeřábek et al 2008a). The study area is located only in a part of the southern Veporic Unit where the cover has already been removed by erosion (Fig.…”

“…The Veporic crystalline basement is composed of Palaeozoic volcano-sedimentary rocks characterised by medium-grade Variscan metamorphic overprint (Vrána 1964;Méres & Hovorka 1991;Kováčik et al 1996;Putiš et al 1997;Jeřábek et al 2008a), which are located in the footwall of high-grade Variscan migmatites and Upper Devonian-Lower Carboniferous I-and S-type granite rocks (~ 370-350 Ma;Siman et al 1996;Michalko et al 1998;Broska et al 2013). …”

“…During the Permian, the basement was intruded by several smaller A-type granitic bodies (e.g., Hrončok Granite; Bezák et al 1999a;Finger et al 2003) and was locally affected by the low-pressure/medium-temperature metamorphism (Finger et al 2003;Jeřábek et al 2008b). The Alpine tectono-metamorphic phase is characterised by Cretaceous amphibolite facies conditions in the structural footwall, which gradually decrease to greenschist facies conditions towards the structural hanging wall Jeřábek et al 2008a). The Cretaceous metamorphism was associated with the development of a subhorizontal mylonitic fabric, which developed during E-W orogen-parallel stretching induced by the northward overthrusting of the Gemeric Unit (Lexa et al 2003;Jeřábek et al 2007Jeřábek et al , 2012Bukovská et al 2013).…”

“…Much progress has been made in recent years towards understanding the processes of Alpine metamorphism during the nappe stacking (e.g., Vrána 1964;Janák et al 2001;Finger et al 2003;Lupták et al 2004;Jeřábek et al 2008aJeřábek et al ,b, 2012, but the Late Cretaceous to Cenozoic evolution is still not well Brought to you by | MIT Libraries Authenticated Download Date | 5/12/18 9:41 PM phase followed by the Late Cretaceous/Palaeocene collapse and the Eocene-Quaternary post-collisional evolution of this part of the Western Carpathians.…”