Pre-Variscan evolution of the Western Tatra Mountains: new insights from U-Pb zircon dating

Abstract

In situ LA-MC-ICP-MS U-Pb zircon geochronology combined with cathodoluminescence imaging were carried out to determine protolith and metamorphic ages of orthogneisses from the Western Tatra Mountains (Central Western Carpathians). The metamorphic complex is subdivided into two units (the Lower Unit and the Upper Unit). Orthogneisses of the Lower Unit are mostly banded, fine- to medium-grained rocks while in the Upper Unit varieties with augen structures predominate. Orthogneisses show a dynamically recrystallised mineral assemblage of Qz + Pl + Bt ± Grt with accessory zircon and apatite. They are peraluminous (ASI = 1.20–1.27) and interpreted to belong to a high-K calc-alkaline suite of a VAG–type tectonic setting. LA-MC-ICP-MS U-Pb zircon data from samples from both units, from crystals with oscillatory zoning and Th/U > 0.1, yield similar concordia ages of ca. 534 Ma. This is interpreted to reflect the magmatic crystallization age of igneous precursors. These oldest meta-magmatics so far dated in the Western Tatra Mountains could be linked to the fragmentation of the northern margin of Gondwana. In zircons from a gneiss from the Upper Unit, cores with well-developed oscillatory zoning are surrounded by weakly luminescent, low contrast rims (Th/U < 0.1). These yield a concordia age of ca. 387 Ma corresponding to a subsequent, Eo-Variscan, high-grade metamorphic event, connected with the formation of crustal-scale nappe structures and collision-related magmatism.

Variscan post-collisional cooling and uplift of the Tatra Mountains crystalline block constrained by integrated zircon, apatite and titanite LA-(MC)-ICP-MS U-Pb dating and rare earth element analyses

“…The oscillatory zonation in the inner portions of the rims dated at 385 ± 8 Ma (Fig. 10a) suggests crystallization from melt, and is similar in age to zircon rims from orthogneisses from the same area and dated at 387 ± 14 Ma (Burda and Klötzli, 2011) and 387 ± 8 Ma found in the internal rims of the zircon crystals from OR sample (Gawęda et al, 2016b). This episode of anatexis at c. 387 Ma is likely related to the metamorphic climax at c. 380 to 370 Ma, in the Sowie Mountains in Southern Poland (Timmermann et al, 2000), Śnieżnik Massif in Sudety Mountains (Anczkiewicz et al, 2007) and the Mariánské Lázně Complex in the Czech Republic (Timmerman et al, 2004), and is interpreted as a phase of partial melting during early Variscan orogenesis (Kryza and Fanning, 2007).…”