Variscan metamorphism and partial melting of sillimanite-bearing metapelites in the High Tatra Mts. constrained by Th–U–Pb dating of monazite

Abstract

The Tatra Mountains of the Western Carpathians are a key area for the study of the eastern continuation of the Variscan basement within the Alpine–Carpathian orogenic belt in Central Europe. Metamorphic zonation in the Tatra Mts. displays an inverted metamorphic sequence related to Variscan thrusting and emplacement of gneisses, migmatites and granites over micaschists. Here we present new results of Th–U–Pb dating of monazite in sillimanite-bearing metapelitic gneisses, migmatite and granodiorite from the High Tatra along with petrological interpretation based on thermodynamic modelling. The metapelitic gneisses show the peak metamorphic assemblage garnet + sillimanite + plagioclase + biotite + muscovite + ilmenite + quartz; inclusions of rutile, phengitic muscovite and paragonite in the garnet core indicate an earlier metamorphic stage. Thermodynamic modelling suggests a clockwise, prograde P–T path via staurolite-to-sillimanite sequence reactions from above 6 kbar to ca. 5–6 kbar and 650–700 °C, at mid-crustal levels. Migmatites, with dominant K-feldspar, plagioclase (An12–35) and quartz in the leucosome, underwent partial melting involving biotite dehydration reactions and formation of peritectic, Mn-rich garnet and/or Ti-magnetite at ca. 7–8 kbar and 760–770 °C, during decompression from lower-crustal levels. Monazite composition in metapelitic gneisses differs from that in leucosome of migmatite. The latter shows pronounced Eu-negative anomalies interpreted as the result of co-crystallization with feldspars and higher Y contents indicating higher temperature of crystallization. Monazite ages are identical within 2σ errors and indicate that both metamorphic and melting events occurred in Early Carboniferous, between 350–345 Ma as a consequence of continental collision and crustal thickening in the course of the Variscan orogeny.