Variscan high-pressure metamorphism of kyanite-bearing paragneisses hosting eclogites in the Veporic unit, Western Carpathians: Evidence from Th–U–Pb dating of monazite

Abstract

In view of a polyphase development of the northern Veporic pre-Alpine basement of the Western Carpathians it is important to determine the age of high pressure (HP) metamorphism known from eclogites occurring in this area. To do this, several monazite-bearing paragneisses were studied petrologically and monazite was dated by chemical (U–Th–Pb) method. Identified were remnants from high-pressure stage, i.e. phengite (Si apfu = 3.2–3.3), kyanite, rutile and Ca-rich garnet rims. Part of the present mineral assemblage may have been preserved from prograde stage (plagioclase) and part reflects re-equilibration during retrogression (biotite, major elements in garnet cores). However, Y and HREE in garnet cores were not homogenised and their profiles indicate origin during temperature rise. Peak metamorphic conditions (P = 21 kbar; T = 650 °C), and retrogression stage (P = 9 kbar; T = 520 °C) were calculated using Zr-in-rutile, and Y-in-garnet and monazite thermometry, phengite composition and pseudosection modelling. Monazite yielded dominant Early Carboniferous age (354 Ma) with subordinate amounts of Ordovician (485 Ma) and Cambrian (505 Ma) ones, and no Alpine record. The monazite position in garnet cores predating the growth of Ca rims suggest that the Carboniferous age of 354 Ma probably predates the peak conditions and refers to the prograde stage of the Variscan metamorphic evolution.