Pages: 227 - 242
Abstract: The age and composition of monazite from ortho- and paragneisses of the Nizke Tatry Mts (Low Tatra Mts) was studied by microprobe chemical method. The monazite shows relatively uniform composition characterized by substitutions to xenotime (7–10 mol. %) and brabantite (6–11 mol. %), much less to huttonite (0–2 mol. %). Biotite-hosted monazite is commonly overgrown by thin REE-epidote rims, in a less foliated sample it is apparently unstable and breaks down to apatite, REE-epidote and a Th-silicate (huttonite?). Empirical thermometers based on Y in monazite give 600–650 ?C for xenotime-bearing samples. Extensive dating by Cameca SX-100 microprobe yielded a range of ages: the majority of data cluster around 340–360 Ma indicating either a Variscan origin of orthogneisses or strong reworking of an older granitic protolith. Because monazite recrystallized at subsolidus temperatures it seems to have failed to record a magmatic age. However, an Ordovician core (ca. 475 Ma) was found in one monazite grain from mylonitic Bystra augengneiss. This partially melted orthogneiss also contains younger monazites (320–330 Ma) which probably recrystallized during this Variscan event. Decompression-related muscovite melting was probably the reason for the partial melt formation. Metamorphic monazites from medium-grade biotite gneisses also record several ages: old inherited cores (450 Ma) and metamorphic events occurring probably at ca. 390, 350 and 330 Ma.
Keywords: Western Carpathians, Nizke Tatry, age, microprobe dating, gneiss, orthogneiss, monaziteDownload PDF document Show Text Version