Pages: 89 - 102
Abstract: The age and chemical composition of monazite from the Permian acid metavolcanic/volcaniclastics of the Southern Gemeric Unit (from the 1st and 2nd volcanogenic horizons of the Roznava Formation) and the Borka Nappe (the Bucina Formation) was studied by the microprobe chemical method. Monazites from the 2nd volcanogenic horizon of the Roznava Formation as well as from the Borka Nappe occur mostly in the form of small irregular grains about 5–10 μm in size always surrounded by the metamorphic white mica-phengite. Although substantial zonality in single grains was not observed, inter-granular variation in composition is much pronounced and it is mainly attributed to the variation in ThO2 concentration in the range from 0.9 to 14.5 wt. % (5.1 wt. % in average). Monazites from the 1st volcanogenic horizon of the Roznava Formation are larger, ~30–40 μm in diameter, and most of them are showing concentric zonality related to the age. Rounded cores mostly of Permian age are surrounded by Alpine rims. ThO2 concentration is more restricted up to 7.2 wt. %. The average is correspondingly lower ~3.5 wt. %. The majority of the monazites from the Permian acid metavolcanic rocks record an Alpine tectonothermal event around 148±8 Ma. Alpine age data divided according to the statistical modeling present two successive sub-events yielding ages of 167±12 and 136±10 Ma. Jurassic event is interpreted as a strong reworking linked to the subduction/accretion processes. Obduction of the Meliata accretionary prism, and strong subduction-related fluid flow over the Southern Gemeric domain including its Permian envelope unit are presumed. The successive Early Cretaceous (136±10 Ma) compression within the Inner Western Carpathians domain was followed by polyphase Alpine tectonic evolution connected with the gradual Cretaceous collision and indentation at about 100 Ma. The evidence for the Early Permian magmatic event at 276±25 Ma in the 1st volcanogenic horizon of the Roznava Formation recorded in the corroded monazite cores is presented for the first time.
Keywords: Western Carpathians, Southern Gemeric Unit, monazite dating, Early Permian metavolcanites, Alpine overprintDownload PDF document