Uranium-rich monazite-(Ce) from the Krivá type granitic boulders in conglomerates of the Pieniny Klippen Belt, Western Carpathians, Slovakia: composition, age determination and possible source areas

Abstract

Monazite-(Ce) is a widespread accessory mineral in granitic cobbles of the Krivá type (Zástranie and Krivá localities) in polymict conglomerates of Cretaceous flysch sequences, the Pieniny Klippen Belt, Western Carpathians, NW Slovakia. The granites show leucocratic muscovite-biotite granodiorite composition and peraluminous calc-alkaline, S-type character. The monazite contains unusually high U, commonly 1 to 3, and in some places up to 6.6 wt.% UO2, together with 5 to 7.7 wt.% ThO2. A cheralite-type substitution [Ca(U,Th)REE–2] is the dominant mechanism of U4+ + Th4+ incorporation into the monazite structure in the Zástranie sample, whereas both cheralite- and huttonite-type substitution [(Th,U)SiREE–1P–1] are evident in the Krivá granitic cobble. Uranium alone prefers the CaU4+(REE)–2 mechanism, whereas Th favours the huttonite substitution. The chemical U-Th-Pb dating of monazite from both granitic cobbles show an Early Carboniferous age (346 ± 2 Ma), which is consistent with the main meso-Variscan, orogen-related plutonic activity in the Central Carpathian area (Tatric and Veporic superunits). Analogous U-rich monazites were detected in some Variscan S-type leucogranites of the Rimavica massif (South Veporic Unit) and the Bojná and Bratislava massifs (northern part of the Tatric Unit). On the basis of structural and palaeogeographic data, the North Tatric Zone is the most plausible source of the monazite-bearing granitic boulders in the Pieniny Klippen Belt. However, the source granitic body was most likely hidden by ensuing tectonic shortening along the northern Tatric edge after deposition of the Coniacian–Santonian Upohlav type conglomerates.