Sobocký T., Ondrejka M., Uher P., Mikuš T. & Konečný P., 2020: Monazite-group minerals and xenotime-(Y) in A-type granitic rocks: chemical composition and in-situ Th–U–total Pb EPMA dating (Velence Hills, Hungary). Acta Geologica Slovaca, 12, 2, 89–106.

Monazite-group minerals and xenotime-(Y) in A-type granitic rocks: chemical composition and in-situ Th–U–total Pb EPMA dating (Velence Hills, Hungary)

Tomáš Sobocký¹, Martin Ondrejka¹, Pavel Uher¹, Tomáš Mikuš² & Patrik Konečný³

¹Department of Mineralogy and Petrology, Faculty of Natural Sciences, Comenius University in Bratislava, Mlynská dolina, Ilkovičova 6, 842 15, Bratislava, Slovakia; sobocky1@uniba.sk; martin.ondrejka@uniba.sk; pavel.uher@uniba.sk
²Earth Science Institute, Slovak Academy of Sciences, Ďumbierska 1, 974 11, Banská Bystrica, Slovakia; mikus@savbb.sk
³State Geological Institute of Dionýz Štúr, Mlynská dolina 1, 817 04 Bratislava, Slovakia; patrik.konecny@geology.sk

Abstract

Accessory monazite-(Ce), -(La), -(Nd), cheralite (monazite-group minerals, MGM) and xenotime-(Y) are rare magmatic accessory minerals in subsolvus A-type granites and related aplite and pegmatite dykes in the Velence Hills, Transdanubian Unit, Hungary. The MGM and xenotime-(Y) form ≤ 200 μm euhedral to subhedral crystals in association with allanite-(Ce), zircon, fluorapatite, and ilmenite. Investigated MGM shows dominant cheralite (Ca²⁺ + Th⁴⁺ <––––> 2REE³⁺) and subordinate huttonite (REE³⁺ + P5⁺ <––––> Th⁴⁺ + Si⁴⁺) substitution with greater extent in pegmatite and aplite than in the host granite. Monazite in-situ Th–U–total Pb electron-microprobe dating of the Velence granite determined 289 ± 3 Ma age (MSWD = 3.22, n = 62) and this confirmed post-Variscan, Permian, (Cisuralian) magmatic crystallization. In addition, xenotime-(Y) from the same rock samples gave 266 ± 5.2 Ma age (MSWD = 1.4, n = 44), and this corresponds to Permian (Guadalupian) post-magmatic (subsolidus) recrystallization of xenotime-(Y) during a younger event and subsequent overprint of the parental granitic rocks. Although monazite-(Ce) remains relatively unaffected, fluid-induced breakdown of xenotime-(Y) produced numerous tiny thorite and cheralite inclusions in the altered xenotime-(Y) domains which can be formed by coupled dissolution reprecipitation reactions between orthomagmatic xenotime-(Y) and younger late to post-magmatic granitic fluids.

Key words: monazite, xenotime, EPMA dating, A-type granite, Velence, Hungary

Manuscript received: 2020-04-29

Revised version accepted: 2020-10-19