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)

摘要

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 dissolutionreprecipitation reactions between orthomagmatic xenotime-(Y) and younger late to post-magmatic granitic fluids.