Detrital and newly formed metamorphic monazite in amphibolite-facies metapelites from the Motajica Massif, Bosnia

The Motajica Massif in Bosnia, situated in the Sava Zone between the Tisza Unit and the Dinarides, hosts a Late Cretaceous flysch series, which is partly preserved in an anchimetamorphic state, partly affected by amphibolite-facies overprint. Samples from both parts contain abundant accessory monazite, which was investigated by means of the electron microprobe with reference to age and composition. In the amphibolite-facies samples new metamorphic monazite has formed. Chemical Th–U–Pb monazite ages indicate that the regional metamorphic event happened in the Late Cretaceous or the Early Palaeogene. In addition, inherited detrital monazite of Permian age has been identified. Based on various observations it is argued that the detrital monazite was not the direct precursor to the metamorphic monazite. The latter may rather have grown from other, more reactive REE carrier minerals, either detrital ones (e.g. detrital allanite) or such that formed during diagenesis and low-grade metamorphism. A later greenschist-facies event that occurred in connection with a ca. 27 Ma old granodiorite intrusion led in some samples to the formation of late fluorapatite–allanite coronas around both detrital and newly formed monazites.The anchimetamorphic samples contain exclusively detrital monazite. These are partly of Permian as well as Jurassic to Early Cretaceous age. Based on their compositional and textural features, the Jurassic to Early Cretaceous monazite relics are considered to be mainly derived from metamorphic rocks. The Permian monazite displays high yttrium content up to ca. 3.5 wt.% Y2O3, indicating a high formation temperature (> 650 °C) and therefore probably a magmatic origin. Apart from almost ubiquitous Permian high-Y monazite, the Motajica flysch also contains large Permian xenotime with magmatic zoning patterns, fluorapatite with parasitic monazite inclusions (formerly REE-rich fluorapatite), and zircon with A-type granite morphology. This accessory mineral spectrum implies that, among others, rift-related REE-rich Permian A-type granites or rhyolites should be considered as a source for the Sava Zone flysch.