Microcrystals of Th-rich monazite (La) with a negative Ce anomaly in metadiorite and their role for documenting Cretaceous metamorphism in the Slavonian Mountains (Croatia)

Microcrystals (1–15 μm) of unusual monazite (La) with 41–47 mol% cheralite ThCa(PO₄)₂] component and a strong negative Ce anomaly are described from a metadiorite from the SW Slavonian Mountains, Psunj, Croatia. The dioritic host rock still shows a relictic igneous fabric on macroscopic scale. However, metamorphic reaction textures can be recognized in thin section. These include partial recrystallization of igneous plagioclase to albite coupled with the formation of epidote. Furthermore, partial replacement of igneous hornblende by a fine-grained orthoamphibole-chamosite-epidote paragenesis can be observed and replacement of ilmenite by titanite. The compositions of the metamorphic minerals indicate upper greenschist facies conditions (460–500 °C according to two-feldspar geothermometry) under a high oxygen fugacity. Microstructures show that the monazite crystals belong to the metamorphic paragenesis and formed at the expense of magmatic allanite. Their negative Ce anomalies reflect the oxidizing conditions of metamorphism. Application of the xenotime in monazite solvus geothermometer provides unrealistically high temperatures of ~500–660 °C which disagree with the greenschist facies metamorphic paragenesis. We interpret that the presence of cheralite has a profound effect on the nature of the monazite-xenotime solvus curve and hence the existing calibrations of this geothermometer may be generally unsuitable for cheralite-rich monazite. An important geological result is that the Th-U-total Pb ages of the monazite grains are uniformingly Upper Cretaceous. Our data thus suggest that the imprint of the Alpine orogeny on the Slavonian Mountains was stronger than presumed until now.