Tales of Monazite, Garnet, and Melt: Linking Mineral Growth and Partial Melting to P–T Evolution

Monazite, a key accessory mineral in high-grade metamorphic rocks, serves as both a robust geochronometer and a primary reservoir for rare earth elements (REE), Th, and U. This study investigates melt inclusions within monazite and garnet to elucidate the processes of crustal melting and element partitioning during granulite facies metamorphism in the Bohemian Massif.
Nanogranitoid inclusions, identified through Raman spectroscopy and electron microscopy, exhibit polycrystalline textures containing quartz and feldspar polymorphs, micas, and occasionally carbonate phases. These inclusions are found in chemically distinct domains of monazite and garnet, offering valuable insights into the interplay between melt entrapment, mineral growth, and metamorphic conditions. Compositional zoning in garnet, characterized by variations in major and trace elements, alongside the corresponding domains in monazite, provides a detailed record of the pressure-temperature (P–T) trajectory during high-grade metamorphism.
Our findings reveal two generations of garnet, each showing distinct chemical zoning and closely linked to monazite inclusions with different ages. Monazite grains dated to ~370 Ma are associated with the first garnet generation (Garnet1), while monazite grains dated to ~340 Ma are associated with the second garnet generation (Garnet2). These monazite inclusions, along with matrix-hosted monazite grains, display contrasting REE and Th/U patterns, reflecting diverse growth conditions and the impact of garnet breakdown during decompression.
The systematic investigation of these inclusions, alongside their textural and chemical context, enhances our understanding of monazite stability in melt-bearing systems and its role in recording the temporal evolution of crustal melting processes. This integrative approach establishes a robust framework for deciphering the intricate relationships between mineral chemistry, melt inclusions, and P–T paths. These findings underscore the critical role of accessory minerals like monazite as indispensable archives of crustal evolution.