Origin of monazite–xenotime–zircon–fluorapatite assemblages in the peraluminous Melechov granite massif, Czech Republic

Abstract

This study describes and discusses the origin of fluorapatite–zircon–monazite–xenotime associations from the Melechov granite massif, Czech Republic. The Melechov massif consists of a series of peraluminous two-mica granite intrusions subdivided into the Stvořidla, Melechov, Kouty, and Lipnice granites. Fluorapatite in these granites occurs in three population groups: (1) large grains (100–600 μm) with variable amounts of monazite and zircon inclusions and grains along the rim of the fluorapatite ranging from many to none; (2) small (<50 μm), inclusion-free grains; and (3) very small (<2 μm) grains scattered throughout the plagioclase, which occur mainly in samples from the Melechov and Stvořidla granites. Rare xenotime inclusions are found in large fluorapatite grains from the Melechov and Stvořidla granites. Both the large and small fluorapatite grains have the same compositional range in Y, the REE, and Mn. Petrographic textures and mineral composition indicate that the monazite and zircon inclusions were overgrown during fluorapatite growth. Evidence for this origin includes: (1) Zircon cannot be metasomatically induced to form as inclusions in fluorapatite; (2) there is no difference either in the Y and the REE or, more importantly, in the Th and U content between monazite grains associated with and not associated with fluorapatite; and (3) distinctive layered, concentric textures in the fluorapatite in which the monazite and zircon inclusions are often seen to concentrate. These observations suggest that the monazite and zircon grains initially grew on the surface of the fluorapatite grain and were later enclosed by new layers of fluorapatite as the grain continued to grow.