Fluorapatite-monazite relationships in granulite-facies metapelites, Schwarzwald, southwest Germany

摘要

Fluorapatite grains with monazite inclusions and/or rim grains are described in two of four samples from a set of granulite-facies metapelites collected from the Variscan Schwarzwald, southern Germany. Fluorapatite in all four samples appears to have experienced some dissolution in the partial granitic melt formed during granulite-facies metamorphism. Monazite inclusions and rim grains are highly deficient in Th and are presumed to have formed from fluorapatite in association with partial melting during granulite-facies metamorphism. Monazite inclusions range from very small (<1 µm) and very numerous to small (1­2 µm), sometimes elongated, and less numerous; both types are evenly distributed throughout the fluorapatite grain interior. Monazite rim grains tend to be 1–10 µm. The formation of monazite inclusions is proposed to be due to dissolution-reprecipitation of the fluorapatite by the aqueous fluids inherent in the granitic melt. We propose that an increase in inclusion size coupled with a decrease in inclusion number is due to Ostwald ripening (interfacial energy reduction), which is greatly facilitated by the presence of an interconnected, fluid-filled porosity in the metasomatized fluorapatite. We further propose that monazite rim grains formed principally during partial dissolution of the fluorapatite in the granitic melt and to a lesser extent by partial dissolution-reprecipitation of the fluorapatite grain rim area allowing for the partial removal of (Y+REE). We conclude that fluorapatite, with monazite inclusions and rim grains, experienced partial dissolution in a H₂O-rich peraluminous granitic melt compared to fluorapatite with monazite rim grains and no inclusions which reacted with a similar, relatively less H₂O-rich melt. In contrast, monazite-free fluorapatite experienced partial dissolution in a comparatively H₂O-poor, subaluminous, possibly peralkaline melt.