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

Published online by Cambridge University Press:  05 July 2018

D. E. Harlov ,
H. R. Marschall  and
M. Hanel
D. E. Harlov*
Affiliation:
GeoForschungsZentrum Potsdam, Telegrafenberg, D-14473 Potsdam, Germany
H. R. Marschall
Affiliation:
Mineralogisches Institut, Universität Heidelberg, Im Neuenheimer Feld 236, D-69120 Heidelberg, Germany
M. Hanel
Affiliation:
Mineralogisches Institut, Universität Heidelberg, Im Neuenheimer Feld 236, D-69120 Heidelberg, Germany
*
E-mail: dharlov@gfz-potsdam.de
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Abstract

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 (1–2 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 dissolutionreprecipitation 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.

Keywords

fluorapatitemonazitemetapelitesgranulitesVariscanSchwarzwaldGermany
Type
Research Article
Information
Mineralogical Magazine , Volume 71 , Issue 2 , April 2007 , pp. 223 - 234
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2007

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Footnotes

†

Present address: Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queen’s Road, Bristol BS8 1RJ, UK

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