Original paper

Origin of fluorapatite-monazite assemblages in a metamorphosed, sillimanitebearing pegmatoid, Reinbolt Hills, East Antarctica

Ziemann, Martin A.; Förster, Hans-Jürgen; Harlov, Daniel E.; Frei, Dirk

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European Journal of Mineralogy Volume 17 Number 4 (2005), p. 567 - 580

published: Jul 25, 2005

DOI: 10.1127/0935-1221/2005/0017-0567

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Abstract

Thermobarometrical and mineral-chemical investigations by electron microprobe and LA-ICP-MS on a sillimanitebearing pegmatoid from the Reinbolt Hills provide important constraints on the P–T–X–age relations of part of East Antarctica during Pan-African tectonism. U–Th–total Pb ages of monazite imply that the pegmatoid of originally Grenvillan age (zircon U–Pb age of ca. 900 Ma) underwent a major, late Pan-African (Cambrian) regional, granulite-facies metamorphism between 500 and 550 Ma. Most of the monazite formed during this event, as result of apatite metasomatism owing to infiltration of high-grade metamorphic fluids. Apatite–biotite and other mineral thermobarometers define the peak metamorphic temperatures and pressures with 850–950 °C and 0.8–1.0 GPa. The F–Cl–OH relations in apatite and biotite, the chemistry of fluid inclusions and the presence of K-feldspar microveins suggest that the metasomatising fluid was a CO2-bearing, diluted KCl brine. The pegmatoid is the first record of monazite-(Ce) formed from fluorapatite that is rich in U (up to 2.6 wt % UO2) and possesses Th/U ratios < 1 (0.09 on average). These chemical signatures are direct reflection of the U and Th concentration patterns in the parental fluorapatite.

Keywords

fluorapatite • monazite • metasomatism • la-icp-ms • pan-african orogeny • east antarctica