XENOTIME-(Y) FROM CARBONATITE DYKES AT LOFDAL, NAMIBIA: UNUSUALLY LOW LREE:HREE RATIO IN CARBONATITE, AND THE FIRST DATING OF XENOTIME OVERGROWTHS ON ZIRCON

Abstract

Xenotime-(Y) is a common accessory mineral in many igneous and high-grade metamorphic rocks, but it is very rare in carbonatite. Uniquely, at Lofdal, Namibia, xenotime-(Y) occurs in many carbonatite dykes. It mantles and replaces zircon in calcite carbonatite and also occurs as aggregates in ankerite carbonatite, aggregates associated with hematite, and crystals associated with monazite-(Ce) and synchysite-(Ce) in highly oxidized iron-rich calcite carbonatites. The paragenetic sequence places the xenotime-(Y) at the end of magmatic activity and certainly into the hydrothermal stage. A U-Pb date of 765 +/- 16 Ma (2 sigma) for xenotime-(Y) overgrowths on zircon obtained by LA-ICP-MS, the first dating of fine overgrowths of xenotime on zircon by this technique, confirms that the formation of xenotime-( Y) is directly related to the crystallization of the carbonatite and provides a date consistent with published dates for Lofdal and Oas syenites. The xenotime-(Y) is heavy-REE-enriched (chondrite-normalized graphs peak at Lu) but can be distinguished from xenotime-(Y) in granitic rocks by the lack of Eu anomaly, higher Gd (reaching > 6 wt%) and lower Yb (below 4 wt%). A monazite-(Ce) - xenotime-(Y) geothermometer developed for metamorphic rocks gives possible but relatively high temperatures of > 450 degrees C for the formation of xenotime-(Y). Overall, the whole-rock compositions are light-REE-enriched, in common with most carbonatites, but the degree of light REE enrichment is less than almost all published datasets (La/Yb-n at Lofdal ranges from 1 to 70), and at 0.5-0.8 wt%, the total REE content at Lofdal is also higher than in many carbonatites. These features are most important in controlling the production of xenotime-( Y) at Lofdal. Exploration for Y in carbonatites should therefore concentrate on rocks that have REE concentrations above 2000 ppm and La/Yb values lower than 70, similar to Lofdal, as well as weathered carbonatite regoliths and carbonatites subjected to extreme hydrothermal conditions, where Y can be concentrated.