Original paper

Stability of REE-bearing minerals in a metaluminous leucotonalite from the Eriksberg gabbro, Transscandinavian Igneous Belt, Sweden

Claeson, Dick T.


This paper describes the accessory mineral assemblage of apatite, monazite, allanite, xenotime, zircon, and an unidentified Th-silicate in a metaluminous leucotonalite from the Eriksberg gabbro, SW Baltic Shield. The leucotonalite dykes are coarse-grained at the contacts with the gabbro and fine-grained in their central part, suggesting that the magma initially was close to or H2O-saturated and experienced a pressure quench. The mineral paragenesis is interpreted to indicate that the magma was initially apatite-saturated and later became saturated in anhydrous phosphates. The loss of volatiles in a pressure quench likely caused the additional crystallization of phosphate species. Zr thermometry suggests an upper estimate of emplacement temperature for the dykes at 794 °C and REE thermometry indicates that solidification of the dykes occurred at temperatures between 752 °C (8 wt % H2O) and 764 °C (6 wt % H2O). Two samples from the leucotonalite dykes plot near the cotectic line in the quartz-albite-anorthite-H2O CIPW-normative system at PH2O = 5 kbar at a liquidus temperature of ≈760-770 °C. Deuteric reaction rims of apatite and allanite developed at the monazite grain-boundaries. An implication of this study is that fluid-rock interaction studies on monazite may in some cases be over-simplistic if xenotime, Th-silicate, and apatite are present as primary inclusions in monazite.


monazitexenotimeallanitemineral stabilitypressure quenchreaction rim