LITHOLOGICAL CORRELATION WITH TRACE ELEMENT ABUNDANCES AND MONAZITE STABILITY: IMPLICATIONS FOR FLUID AVAILABILITY IN THE METAMORPHIC AUREOLE OF THE BALLACHULISH IGNEOUS COMPLEX

Monazite abundances and textures, and whole-rock compositions are documented and interpreted as a function of temperature and fluid availability in three transects of the Creran Succession and Appin Quartzite-Ballachulish Slate transition series in the aureole of the Ballachulish Igneous Complex, Scotland. In a transect sub-perpendicular to the regional foliation of the Creran Succession, FeO and MgO increase with increasing SiO₂, while Al₂O₃ decreases. There are no systematic changes in major element abundances with temperature, although REE abundances show a slight decrease with increasing temperature. In a transect parallel to foliation, FeO and MgO increase while Al₂O₃ and SiO₂ decrease with increasing temperature. REE abundances increase from regionally to thermally metamorphosed samples, reaching a maximum at 615°C and remaining nearly constant thereafter. The third transect, collected foliation parallel in the Appin Quartzite-Ballachulish Slate transition series, has increasing FeO and MgO and decreasing Al₂O₃ with increasing SiO₂ content, but no systematic variations in bulk rock compositions or REE abundances with temperature are observed.

Monazite abundances are variable and relatively low in the lowest grade rocks (0.00042-0.00118% Vol.), and, generally decrease to ≤0.00217% Vol. at ~600°C. In samples metamorphosed at >600°C, an increase in monazite abundances occurs in all all transects (≤0.013% Vol.). From regional grade through ~610°C in rocks of the Creran Succession transect collected foliation-parallel, monazite occurs as individual grains or inclusions. In equivalent rocks of the other transects, monazite occurs as elongate clusters and frequently in association with biotite. Monazite above 610°C in all transects shows no systematic variation in grain size or distribution.

Monazite occurrence and abundance correlate with temperatures through 600-620°C, but not necessarily with whole-rock major or rare earth element abundances. In the Creran Succession, the transect parallel to foliation shows increased monazite abundances with higher REE abundances. However, in the transect sub-perpendicular to foliation the pattern is non-systematic. This suggests that there is a second-order process or REE-mineral controlling the local distribution and abundance of monazite.