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 two transects of the Creran Succession and one transect in the 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 relatively low in the lowest grade rocks (0.00042-0.00118% Vol.), variably increase from 550-600 ˚C, and are generally ≤0.00217% Vol. at ~600°C. In samples metamorphosed at >600°C, an increase in monazite abundances occurs in 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 commonly 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.