Texture, composition, and origin of rutile in the South Mountain Batholith, Nova Scotia

The South Mountain Batholith (SMB) is a peraluminous granitic complex containing characteristic minerals such as biotite, muscovite, garnet, cordierite, andalusite, ilmenite, and rutile. Rutile occurs as a rare foreign phase in the most primitive rocks of the batholith, and as a more common primary magmatic phase in the moderately to highly evolved rocks of the SMB. Grain sizes range from 0.04 to 0.28 mm (rarely to 0.54 mm), and grain shapes range from rare euhedral to commonly anhedral. In addition, many of the late grains of rutile show patchy and oscillatory zoning. With increasing chemical differentiation of the SMB, the rutile grains have increasing concentrations (average low-high values in wt.%) of Nb2O5 (0.36-2.28), Ta2O5 (0.04-0.63), FeO + MnO (0.22-1.36), SnO2 (0.02-0.09), WO3 (0.03-0.67), and ZnO (0.01-0.04). In pegmatitic rocks, rutile has high (Nb + Ta) concentrations and low Nb/Ta values, possibly reflecting variable partitioning of Nb and Ta into a fluid phase. Magmatic rutile grains show some mixed (Nb + Ta) and (Fe + Mn) substitution, with rutile from pegmatites showing a substitution toward columbite-tantalite end-members. We conclude that rutile became a primary magmatic phase in the SMB in the middle stages of its chemical evolution, that rutile is a sensitive monitor of Nb-Ta-Sn-W in the batholith, and that oscillatory zoning in rutile is the result of either alternations between fluid-present and fluid-absent conditions or diffusion-controlled growth.