A Study of the Fe–Ti Oxides in the Whitestone Anorthosite, Dunchurch, Ontario

The Fe–Ti oxides hemo-ilmenite, magnetite, and minor ferrian ilmenite occur predominantly in the foliated oxide-rich, garnet-bearing border facies of the Whitestone anorthosite. The rhombohedral phases comprises 80–90 modal % of the oxides, and shows variation in bulk composition from Ilm 90 Hem 10 to Ilm 50 Hem 50 (mol %), which we interpret to be a result of local variation in [Formula: see text]. Hemo-ilmenites with bulk compositions more ilmenite rich than approximately Ilm 85 Hem 15 exsolve predominantly by a continuous exsolution mechanism, whereas more hematite rich phases exsolve discontinuously. Along the hematite limb of the solvus, the exsolution mechanism changes at a bulk composition of approximately Hem 60 Ilm 40 . The compositions of hematite lamellae and host phase ilmenite in hemo-ilmenite are constant at Hem 72 Ilm 28 and Ilm 95 Hem 5 respectively, as indicated by electron-probe analyses. This suggests that the position of the hematite–ilmenite solvus is at more ilmenite rich compositions than was determined by Carmichael (1961).Magnetite contains less than 3.5 mol % Usp, and the average unit cell dimension of 14 samples is 8.396 ± 0.004 Å. In hemo-ilmenite grains adjacent to magnetite, hematite lamellae thin and disappear leaving a zone up to 50 μ wide of homogeneous ferrian ilmenite (Ilm 95 Hem 5 ). This ilmenite was added during cooling, as the result of oxidation of ülvospinel in magnetite. We envisage that oxides of the Whitestone anorthosite cooled slowly, from an initially high temperature stage in a tectonically quiet environment.