The Tanco pegmatite at Bernic Lake, Manitoba. XIV. Internal tourmaline

The zoned petalite-subtype Tanco granitic pegmatite, located about 180 km e ast-northeast of Winnipeg, near the Manitoba-Ontario border, intrudes amphi bolite. Tourmaline is not abundant, but is widespread in most of the pegmat ite zones. Tourmaline is black, brown or rarely green in the border zone (1 0), wall zone (20), aplitic albite zone (30) and central intermediate zone (60); it is pink and rarely green in the petalite-bearing lower and upper i ntermediate zones (40) and (50). On the basis of paragenesis and compositio nal zoning within individual tourmaline crystals, tourmaline at Tanco is ch aracterized by the following sequence of crystallization: foitite - schorl --> (+/- schorl - foitite) --> Al-rich schorl (+/-Mg) --> schorl (+/-Mg) -- > schorl - elbaite (+/-Mg) --> "fluor-elbaite" - schorl (+/-Mg) --> Fe-rich "fluor-elbaite" --> Mn-bearing "fluor-elbaite" --> rossmanite --> elbaite --> elbaite --> rossmanite --> (+/- Ca-bearing "fluor-elbaite", Ca-bearing "fluor-elbaite" - rossmanite). The dominant substitution at the X site is N a reversible arrow square, and there is a positive correlation between prop ortions of Na and F. The dominant substitution at the Y site is 3Fe(2+) rev ersible arrow 1.5A1+ 1.5Li, which is controlled by fractionation of the peg matite-forming melt. Magnesium and Ti incorporated from the host amphibolit e into the pegmatite-forming melt generated endomorphic Mg-bearing schorl t o Al-rich Mg-bearing schorl to Mg-bearing schorl - elbaite to Mg-bearing el baite - schorl in zones (10), (20) and (30). Sodium, Mn and F all increase from foitite schorl to "fluor-elbaite" - schorl and Fe-rich "fluor-elbaite" in zones (10), (20), (30) and (60). In late-stage tourmaline, Ca and F inc rease from rossmanite-elbaite to Ca-bearing "fluor-elbaite" - rossmanite to Ca-bearing "fluor-elbaite" in zones (40) and (50). Late Ca-enrichment is d ue to sequestering of Ca in the melt as fluoride complexes. This is the fir st occurrence of foitite, rossmanite and late-stage Ca-enrichment in tourma line reported a petalite-subtype pegmatite.