The petalite-subtype Tanco granitic pegmatite, southeastern Manitoba, intru
des an amphibolite (metagabbro), which is metasomatically altered to tourma
line and magnesian annite close to the contact. The composition of exoconta
ct tourmaline depends on the composition of the host rock and on the compos
ition and amount of fluid injected from the pegmatite-forming melt into the
host rock. Two compositional groups of tourmaline occur in the exocontacts
at Tanco: (1) common feruvite - schorl - dravite (Ca- and Mg-rich), and (2
) rare intermediate ternary tourmaline: elbaite - schorl - dravite (Na-, Al
- and Li-rich). The Ca, Mg and Ti contents of group-1 tourmaline increase w
ith distance from the contact between the pegmatite and the host rock. Grou
p-1 and -2 tourmalines crystallize as a result of Na-, Al-, Li- and B-rich
pegmatite-derived fluids infiltrating the Ca-, Mg- and Fe-rich host amphibo
lite, or as a chemical reaction between the pegmatite-forming melt and the
host rock. The exocontact tourmaline is zoned, with a (Fe, Mg)-rich group-1
core and a Li-rich group-2 rim. Early consumption of Ca, Mg and Fe by crys
tallization of feruvite - schorl - dravite increased the chemical potential
of Na, Al and Li in the fluid and promoted crystallization of elbaite - sc
horl - dravite. The rare ternary tourmaline of group 2 crystallized in the
tourmaline aureole from fluids with high chemical potential of B, Al, Na an
d Ca, which are major constituents in tourmaline, but absent or minor in ho
lmquistite. The influx of B-rich acidic fluids promoted crystallization of
tourmaline and prevented crystallization of holmquistite.