3 NOVEL ISOGRADS IN METAMORPHOSED SILICEOUS DOLOMITES FROM THE BALLACHULISH AUREOLE, SCOTLAND

Isograds were mapped in siliceous dolomites from the Ballachulish aure ole on the basis of the formation of geikielite (MgTiO3), baddeleyite (ZrO2), and qandilite (Mg2TiO4) by the following model reactions: ruti le + dolomite = geikielite + calcite + CO2, zircon + 2 dolomite = badd eleyite + forsterite + 2 calcite + 2 CO2, and geikielite + periclase = qandilite. The (T, X(CO2)) conditions of reaction inferred from (1) m ineral equilibria in pelitic rocks, (2) calcite + dolomite thermometry , and (3) the diopside + dolomite + forsterite + calcite + tremolite a nd dolomite + periclase + calcite equilibria are: geikielite isograd ( 640-655 degrees C, 0.76-0.80); baddeleyite isograd (660-710 degrees C, 0.76-0.95); qandilite isograd (725-755 degrees C, <0.08). These T-X(C O2) conditions for the geikielite and baddeleyite isograds are the sam e within error of those independently estimated from rutile + dolomite + geikielite + calcite and zircon + dolomite + baddeleyite + forsteri te + calcite equilibria using Berman's thermodynamic data base. Rutile , zircon, geikielite, and baddeleyite are common in dolomites from at least two other contact aureoles in Scotland and Montana. Although the minerals occur in concentrations <0.01%, they appear to have been in local equilibrium during metamorphism both with each other and with co existing carbonates and silicates and are potentially useful but curre ntly unexploited records of the physical conditions of metamorphism.