Three novel isograds in metamorphosed siliceous dolomites from the Ballachulish aureole, Scotland

Abstract

Isograds were mapped in siliceous dolomites from the Ballachulish aureole on the basis of the formation of geikielite (MgTi03), baddeleyite (Zr02), and qandilite (Mg2Ti04) by the following model reactions: rutile + dolomite = geikielite + calcite + C02' zircon + 2 dolomite = baddeleyite + forsterite + 2 calcite + 2 C02' and geikielite + periclase = qandilite. The (T, Xco,) conditions of reaction inferred from (1) mineral equilibria in pelitic rocks, (2) calcite + dolomite thermometry, and (3) the diopside + dolomite + forsterite + calcite + tremolite and dolomite + periclase + calcite equilibria are: geikielite isograd (640-655 °C, 0.76-0.80); baddeleyite isograd (660-710 °C, 0.76-0.95); qandilite isograd (725-755 °C, <0.08). These T-Xco2 conditions for the geikielite and baddeleyite isograds are the same within error of those independently estimated from rutile + dolomite + geikielite + calcite and zircon + dolomite + baddeleyite + forsterite + 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 coexisting carbonates and silicates and are potentially useful but currently unexploited records of the physical conditions of metamorphism.