mineral equilibria in the leven schists near fort william, inverness-shire

Models of upper mantle and crustal processes relating to metamorphism can be proposed if the distribution of temperature and pressure during a metamorphic event is known. Knowledge of the distribution of temperature and pressure during the Dalradian metamorphism in the west and south west Scottish Highlands has not materially increased since Elles and Tilley (1930) applied Barrow's (1912) zones to this area. This study was initiated to improve this situation.
Rocks from an area 3 miles by 2 miles, east and south east of Spean Bridge, 15 miles east of Tort William, Inverness-shire, were studied to find the conditions of metamorphism, temperature and pressure and the composition and behaviour of the fluid phase attending metamorphism. The rocks are siliceous dolomites, carbonate-bearing pelites (calc-pelites), Ca-rich, but carbonate-free pelites, graphitic pelites and graphitic calc-pelites of the Leven Schists metamorphosed in the garnet zone of the epidote amphibolite facies. Characteristic assemblages are :
pelites: quartz-oligoclase-biotite-chlorite-muscovite-garnet-epidote-ilmenite
siliceous dolomites: quartz-oligoclase-phlogopite-chlorite-talc-tremolite-calcite-dolomite-rutile
calc-pelites: quartz-oligoclase-biotite-chlorite or muscovite-epidote-calcite-calcite-dolomite-sphene and/or rutile
The rocks show a considerable variety of assemblages which appear to be in textural equilibrium. The preponderance of low variance assemblages and the occurrence of apparently incompatible assemblages, even on a thin section scale, suggest that the fluid phase composition was buffered by the assemblage. The relationship of fluid pressure to solid pressure and the composition of the fluid phase are discussed and it is concluded that Pfluid = Ptotal and PCO2 + PH2O = Pfluid are good approximations except in the graphitic rocks where methane was an important component of the fluid phase. The metamorphic variables that control the assemblages are considered to be temperature, pressure and XCO2 (and XCH4 in the graphitic rocks).
The descriptions and calculations on the assemblages are considered in terms of a model system, K2O-CaO-MgO-Al2O3-SiO2-CO2-H20, with the phases quartz, anorthite, calcite, dolomite, talc, tremolite, clinochlore, muscovite, phlogopite, clinozoisite and grossularite which are components of minerals found in the assemblages.
A thermodynamic background for the calculation of metamorphic conditions is presented. The calculations are only as reliable as the thermodynamic data. The errors on the best calorimetric determinations of enthalpies of minerals become unacceptably large when applied to the enthalpy change of a reaction because the enthalpy of reaction is often an order of magnitude smaller than the enthalpy of individual minerals. The corollary is that precise relative thermodynamic data can be calculated from experimentally reversed reactions. A calculation method is presented and is used to derive themodynamic data for the model system phases and, in addition, forsterite, enstatite, sphene, wollastonite, annite, Fe-chlorite, almandine, Fe-staurolite, Fe-chloritoid and Mg-cordierite. These data and the activities of the model system components in microprobe analysed minerals are used to infer the metamorphic conditions.
The % MgCO3 in calcite in equilibrium with dolomite provides a geothermometer with the experimental calibration of the calcite-dolomite solvus in MgCO3-CaCO4 by Goldsmith and Newton (1969). The natural carbonates contain iron, but Fe is shown to have little or no affect on the solvus. About 20 calcites from each of seven calcite-dolomite bearing rocks were probe analysed to apply the calcite-dolomite geothermometer.
There is evidence from the area of two metamorphic events, a main event affecting the whole area and a late event affecting only the siliceous dolomites.
The calcites from the rocks, apart from the siliceous dolomites show a narrow range of composition, representing a temperature of 546°C at 2 kbars or 530°C at 6 kbars. There is no indication of a temperature gradient across the area. The analysed assemblages are all univariant in the model system; one variable, tempeature, is fixed. so that the others can be calculated.
A graphitic assemblage
quartz-oligoclase-biotite-chlorite-clinozoisite-tremolite-calcite-dolomite-graphite
is univariant if the fluid phase contained methane. Applying the calcite temperature, this assemblage formed at about 5.6 kbars with XCH4 = 0.30 and XCO2 = 0.40.
Two calc-pelitic assemblages
quartz-oligoclase-biotite-chlorite-epidote-calcite-dolomite
quartz-oligoclase-biotite-muscovite-epidote-calcite-dolomite
give pressures of 4.8 and 5.0 kbars and XCO2 = 0.55 and 0.60 respectively.
The fine grained parts of the siliceous dolomites appear not to have been affected by the late event. Two different assemblages
quartz-oligoclase-tremolite-talc-dolomite-clinochlore
quartz-oligoclase-tremolite-calcite-dolomite-clinochlore
give pressures of 4.5 and 4.7 kbars and XCO2 of 0.3 and 0.8 respectively.
The carbonate-free pelitic assemblages are compositionally far removed from the model system. Although they are model invariant (if XCO2 = 0), reactions that intersect to form the invariant point do not converge in the pressure range of metamorphism. This is interpreted as being due to the failure of the method for calculating activities at this degree of dilution.
The conditions of metamorphism for the main metamorphic event from 1, 2 and 3 above are concluded to be about 5 kbars at 535°C with variable fluid phase composition.
The siliceous dolomites have been recrystallised along irregular surfaces on which large tremolites are developed. The assemblage in the coarse grained part of the siliceous dolomites is model invariant,
quartz-albite-phlogopite-talc-chlorite-tremolite-calcite-dolomite
This represents metamorphic conditions of about 420°C and 1 kbar. The calcites from the siliceous dolomites have a wide spread in %MgCO3. This is interpreted as re equilibration at a lower temperature than the main event, probably between 400°C and 450°C. Fluid inclusion studies (by Mr F.W. Smith , Durham University) on a semi-conformable quartz vein from within the area defines a pressure temperature band for the formation of the quartz vein. This intersects the band of calcite temperatures and corresponds with the calculated conditions for the siliceous dolomite assemblage. The late metamorphic event, at about 420°C and 1 kbar, is interpreted as a late flushing through of the area by magmatic or metamorphic iLuids along fractures through the pelites and into the more permeable rocks along minor planes of weakness to form the coarse assemblage of the siliceous dolomites and partly re-equilibrate the calcites.
The distribution of sphene and rutile in calcite-quartz bearing assemblages cannot be interpreted as equilibrium if the above calculations are crrrect. For sphene to have been stable, the XCO2 must have been less than 0.03. It is considered that sphene is metastable in many of the calcite-quartz bearing assemblages, casting some doubt upon the conclusions of Ernst (1971) on the fluid phase composition during the Franciscan and Sanbagawa metamorphism.