Metamorphic conditions and uplift history in central Perthshire: evidence from mineral equilibria in the Foss celsian–barite–sulphide deposit, Aberfeldy

Interlayered lithologies of diverse mineralogy enclosing the Middle Dalradian syngenetic Ba–Zn–Pb deposit at Foss, S of Loch Tummel in Perthshire, allow estimates of the P–T conditions of metamorphism using several new geothermometers and geobarometers. Temperatures of 500–560°C are derived from calcite–dolomite geothermometry and from data on Mg–Fe partitioning within garnet–hornblende and garnet–biotite pairs. Plagioclase–biotite–garnet–muscovite and biotite–muscovite–chlorite equilibria indicate peak metamorphic pressures of 8–10 kbar which are in agreement with the results of sphalerite geobarometry.

Textural evidence indicates that the rare platy mineral cymrite (BaA1₂Si₂O₈ H₂O) was widespread in mineralized lithologies during penetrative deformation, and has since largely been replaced by celsian. The P–T location of the experimentally determined reaction line, cymrite_ss = celsian_ss + H₂O, places further constraints on the peak metamorphic conditions and suggests that the final episode of penetrative deformation (D₃) was followed by a period of near-isothermal uplift during which celsian became stable. The cymrite field appears to have been re-entered during late brittle deformation. Peak metamorphic conditions recorded by mineral equilibria at Foss differ from those of areas 10 km to the N and S.