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Grant Institute of Geology, University of Edinburgh, West Mains Road, burgh EH9 3JW, Scotland
Interlayered lithologies of diverse mineralogy enclosing the Middle Dalradian syngenetic BaZnPb deposit at Foss, S of Loch Tummel in Perthshire, allow estimates of the PT conditions of metamorphism using several new geothermometers and geobarometers. Temperatures of 500560°C are derived from calcitedolomite geothermometry and from data on MgFe partitioning within garnethornblende and garnetbiotite pairs. Plagioclasebiotitegarnetmuscovite and biotitemuscovitechlorite equilibria indicate peak metamorphic pressures of 810 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(BaA12Si2O8 H2O) was widespread in mineralized lithologies during penetrative deformation, and has since largely been replaced by celsian.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 PT location of the experimentally determined reaction line, cymritess = celsianss + 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.
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