Chemistry and microstructure of orthoamphiboles from cordierite-amphibole rocks at

ABSTRACT. Orthoamphiboles from the cordierite amphibole-bearing rocks at Outokumpu span a large part of the gedrite-anthophyllite solid-solution series. The amphiboles grew at temperatures near to or just above that of the solvus crest with post-metamorphic cooling resulting in sub-solvus exsolution, the distribu-tion of which is compatible with an asymmetric solvus in which the anthophyllite limb is the steeper. Meta-morphic onditions in the area imply that the crest of the solvus is unlikely to be at a temperature in excess of about 600 ~ Electron microprobe traverses and A1 K~ scans how that he amphibole compositions vary rapidly both along and across grains. These variations can be explained by varying availability of Na controlling the extent of the edenitic substitution i the solid solution series. In this case this variability in Na availability may be explained by the external control of #Na through the fluid phase. TH ~ existence of a solvus in the orthoamphibole solid-solution series between anthophyllite and gedrite has been demonstrated by a number of workers, notably Robinson and co-worker