PT PATHS FROM MODAL PROPORTIONS - APPLICATION TO THE KORALM COMPLEX, EASTERN ALPS

Thermodynamic pseudosections portray those parts of a petrogenetic gri d that are relevant to a given bulk composition and the reactions appe aring on them can therefore be used directly to infer the PT path that the rock followed. However, for many 'normal' bulk compositions the u se of pseudosections is hampered by the fact that they display only fe w large fields of high thermodynamic variance in the PT range of inter est. Here it is discussed how modal information on reaction progress w ithin these fields call be used to determine PT path information for t hermodynamically high variant metamorphic assemblages. We use this inf ormation on reaction progress to contour pseudosections for modal prop ortions of minerals using the software package THERMOCALC. The approac h is applied to di- tri- and quadrivariant assemblages from the Koralm complex in the eastern Alps. A PT path for these rocks is derived fro m modal considerations and compared with interpretations of mineral co mposition contours on the same pseudosection and with conventional the rmobarometry. It is shown that at least part of the complex must have cooled initially near isobarically from prevalent peak conditions arou nd 700 degrees C and 14 kbar before the rocks commenced a Barrovian-ty pe decompression path.