FIELD RECOGNITION OF ROCKS TOTALLY RETROGRESSED FROM GRANULITE-FACIES- AN EXAMPLE FROM ARCHEAN ROCKS IN THE PAAMIUT REGION, SOUTH-WEST GREENLAND

Since the middle of the 1970s, field criteria have been used in the Ar chaean high-grade gneiss complexes of southern West Greenland to disti nguish amphibolite-facies rocks retrogressed from granulite facies fro m amphibolite-facies rocks that never reached granulite facies. Exampl es of the textural criteria are figured from different areas. Many of them are the result of growth, during dehydration partial melting, of large grains of orthopyroxene that were subsequently retrogressed to l arge grains or aggregates of hornblende. An example of the use of this field technique is given from the Paamiut region in the southern part of the Archaean craton. Rocks metamorphosed to granulite facies, but partly or totally retrogressed to amphibolite facies, have been found to crop out over about half of the Paamiut region. The metamorphic gra de in most of this geologically well-mapped region was previously cons idered not to have exceeded amphibolite facies. The Paamiut region can now be shown to be made up of three tectonically-bounded blocks. The Sioraq block in the north and the Neria block in the south, which were metamorphosed to granulite facies, overlie the Paamiut block, which w as not metamorphosed above amphibolite facies. (C) 1997 Elsevier Scien ce B.V.