Field recognition of rocks totally retrogressed from granulite facies: an example from Archaean rocks in the Paamiut region, South-West Greenland

Abstract

Since the middle of the 1970s, field criteria have been used in the Archaean high-grade gneiss complexes of southern West Greenland to distinguish amphibolite-facies rocks retrogressed from granulite facies from amphibolite-facies rocks that never reached granulite facies. Examples 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 large 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 grade in most of this geologically well-mapped region was previously considered 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 was not metamorphosed above amphibolite facies.