The petrology and structure of the north eastern Neria area, south-west Greenland

Abstract

The rocks of two areas of the pre-Gambrian basement of S.W. Greenland are described. The suite of plutonic rocks comprises gneisses, amphibolites, ultra-basic rocks, schists, and granites. Particular attention is devoted to the petrology and geochemistry of the amphibolites and it is shown that mineralogical differences between the amphibolites of the two areas cannot be attributed to differences in bulk rock composition. The origin of the amphibolites is discussed in the light of Z-ray fluoreacence rock analyses and it is concluded that they are, probably igneous in origin. The amphibolites of one area contain stable garnet and clinopyroxene, but garnet and stable clinopyroxene are not found in the amphibolites of the other (none of the amphibolites contains stable orthopyroxehe). This difference in mineralogy is interpreted as being the result of "wet" metamorphism near a large autochthonous granite, and "dry" away from it. The grade of metamorphism is revealed by the presence of sillimanite in the schists as being of the upper amphibolite facies. The presence of cordierite in the same rock shows that pressure was only moderate. The country rocks were migmatised throughout what was probably a long period of plutonism. The plutonism ended with the formation of a large autochthonousgranite. Small allochthonous granites surround the autochthonous mass. The large and small scale structures of the areas are described, and the deformational history elucidated as far as is possible. Three periods of Plutonic deformation are recognised, with the main migmatisation having occurred during the second. Numerous dykes, some of which are fresh olivine dolerites and others metadolerites, intrude the plutonic rocks and these also are briefly described.