The earlier gardar igneous rocks of the Ilimaussaq area of South Greenland

Abstract

During the earlier part of the Gardar Period, contental sandstones and volcanics (principally flows of olivine basalt) were laid down in the Ilimaussaq region; strata with a maximum thicjness of over 3km. are preserved. The region was characterised by block faulting with sizeable dislocations and the stratigraphic succession on each fault block is different. The geology of each of the blocks is discussed in turn and, in a final synthesis, the successions of the blocks are correlated and a preliminary analysis made of the fault system. In addition to the basalt magma, which was erupted quietly in great quantities, a quite separate volatile-fich ultramafic magma was available in the region at intervals during the early Gardar. It was responsible for the drilling of numerous diatremes and for a phase of intense, explosive volcanic activity. There is clear evidence of the presence of a related, concealed carbonatite body, located near Qagssiarssuk, in the north of the area. The associated lampropbyric rocks in this vicinity have been subjected to calcitic carbonatisation, followed by ankeritic carbonatisation, while the country rocks have been subjected to potash feldspathisation. Necks, sills and flows of carbonatissd uncompahgrite occur. The petrography of the volcanic rocks is discussed in some detail and chemical analyses are provided. Separate studies, in which X-ray techniques were used extensively, have been made of the carbonates, the alkali feldspars and the trace elements, in rocks associated with the carbonatite. The lamprophyre-carbonatite vulcanism has many parallels in the volcanic fields of East Africa, but the presence of olivine basalt magnia in the area at the same time is an unusual feature. On occasions, the two magmas were erupted almost simultaneously, yet there is no indication of intermingling or mutual contamination.