The Midsommersø Dolerites and Associated Intrusions in the Proterozoic Platform of eastern North Greenland—a Study of the Interaction between Intrusive Basic Magma and Sialic Crust

Three types of intrusive rocks occur in the Proterozoic sandstones of the Independence Fjord Group in eastern North Greenland: (1) dolerites, (2) granophyric rocks, and (3) mobilized sandstones, here termed ‘rheopsammites’. Many of the dolerites and the acid rocks are strongly altered, and are red-brown to brick red in colour. This alteration is due to hydrothermal activity associated with the intrusion of the rocks at c. 1230 Ma. The rocks show a wide variation in chemical composition due to: (1) fractionation of the basic magma at depth, (2) anatexis of basement gneisses and sandstones under the influence of the basic magma, (3) assimilation of material from the basement gneisses and the sandstones by the basic magma, (4) hydrothermal metasomatism after emplacement of the intrusions, and (5) in some cases fractionation after intrusion. Study of the field relations, petrography, major element compositions, selected trace elements and Rb–Sr whole-rock isotope data in most cases gives a general impression of the relative importance of these processes in the formation of individual intrusions. The relationships are, however, complex and some aspects of the mechanism of contamination remain unsolved.