THE MESOPROTEROZOIC MIDSOMMERSø DOLERITES AND ASSOCIATED HIGH-SILICA INTRUSIONS, NORTH GREENLAND: CRUSTAL MELTING, CONTAMINATION AND HYDROTHERMAL ALTERATION

Abstract

The Midsommersø dolerites and the flood basalts of the Zig-Zag Dal Basalt Formation, eastern North Greenland, represent a major Mesoproterozoic (~1,380 Ma) igneous event. The intrusive rocks form large sheets within a succession of feldspathic sandstones which underlie the basalts. The geochemistry of the basalts has recently been re-investigated and reported elsewhere in this Journal; here we present new trace element and Nd-, Sr- and Pb-isotopic data for the intrusive rocks. Unlike the basalts, the intrusions yield evidence of considerable interaction and contamination with upper crustal rocks, especially the sandstones. High-silica rocks (80–90 wt% SiO2) occur in sheets, up to 60 m thick. They were formed by mobilisation of sandstones, and indicate a very high rate of emplacement of hot basic magma into the sandstones at depth. These mobilised sandstones (‘rheopsammites’) are among the most SiO2-rich intrusive rocks on earth. Sheets of remobilised granitoid rocks from the crystalline basement (~70% SiO2) are also present. Hydrothermal activity, associated with the igneous event, significantly changed the compositions of the silicic rocks as well as that of many dolerites. Sheets of hydrothermally altered (‘red’) dolerites and silicic rocks invariably have borders of dark, fresh dolerite; this is interpreted to be the result of intrusion from zoned magma chambers. Nd isotope data confirm the crustal origin of the silicic rocks as well as the contamination of some dolerites by components derived from crustal sources, while Sr- and Pb-isotopic systems are strongly affected by the hydrothermal alteration, and give little information on the petrogenesis of the rocks. Recent loss of Sr from hydrothermally altered rocks further affected the Sr isotope systems, and earlier age determinations by the Rb–Sr whole-rock isochron method (1,230 Ma) have proved to be in error. The dolerites and the basalts are geochemically very similar, but most dolerites have moderately negative Eu anomalies that are not observed in the basalts. Eu anomalies in the dolerites could be related to contamination by sandstone at depth, but it is not clear why the basalts escaped a similar contamination.