The geochemistry and petrology of tertiary basalt lavas from east Greenland

Abstract

The 2,660m-thick basalt pile described in this thesis represents one of the upper parts, which were extruded predominantly under subaerial conditions, of the Tertiary basalt sequence in East Greenland. The pile rests on Lower Cretaceous sediments and is overlain by the sediments of Kap Dalton Formation Chemically (XRF analysis), the basalts can be classified as ocean-island tholeiites and are broadly comparable to the Tertiary tholeiites from mid-west Iceland and the tholeiites of the Neovolcanic zone of Iceland. Although they have rather limited compositional ranges, they show evidence of two episodes of fractionation. The magma compositions were controlled largely by olivine fractionation in the initial stage and subsequently by plagioclase fractionation. Comparisons between bivariate and multivariate analyses have been made and it is found that in the case of rather narrow compositional ranges, the multi variate analysis (R-mode factor analysis) is the best method for illustrating geochemical patterns. Petrographically, the basalt suite consists chiefly of plagioclase (average labradorite), augite and Fe-Ti oxides with small amounts of olivine, pigeonite and interstitial glass, except for pillow lavas where the groundmass is entirely glass. The textures are variable from glassy to coarse-grained types. Thus they can be divided, in terms of textures and field observations, into four main groups and then subdivided into a number of rock types. The majority appear to be aphyric types. The principal microphenocryst and/or phenocryst assemblages are plagioclase, plagioclase + olivine, plagioclase + olivine + augite, Fe-Ti oxides, or Fe-Ti oxides + plagioclase. The plagioclase compositions (electron probe analysis) vary from An(_83.4)Ab(_16.3)Or(_0.3) to An(_27.3)Ab(_68.7)Or(_4.0). The great majority of analysed clinopyroxenes are augite and they follow the equilibrium fractionation trend typified in the Skaergaard tholeiitic intrusion. Olivine microphenocrysts and groundmass olivines were analysed and have chrysolitic and hyalosideritic compositions, respectively. Temperatures and oxygen fugacities, which have been estimated from the coexisting phases of titaniferous magnetite and ilmenite, are comparable to those of the Icelandic Thingmuli tholeiitic suite. The origin of these East Greenland tholeiites is attributed to low-degree partial melting of undepleted mantle beneath the region on the present seaward side of the Blosseville coast and, subsequently, low-pressure crystal fractionation.