Abstract
The Kûgánguaq Member is a lava and tuff sequence comprising about 7.S km3 of magnesian silicic basalts (92%), magnesian andesites (7%) and feldspar-phyric silicic basalts ( 18%) through reaction with crustal rocks, presumably Cretaceous sand or siltstones. The reaction led to sulphide fractionation and to magma modification which cannot be explained in terms of crystal fractionation or by simple mixing between magma and contaminants. The transition element (Fe, Ti, Vand Cr) concentrations in rocks, glasses and minerals indicate that the Kûgánguaq Member rocks equilibrated at oxygen fugacities several orders of magnitude below the FMQ buffer, but above the IW-buffer. The partitioning of vanadium between olivine and glass and between chromite and glass show particularly large variations and appears to be the most sensitive recorder of fo2 variations in the range between the FMQ and IW buffers at high igneous temperatures.
Highlights
The lower Tertiary vo1canic provinee of West Greenland is notable for its large volumes of picritic vo1canic rocks (Clarke & Pedersen, 1976) and for its contaminated and strongly reduced vo1canic rocks which share many features with extratel1uric materials.In the Disko and Nugssuaq regions the early vo1canic sequence - the Vaigat Formation (Hald & Pedersen, 1975; Pedersen, 1985) - is largely composed of picritic lavas and hyaloclastites with minor interlayered sequences of contaminated lavas
The lavas forrned as the result of major geotectonic processes leading to crustal foundering in the Baffin Bay region (e.g. Clarke & Upton, 1971; Van der Linden, 1975; Henderson et al, 1981)
As the vo1canism progressed increasing igneous diversification took place (Clarke & Pedersen, 1976), but in the early stages of activity, corresponding to the lower part of the Vaigat Formation, the magmatism seems to have been very simple: primitive mantle-derived picritic melts low in LIL elements ascended rapidly through a continental crust composed of a Precambrian substratum and a cover of Mesozoic to early Tertiary sediments
Summary
The lower Tertiary vo1canic provinee of West Greenland is notable for its large volumes of picritic vo1canic rocks (Clarke & Pedersen, 1976) and for its contaminated and strongly reduced vo1canic rocks which share many features with extratel1uric materials (see Bøggild, 1953; Pauly, 1969; Pedersen, 1975, 1981). Some contaminated sequences contain extremely reduced basalts and andesites with metallic iron and graphitic pyrometamorphosed shale xenoliths, as described from the Asuk Member (Pedersen, 1978b, 1979a, 1985). In order to be able to characterize all these rocks in terms of their predominant contaminants, the best exposed and most variable iron-free contaminated lava sequence in the Vaigat Formation has been selected for a detailed study; this is aminor vo1canic system on Disko, formalized as the Kuganguaq Member (Pedersen, 1985). The present paper on silicic basalts and magnesian andesites from the Kilganguaq Member forms part of the current investigation on Disko and Nilgssuaq of the contaminated volcanic rocks and their xenoliths, with particular emphasis on their unusual and strongly reduced igneous and pyrometamorphic character. A few kilometres north of the eruptive centre, coastal exposures of the pre-volcanic basement of Cretaceous sediments (fig. 1) have revealed an old escarpment a
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