Palaeocene ignimbrite from Ubekendt Ejland,west Greenland

An intensely-welded ignimbrite has been identified in the upper part of the thick Palaeocene lava pile of Ubekendt Ejland, west Greenland. It consists of 2.3 volume present of phenocrysts and 2 percent of lithic fragments (basalt and trachyte) in a compact nitreous matrix of flattened and welded rhyolitic pumice fragments and glass shards. The phenocrysts are mostly of plagioclase (An₁₇), together with minor amounts of sanidine (Or₈₇), hydrothermally-altered olivine and augite, ilmenite and zircon. The major elements chemistry of the magmatic fraction of the rock is deduced from microprobe analyses of its constituent phases. It is suggested that this erupted pyroclastic rock may be a surface expression of high-level granite emplacement in southern Ubekendt Ejland. The source could have been an early acid member of the currently-exposed Sarqâta qáqâ plutonic complex. Recent Rb/Sr isotopic studies have shown that the acid and basic rocks of this intrusive centre, together with the surrounding basaltic lavas, define a single isochron (65 m.y.) with an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.7045, appropriate to the upper mantle. The ultimate source of the Ubekendt Ejland ignimbr.es may therefore be sub-crustal.