Abstract

The Lilloise intrusion is one of the major Tertiary plutonic complexes in east Greenland, The intrusion is composed of layered cumulates, beginning with a great thickness of feldspar-free peridotitic rocks and passing upwards into olivine pyroxene plagioclase cumulates in which small scale rhythmic layering is well developed. The mineral assemblage and composition of the Ca-rich pyroxenes is indicative of an alkali basalt parent magma. The layered rocks are cut by numerous veins and dykes of cognate syenitic material. The most voluminous and advanced stages of differentiation contain aegirine augite and annite-rich mica, while the intermediate members are characterized by kaersutite. These late stage veins and dykes are comparable to the products of differentiation of alkali basalt magma as found elsewhere in shallow intrusive bodies.

Major layered ultrabasic or gabbroic intrusions of alkali basalt parentage are rare, indeed none are unequivocally recorded in the recent summary by Wager and Brown (1968). The Lilloise intrusion, which is one of the major Tertiary plutonic complexes in east Greenland, is such a body. From morainic material collected by Wager (1934) the intrusion was previously believed to be an undersaturated syenite and is recorded as such on published maps. The syenite specimens collected by Wager almost certainly originated in the Borgtinderne complex rather than in the Lilloise.

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