The petrology and mineralogy of the South Qȏroq centre, Igaliko complex, south Greenland

Abstract

The South Qȏroq Centre is one of four high-level, major intrusive centres comprising the Igaliko Nepheline Syenite complex. Three elliptical stocks of foyaite were emplaced in fairly rapid succession by ring fracture and block subsidence, followed by a partial ring dyke of augite syenite. Petrographic and mineralogical data shows that the intrusions become successfully less differentiated with time. Inward-dipping microsyenite sheets appear to be associated with the ring dyke and four earlier, satellitic stocks occur around the periphery of SS2. Fieldspar, nepheline, iron-titanium oxide, clinopyroxene and amphibole have been investigated in each of the principal rock units by electron-microprobe. Most minerals show a considerable range in composition often with chemical zoning, forming continuous series from the augite syenites through the foyaites. Variations within the three individual foyaites are usually slight, but the alkai feldspar compsitions. Alkali feldspars provide estimates of P(_H(_2)0) and mafic minerals give an indication of f(0(_2)) conditions. Major and trace element analyses of the rocks were made by X-Ray florescence. Variation diagram trends are interpreted mainly in terms of fractionation of feldspar and the mafic phases. Trace element distributions are highly characteristic of fractional crystallisation series, but may not be compatible with progressive partial melting. The analyses are compared with phase equilibria in the experimental systems Q – Ne – Ks and Na(_2)0 – Fe(_2)0(_3) – Al(_2)0(_3) – Si0(_2). It is suggested that the centre evolved from an underlining, differentiated magma chamber, formed by crystal accumulation, possibly with associated liquid fractionation. Successively lower portions of the chamber were tapped, producing batches of fractionated magma. Post-emplacement differentiation was restricted mainly to slight outward diffusion of alkalis and volatiles under a thermal diffusion gradient. Physico-chemical conditions during recrystallisation near to the Igdlerfigssalik Centre are interpreted from textural, geochemical and mineralogical changes.