The South Qôroq Centre nepheline syenites, South Greenland. Petrology, felsic mineralogy and petrogenesis

Abstract

The South Qoroq 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. Inward-dipping microsyenite sheets appear to be associated with the ring-dyke; and four earlier, satellitic stocks occur around the periphery of the centre. Petrographic and mineralogical data show that the intrusions become successively less differentiated with time. Felsic mineral phases (alkali feldspar, nepheline and sodalite) constitute over 80% of most rocks from the centre. Electron-microprobe analyses demonstrate the major role of felsics in the fractionation of the magma and, together with estimates of feldspar structural state from 2V measurements, give indications of the history and conditions of crystallisation. Nepheline compositions fall within a small range of decreasing Si content, but are outside the Morozewicz-Buerger convergence field and are not affected by sub-solidus alkali exchange. Feldspars form a continuous series from Or₁Ab₆₇An₃₂ to a K-enriched alkali feldspar Or₇₂Ab₂₈An₀. From comparison with other rock suites, this extended feldspar trend seems to be associated with the co-precipitation of nepheline, and increasing peralkalinity of the magma. Major and trace element analyses of the rocks, made by X-ray fluorescence, give variation trends which may be interpreted mainly in terms of fractionation of feldspar and the ferromagnesian phases. In particular, trace element distributions are highly characteristic of fractional crystallisation series, but may not be compatible with fractional melting. Analyses of rocks with co-existing felsic phases compare favourably with phase equilibria in the experimental system Q-ne-ks at 1kb. It is suggested that the centre evolved from an underlying differentiated magma chamber, formed by crystal fractionation and accumulation. Successively lower portions of the chamber were tapped, producing batches of fractionated magma. Later stages of crystallisation were influenced by a build-up in volatiles consisting of H₂O, CO₂, Cl and F, and post-emplacement differentiation was implemented mainly by outward diffusion of these volatiles together with alkalis under a thermal diffusion gradient. Temperatures of crystallisation deduced from the nepheline geothermometer (Hamilton, 1961) and from phase equilibria in the Ab-Or-H₂O system are in reasonable agreement. Assuming a P_H2O of about 1 kb, the foyaite feldspars crystallised at about 850°C and augite syenite feldspars slightly higher. Nephelines commenced crystallisation within the range 900-850°C but stabilised at 775-700°C irrespective of rock-type. Physico-chemical conditions during recrystallisation attributable to the later Igdlerfigssalik Centre are inferred from textural, geochemical and mineralogical changes. The recrystallised rocks provide evidence for the behaviour of trace elements during the initial stages of remelting under hydrous conditions in an open system.