Session O08:4P

Lujavrites from the Ilimaussaq Intrusive Complex in South Greenland: Crystallization Conditions and Composition of the Late-Magmatic Fluid Phase

Holger Sommer (markl@ruf.uni-freiburg.de) &

Gregor Markl

Institut für Mineralogie, Albertstrasse 23 B, 79104-Freiburg, Germany

Geological and geochemical evidence suggests that the Ilimaussaq intrusion in the Precambrian Gardar rift province of South Greenland was emplaced as three main pulses - nepheline-augite syenite, quartz-bearing granite and agpaitic rocks. The agpaitic rocks in the Ilimaussaq Intrusion have been dated to ~1168 Ma (Rb-Sr w.r., Blaxland et al., 1976). The third, and main, pulse of peralkaline, silica undersaturated magma lead to the formation of foid-bearing syenitic rocks (sodalite foyaite, naujaite, kakortokite) and extremely fluid-enriched residual melts, the lujavrites. Field and geochemical observations indicate the establishment of two or more, largely isolated sub-magma chambers of lujavrite. Changes from green ägirin lujavrite to black arfvedsonite lujavrite record changes in fO₂, fH₂O and possibly in fHF and fHCl. Interestingly, textural relationships within various samples suggest alternation from aegirin to arvedsonite and back to aegirine stability indicating oscillating intensive parameters of fluid and/or liquid. The final stages of magmatism are marked by the emplacement of arfvedsonite lujavrite and lujavrite extremely rich in villiaumite (NaF) and naujakasite. Reaction textures at naujaite-lujavrite interfaces and the occurrence of hydrothermal veins with abundant analcime, aegirine or arfvedsonite record the expulsion of late-magmatic, relatively low-temperature fluids in the analcime stability field from the lujavrites. These textures involve formation of abundant needle-shaped crystals of arfvedsonite and/or aegirin in veins and at the interfaces growing into the naujaite. On the other hand, aegirine and arfvedsonite in the lujavrites are in many places converted into each other in areas close to naujaite xenoliths. This indicates involvement of fluids originally stored in the naujaite and it may indicate that the lujavrite intruded while the naujaite was not yet fully crystallized (despite sharp borders of naujaite xenoliths in lujavrite). In order to understand the compositional and intrinsic variability in the lujavrites and especially their fluids, new data on F-amphibole and other halogen-bearing minerals are combined with microthermometry results from fluid inclusions and geothermometry based on various amphibole-feldspar-nepheline equilibria. The halogen-rich nature of the fluids is demonstrated by the occurrence Cl-rich ussingite, villiaumite, abundant sodalite and extremely saline fluids with up to 60 wt.% NaCl equivalent.

Blaxland et al., Lithos, 9, 31-38, (1976).

Crystallization Conditions of the Augite Syenite, Ilimaussaq Intrusion, Greenland

Michael Marks (markl@ruf.uni-freiburg.de) &

Gregor Markl

Institut für Mineralogie, Albertstrasse 23 B, 79104 Freiburg, Germany

The Ilimaussaq intrusion is part of the late Precambrian Gardar rift province of South Greenland. The Gardar intrusive complexes fall into two categories: those involving silica-saturated rocks, such as quartz syenite and granite and those involving nepheline-bearing syenitic rocks. Only in the Ilimaussaq intrusion both rock types occur together. The main rock types of the complex are nepheline-bearing augite syenite, quartz-bearing alkali granite and nepheline-bearing syenitic rocks (agpaitic rocks named naujaite, sodalithe foyaite, lujavrite and kakortokite). Geological and geochemical evidence suggest that the intrusion was emplaced as three main pulses: 1) augite syenite(dated by Sm-Nd mineral isochrone to 1.13±0.05 Ga), 2) alkali granitic rocks, and 3) agpaitic rocks. In order to constrain the early evolution of the complex and to be able to compare (and relate?) the various rocks to each other, the augite syenite was examined in detail at various localities. The augite syenite consists of alkali feldspar, olivin, clinopyroxene, amphibole, biotite, nepheline, apatite and magnetite. It forms a shell around the southeastern, southern and western margin of the Ilimaussaq intrusion. This shell is between 10 and 500 m thick and it is commonly separated from the inner agpaitic rocks by an agpaitic border pegmatite. Xenoliths of the augite syenite are found in all other rock units of the intrusion. In addition, the augite syenite forms a horizontal sheet in the roof zone northeast of Lake Taseq. The rock is chilled against the country rocks which consist of Julianehåb granite and Gardar sandstones and volcanics. It shows increase in grain size towards its inner contacts within the complex. This textural trend is accompanied by chemical trends: From the outer to the inner part, the Fe and Mn content in olivine and clinopyroxene increases systematically from Fa₈₁ to Fa₉₄ and from Ac₄Di₅₀Hd₄₆ to Ac₄Di₄₁Hd₅₅ indicating decreasing temperatures with continuous fractional crystallization. Nepheline becomes more abundant with increasing distance from the outer contact indicating decreasing SiO₂ activity. Detailed phase equilibrium studies using the QUILF technique combined with microthermometry of fluid inclusion are used to discuss the intensive parameters (P, T, fO₂, aSiO₂) during crystallization of the augite syenite and its relation to the agpaitic rocks within the Ilimaussaq complex.

Geochemical Behaviour of F and Cl During Crystallization of the Nepheline-Syenitic Units of the Ilimaussaq Intrusion, Greenland

Gregor Schwinn (markl@ruf.uni-freiburg.de) &

Gregor Markl

Institut für Mineralogie, Albertstrasse 23 B, 79104-Freiburg, Germany

The Ilimaussaq Intrusion belongs to the alkaline, probably rift-related intrusions in the Gardar Province of South Greenland. The approximately 1.18 Ga old intrusion exhibits the greatest diversity of different rock types of all Gardar intrusions and consists of principally three rock suites: nepheline-bearing augite syenite, quartz-bearing alkali granite and a wide variety of foid-bearing syenitic rocks. These three main rock suites are considered to represent three pulses of different kinds of magmas. The alkaline granite is found in the highest portions of the intrusive complex. It intrudes the augite syenite which forms an early shell into which - partly separated from the interior by a border pegmatite - the later foid-syenitic to agpaitic magma intruded. This central part consists of a sequence of different agpaitic foid syenites from sodalite-foyaite at the top, via naujaite, lujavrite to kakortokite at the bottom. The genetic relationship among these various rock types is still not understood. They are considered to represent flotation cumulates (sodalite foyaites, naujaites with up to 80 vol.% sodalite), bottom cumulates (kakortokites, but unclear of which magma, maybe of the naujaitic magma?) and very fluid-rich residual, highly differentiated melt (lujavrites). Early anhydrous mineral assemblages (pyroxene, olivine, feldspars, nepheline) in the foyaite develop towards hydrous assemblages with F-bearing amphibole and with aenigmatite in foyaite, kakortokite and naujaite, and finally to the extremely volatile (F and Cl)-rich lujavrites. The early changes in melt composition are indicated by the chemical changes from low-Zr to high-Zr aegirin augite that is overgrown by F-amphibole which in turn is again overgrown by Zr-free aegirine. These various phase assemblages reflect variability in the composition of both the coexisting fluids and the coexisting melt. The early stages of the crystallization history of the central portion of the Ilimaussaq intrusive complex is deduced from phase equilibria among the early, anhydrous minerals. Fluid compositional changes in terms of fO₂, fH₂O, fHF and fHCl are critically important for the development of agpaitic rocks. Further insight into the variations in these parameters is gained by the combination of these calculations with fluid inclusion microthermometry and with calculations on halogen-bearing minerals (mainly amphiboles) involving aegirine. We present systematic data from all units of the central part of the Ilimaussaq intrusion complex.