Igneous layering in the syenites of Nunarssuit and West Kûngnât, South Greenland

• Main Author: Hodson, Mark Edward
• Published: University of Edinburgh 1994
• Subjects: 552
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.652493

Field work was carried out to record the layering phenomena exhibited by the two syenitic bodies. In both cases the layering developed close to the edge of the intrusion. The rhythmic layers are c.20cm thick and have a thin, c.3cm, melanocratic base. Over a short distance this grades into leucocratic syenite which has the same modal composition as homogeneous syenite above and below the layered series. Cumulus phase are pyroxene, olivine, feldspar, apatite and opaque oxides and sulphides. The occurrence of the Nunarssuit layered syenite is cyclic. In each cycle the bases of layers become more melanocratic up section. Each cycle is capped by a thick melanocratic layer. Two fully developed cycles are present. A third cycle starts to develop but then layering fades and the syenite becomes essentially homogeneous. Troughs, rich in mafic phases, and unconformities between layers are present in both layered series. Slumps and slump breccias are present in Nunarssuit. Samples from across individual layers, groups of adjacent layers, distantly spaced layers, slump structures and breccias were collected. Electron probe, synchrotron XRF-microprobe and XRF whole-rock analysis were used to determine mineral and rock chemistry. δ<SUP>18</SUP>O values were determined for Nunarssuit samples. Grain size analysis, cathodoluminescence studies, and SEM and electron probe BSE image analysis were used for textural analysis. The composition of the magmas from which the syenites crystallised was estimated and its physical and chemical properties determined. The samples from the Nunarssuit layered syenite become more magnesian up section. In West Kûngnât the samples become more ferroan up section. In both intrusions samples from trough layers are more magnesian than samples from adjacent syenite. Fluid dynamic calculations indicate that the crystallising magma convected turbulently. Crystals would have been kept in suspension in the body of the magma but may have settled in stagnant boundary layers along the bottom and sides of the chamber. Non-linear analysis indicates that the layer producing mechanism was chaotic. Textures and mineral chemistry were modified after crystallisation due to interaction with fluids. At high temperatures, below the solidus, small grains of olivine and pyroxene were absorbed by larger grains of the same mineral species. At temperatures as low as c.450°C fluids reacted with the syenites and: 1) pyroxenes were partially altered to amphiboles and pyroxene rims were enriched in the acmite component, 2) olivines were altered to biotites, 3) feldspars exsolved and, 4) apatites and zircons lost some or all of their original zoning.