Abstract

SUMMARY: The sediment-like features of two layered syenites are described and reviewed in the light of recent suggestions that layering in gabbros maybe produced by in situ crystal growth in a near-rigid medium. In the Nunarssuit Syenite igneous layering is normally graded (sharp mafic bases becoming more feldspathic upwards) and channel structures, some with breccias and extreme concentrations of mafic phases, are evidence of magmatic currents. Contorted bedding provides evidence of c. 15 m of crystal mush. In the chemically similar Klokken Laminated Syenite the layers are inversely graded with well-sorted horizons of hedenbergite or fayalite at the top of graded units. Current bedding and rare channels are superimposed o n this inverse grading, and striking load structures occur at junctions with interbedded horizons of Granular Syenite, demonstrating the presence of >3 m of crystal mush. The Klokken Laminated Syenites are slightly more fractionated than Nunarssuit and approach the water-rich termination of fractionation of olivine, clinopyroxene and ternary feldspar. Cumulus phases in Nunarssuit layers are chemically less fractionated than those in ‘normal’ rock and occur with cumulus chevkinite and zircon, consistent with separation of suspended crystals from melt, but cumulus minerals at Klokken do not show this type of chemical contrast. Calculated magma densities and viscosities show that crystal settling of all phases was feasible in both complexes if the magmas were hydrous. Druses at Klokken suggest that the intercumulus liquid was water saturated during load structure formation. Sorting of locally derived crystals by currents accounts for the Nunarssuit layering, but at Klokken the crystal supply was controlled by order-of-nucleation and growth rate effects, although crystal settling occurred.

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