Published online by Cambridge University Press: 01 May 2009
The Vesturhorn Intrusion of SE Iceland contains a tilted block consisting largely of size-graded, igneous-layered, eucritic gabbro, which is described in some detail. Size-graded, igneous layering is remarkably uncommon and at present there appear to be only two known occurrences. The physical properties of basalt magma suggest that most igneous layering is likely to form from currents moving by laminar flow. Considerations of flow in such a simple system do not satisfactorily account for the scarcity of this structure. It is suggested that size-graded, igneous layering may reflect the availability of convecting magmas containing crystals with appreciable proportions of different grain sizes of each mineral species. Such a grain size distribution, together with other features considered here, may indicate primocryst growth outside the chamber of deposition.