Layering and Net Veining in Hornblende Lamprophyre Intrusions from the Coast of Labrador

Abstract

Dikes of hornblende lamprophyre up to 10 m thick occur extensively southeast of Cape Harrison, on the coast of Labrador. Many exhibit well-marked layering parallel to their walls. Individual layers can be traced for more than 300 m in flat-lying dikes, but layering is absent in those dipping at more than 30°. Some layers are isomodal, some show an increase in grain size upward, while others decrease upward. Other layering results from variations in the ratio of hornblende to plagioclase. Both normal and reverse-density grading occur. The habit of the hornblende crystals is equant in some layers and acicular in others. Both plagioclase and hornblende are often strongly zoned and partly altered. Some of the intrusions contain disturbed and net-veined layers adjacent to undisturbed layers. In the deformed layers, feldspathic material forms a matrix for fragments of hornblendic layers. Layering in igneous rocks has usually been attributed to crystal settling from a convecting magma. However, in these rather t...