On Labradorite - Norite with Porphyritic Labradorite Crystals: a Contribution to the Study of the Gabbroidal Eutectic

Abstract

The object of this paper is to contribute to the investigation, on a physico-chemical basis, of the sequence of crystallization in the gabbroidal eruptives. The Lofoten and Vesteraal Islands on the northern coast of Norway (lat. 67° 50′ to 69° N.) constitute, as is well known, a petrographical province with a very considerable number of eruptives, among which orthoclase-plagioclase-rocks, especially monzonite and banatite, are the most frequent. On one of the islands, Flakstadö, there is an area of labradorite-rock comparatively poor in ferromagnesian silicates and magnetite, besides labradorite-norite with somewhat larger quantities of the same. Under the name ‘labradorite-norite’ is denoted a rock which lies chemically and mineralogically on the boundary between labradorite-rock and norite. The field-exposure of the labradorite-rock with the labradorite-norite has a length of sisteen and a half kilometres, a breadth averaging from two and a half to three and a half kilometres, and an area of about 50 square kilometres. To this must be added its continuation under the sea. At the northern margin, towards the Archæan in the neighbourhood of Napp Farm on Flakstadö, the labradorite-norite occurs with a porphyritic development, that is, with phenocrysts of labradorite-felspar, often of remarkably large dimensions. We will now examine this latter rock more closely. The porphyritic labradorite-norite of Napp, like most of the rocks belonging to the eruptive province in question, generally shows little or no sign of pressure. It is in most places coarsely crystalline, with labradorite-phenocrysts measuring as much as 15 or 18 centimetres in length and from 6 to 8 in breadth. It has