Abstract

North of Davis Inlet, Labrador, an adamellite body of batholithic proportions cuts basement complex gneisses and anorthosite. The adamellite can be subdivided into hypersthene-, fayalite-, hornblende-, and biotite facies. Differences in texture and composition distinguish interior and marginal zones. Dike rocks can be correlated with the intrusive. Hornblende adamellite makes up the bulk of the intrusive. The biotite facies is a late differentiate and in places is pegmatitic. The hypersthene facies is generally marginal or in dikes and was formed early. The exact position of the fayalite facies is uncertain, and its period of formation may overlap that of other facies.

Variations in amount and composition of the minerals in the different facies indicate differentiation without important introduction of material. Field evidence indicates that the adamellite was intruded as a liquid of low viscosity, with much fracturing of the wall rocks and the formation of abundant inclusions. In this respect it differs radically from the anorthosite which is essentially concordant with the older gneisses, suggesting high viscosity of the anorthosite. Indications of primary flow are meager in the adamellite, and evidence of post-consolidation dynamic metamorphism is absent.

Mineral intergrowths are prominent in the adamellite. Hornblende is diffusely poikilitic, and in places biotite and pyroxene are poikilitic. Plagioclase and K feldspar are intimately intergrown in the interior of the adamellite body, and the K feldspar has apparently replaced the plagioclase. Quartz penetrates all these minerals. Textural details indicate replacement.

The contradiction between consolidation of a magma without introduction of material, and widespread replacement by K feldspar and quartz may be explained if the textural relations are interpreted as primary intergrowths modified by deuteric solution which has transferred material to near-by points of greater stability.

Diabase dikes are the only intrusives younger than the adamellite.

The anorthosite north and south of the adamellite is a gabbroic olivine-bearing facies. It is purer northeast of the adamellite and contains chatoyant plagioclase. Flow structures are absent in the anorthosite adjacent to the adamellite, indicating the adamellite penetrated through the marginal zone of the anorthosite.

The gneisses north and east of the adamellite are a variable tonalitic and dioritic series containing both hornblende and pyroxene. Those west of the adamellite are pale garnetiferous biotite gneisses containing variable proportions of K feldspar, oligoclase, and quartz. Graphite is present, and they are of sedimentary origin.

In the western gneisses, contact metamorphism by the adamellite has transformed garnet into a myrmekitic intergrowth of vermicular hypersthene in a cordierite mosaic.

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