ANORTHOSITE-ADAMELLITE COMPLEX OF NAIN, LABRADOR

Abstract

The anorthosite-adamellite complex has invaded basement-complex gneisses which have a low Na 2 O/K 2 O ratio and show many features of the granulite facies. Ultramafic rocks of the Alpine type occur. Assignment of the metamorphic rocks to the Grenville Province is at least open to question. The anorthositic rocks are subdivided into three facies. A dark facies is characterized by dark plagioclase and olivine. A pale facies is characterized by pale plagioclase and a low content of hypersthene with thin exsolution lamellae. A buff-weathering facies is characterized by antiperthitic inclusions in plagioclase, more abundant pyroxenes with exsolution lamellae indicating inversion from pigeonite, and accessory quartz. There is an imperfect progression from the dark facies in the north and east through a central area of pale facies to the buff-weathering facies in the south and west. Adamellite margins are variable in character near anorthosite and in places show clearly intrusive relations. A short distance within the margin a fayalite facies appears. In it plagioclase and pyroxenes resemble those minerals in buff-weathering anorthosite. It is greenish gray when fresh but rusts readily. The fayalite facies is succeeded westward by a hornblende facies in which much of the hornblende is poikilitic. There is also a biotite facies with intrusive relations toward the fayalite facies. Possibly it is more abundant toward the west margin of the complex. Small intrusive bodies occur, in many cases in gneiss zones involved with the anorthosite-adamellite complex. Many of them show affinities with the adamellitic rocks. Some olivine gabbroic rocks may be related to the anorthosite. Variations in composition of rocks and minerals of the complex could result from repeated intrusion of magmatic material evolving through differentiation by partial crystallization, with the zone of intrusion migrating westward. Faulting occurred in the region, but field data are insufficient for evaluation.