FELDSPATHIC INCLUSIONS IN THE GARDAR IGNEOUS ROCKS OF SOUTH GREENLAND AND THEIR RELEVANCE TO THE FORMATION OF MAJOR ANORTHOSITES IN THE CANADIAN SHIELD

Abstract

Anorthosite xenoliths and plagioclase megacrysts occur in Gardar igneous rocks over an area of approximately 50 000 sq km in South Greenland. The anorthosite inclusions represent a top accumulate of labradorite, which crystallized at a relatively early stage from the developing Gardar magmas at depth, while the megacrysts are regarded as feldspars that remained suspended in their parent magmas for a considerable period at a later stage when the Gardar magmas had developed their dominantly alkaline character. The formation of anorthosite was a continuous process, which probably started about 1300 m.y. ago (the age of the earliest Gardar intrusions) and continued to about 1200 m.y.The Gardar anorthosites formed in the period immediately preceding the development of the Grenville metamorphic belt of the Canadian Shield. They are particularly relevant to the problem of the anorthosites in the Canadian Shield, since they provide evidence for the formation of anorthosites in pre-Grenville, post-Elsonian non-orogenic conditions. It is suggested that the main factor controlling the formation of the two main types of primary anorthosites—those of layered intrusions and those of the almost monomineralic Adirondack type—is the relative specific gravity of the plagioclase and its parent magma. In most layered intrusions, formed from uncontaminated basaltic magmas, the first-formed plagioclases are bytownitic and would sink in their parent magma. In contrast, in alkali provinces such as the Gardar and in orogenic environments where original basaltic magmas may have been contaminated, the first-formed plagioclase is commonly either labradorite or andesine. This will rise in either basaltic or dioritic magma. The separation of plagioclase by flotation from complementary mafic minerals provides a mechanism by which anorthositic rocks can form from basic magmas apparently unaccompanied by mafic layers.Once formed, anorthositic masses of this type would be extremely resistant and could survive several later plutonic episodes as large monomineralic masses in deep-seated mesozonal rocks or the upper katazonal rocks. In contrast the complementary layered mafic rocks would form layered masses in more deep-seated katazonal gneisses.