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Abstract

Ilmenite deposits occur in anorthosite in the vicinity of Allard Lake, about 25 miles north of Havre St. Pierre (Quebec) on the north shore of the St. Lawrence River. Three main rock types are exposed in the 30-square-mile area examined, which straddles the northeast border of the anorthosite massif: (1) Coarsegrained, massive anorthosite, generally more than 95 per cent calcic andesine; in thin section displays considerable evidence of cataclasis. Locally the anorthosite contains large plagioclase phenocrysts similar in composition to the matrix plagioclase. Irregularly distributed bronzite occurs as (a) interstitial frameworks, poikilitically enclosing plagioclase; (b) approximately aligned aggregates of coarse crystals; and (c) coarse crystals along joint surfaces which transgress primary foliation in the anorthosite. (2) Medium-grained, gneissoid, oxide-rich norite, consisting (in paragenetic order) of 3–50 per cent protoclastically deformed plagioclase (An40-45), 0–40 per cent pyroxenes (hypersthene < augite), 8–10 per cent apatite, 20–54 per cent oxides (hemo-ilmenite < magnetite). This rock occurs in three moderately to steeply dipping sheets, as much as 4 miles long and 3000 feet thick, intruded into the anorthosite. The composition and deformation of the plagioclase suggest that the norite resulted from the incorporation of crushed anorthosite by an oxide-pyroxene-apatite magma. An irregular antipathetic relation between the amounts of pyroxene and oxide in individual samples is unrelated to the position of the samples in the norite sheets. It is interpreted as resulting from immiscibility between the oxide and silicate fractions of this mafic liquid. There is an enrichment in hemo-ilmenite toward the base of two sheets (3) Pvroxene svenite meiss bordering; the anorthosite massif The contact zone is characterized by conformable gneissosity and enrichment in mafic minerals in both the anorthosite and the syenite, and a progressive change in mineralogy toward the syenite. Similar conformably zonal relations between anorthosite and bordering svenites or ovroxene granites are evident in oublished descriptions of other anorthosite massifs.

It is postulated that the anorthosite suite originated by intrusion of a hydrous gabbroic anorthosite magma into a catazonal environment in which the predominant country rocks were salic gneisses. Crystallization resulted primarily from slow outward diffusion of volatiles whereby the bulk of the mafic constituents were concentrated toward the margin. Addition of volatiles to the country rock facilitated its partial fusion to a granitic magma that reacted metasomatically with the anorthosite to produce the pyroxene syenite gneiss.

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