Some Chemical Characteristics and Variations of Gneisses, Intermediate Intrusive Rocks, and Anorthosite in the Lac Rouvray Map-area, Quebec

Abstract

A sequence of high-grade gneisses acted as the host for the intrusions of anorthosite and intermediate rocks in the Lac Rouvray area. Based on structural data both these intrusive episodes are closely related spatially as well as temporally.Differentiation trends of the intermediate rocks show enrichment in alkalies relative to iron and magnesia. General agreement exists between the Lac Rouvray series and average calc-alkalic trends of differentiation, suggesting magma generation at considerable depth with emplacement of the intrusions into the crust at some horizontal distance from the most actively deforming part of the orogen.It is now generally accepted that anorthosites are crystal cumulates and therefore compositionally different from their parental magma. Anorthositic rocks have been included in some of the figures for completeness. Owing to their unique composition and general lack of variation, they occupy isolated regions in the projections.High contents of silica typify the gneisses. With increasing silica, variation trends of major oxides are in most respects similar to those of the intermediate suite. Structural and textural evidence, however, demonstrates these rocks to be unrelated.Soda contents for the gneisses and intermediate rocks are nearly constant over a considerable range in potash and lime. Linear decrease in iron and magnesia with increasing alkali content is reflected by all rocks. Anorthositic rocks are restricted to the high-alkali end of this trend, followed by gneisses at lower values of alkalies. The intermediate suite dominates the iron- and magnesia-rich portion of the trend.Potash values for gneisses and intermediate rocks are relatively high over a wide range of silica percentages. Normative feldspar compositions for these rocks, however, show that they are not unusually potassic. Comparison of their alkali-lime indexes shows a significantly higher alkalinity for the intermediate rocks. The index reflects mainly the variations in lime and potash, owing to the nearly constant soda contents.