Abstract

The results of detailed mapping and sampling of two sections of Blake River Group volcanic rocks in the Noranda, Quebec region are presented and discussed. The Duparquet section, west of Lac Duparquet, crosses the north limb of a regional synclinorium where the assemblage is more than 40 000 ft thick. Continuity in upper levels of the succession is disrupted by folds, and the relative stratigraphic position of a sequence south of the folds near the synclinorium axis is uncertain but is interpreted as younger. The base of the section is the Porcupine-Destor fault. A subordinate section 40 miles to the east comprises a 12 500-ft thick assemblage overlying Kewagama sediments. Both sections were sampled at 400–500 ft stratigraphic intervals. The samples were analyzed for major elements by X-ray fluorescence and rapid chemical methods and the results checked by classical chemical analyses.Basalts and andesites are predominant rock types. Acidic volcanic rocks, prominent in assemblages a few miles east of the Duparquet section, may be represented by mixed acid–basic fragmental layers that occur in upper levels of the Duparquet section. Pumpellyite and prehnite are indicative of the low metamorphic grade of the rocks.Salient features of the chemistry of the Duparquet section are: (1) Al2O3 increases steadily with stratigraphic height throughout the lower 40 000-ft succession, the upper 15 000 ft of which are high alumina lavas, but is of normal content in the presumed higher levels near the synclinorium axis; (2) iron content and color index decrease markedly and MgO and TiO2 contents moderately with stratigraphic height, but all revert to more normal values in the supposedly higher sequence; (3) the remaining constituents show less distinct trends, but SiO2 and K2O are generally higher in upper levels of the section. In the subordinate section compositions are fairly constant and match those in the lower part of the Duparquet section. Noranda rocks resemble circumoceanic basalts except for markedly lower Al2O3 and K2O.Speculation on causes of the stratigraphic variations in composition are based on recent high-pressure studies. Increasing alumina content with stratigraphic height is attributed to a secular rise in the depth of magma generation from below about 35 km to above. Late production of acidic magmas may be due to eventual melting of parts of the crust.

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