GEOCHEMISTRY OF THE YELLOWKNIFE VOLCANIC ROCKS

• Published in: Canadian Journal of Earth Sciences
• Main Author: Baragar, W. R. A.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Canadian Science Publishing 1966
• Subjects: Yellowknife
• Online Access: http://dx.doi.org/10.1139/e66-002; http://www.nrcresearchpress.com/doi/pdf/10.1139/e66-002

Results of rapid-method chemical analyses of samples taken at about 500-ft stratigraphic intervals through two sections of Yellowknife Group volcanic rocks are presented in graphical and composite form. The Yellowknife section is about 40 000 ft thick with the base undefined; the Cameron River section, about 45 mi northeast, is about 7 000 ft thick and may be complete.Two aspects of the volcanic chemistry considered are (1) variation in composition with stratigraphic height; (2) bulk composition of the volcanic rocks.Chemical data of the Yellowknife section define two volcanic cycles in each of which mafic lavas show a small but significant increase in sialic components with stratigraphic height culminating abruptly in acidic layers. The Cameron River section shows a similar but less well-defined trend. Iron–magnesium ratios stage a succession of systematic increases, each persisting for a few thousand stratigraphic feet, but no overall systematic variation. The two types of chemical variation correspond to calc-alkali and tholeiitic differentiation trends respectively. The tholeiitic trend is attributed to fractionation in high-level magma chambers, demonstrated for Yellowknife magma by the Kam Point sill, and the calc-alkali trend to contamination of tholeiitic magma by sialic crust.Frequency distribution diagrams show Yellowknife volcanic rocks to be similar to Chayes' circumoceanic basalts in TiO 2 , CaO, and MgO and to his oceanic basalts in Al 2 O 3 . The characteristic rock type is basalt.