The volcanic stratigraphy and petrochemistry of the Gods Lake Subgroup, Knee Lake, Manitoba and the diagram MgO/Al2O3 versus (Na2O + K2O)/(Total FeO + TiO2) : a distinct separation of the calc-alkaline and tholeiitic rock series

Abstract

The predominantly volcanic-volcaniclastic Hayes River Group of Archean age outcrops within a northeasterly trending synclinorium on Knee Lake in northern Manitoba. The greenhouse belt was mapped on a scale of 1 inch to 2640 feet; three acid volcanic sections were mapped on a scale of 1 inch to 500 feet. All inland and shoreline outcrops were sampled where feasible. Seventy-six chemical analyses of Hayes River Group volcanic and pyroclastic rocks are presented in graphical and composite form. Stratigraphic and petrochemical studies (in Part I) indicate that the Hayes River Group can be compared with the upper volcanic and sedimentary sequences of the Barberton model for Archean evolution... A new method of petrochemical representation is presented in Part II which discriminates plots of calc-alkaline versus tholeiitic rocks and their differentiation trends, and which: (1) is applicable to a majority of sub-alkaline igneous provinces, (2) minimizes the effect of alteration, and (3) provides a 95 percent effective separation of the two sub-alkaline rock series. In application, the new diagram successfully distinguishes between high-alumina olivine tholeiites and high-alumina calc-alkaline basalts. The diagram was devised to overcome inconsistent results obtained from the conventional methods of plotting chemical data.