Stratigraphy and flow morphology of Archean tholeiites, northeastern Ontario

Abstract

Archean (2.7 Ga old) subaqueous basalt flows of the lower Blake River group, at Lava Flow Mountain, Ontario, are dominantly of a thick, massive flow morphology with hyaloclastic flow-top breccias. Pillow lavas are a subordinate component (< 20%) of the stratigraphy. A 3050 m, vertically-dipping, south-facing sequence consists of alternating and interfingerling units, 300 to 500 m thick, of light green low-Fe lava flows and dark green to black high-Fe flows. High-silica variolites occur within the latter. These three lithologies occur in the approximate proportions 40:50:10. Rare indicators of paleoflow direction suggest an eastern provenance for both low- and high-Fe types. Massive, pillowed and hyaloclastic flow morphologies may be laterally equivalent and probably represent separate facies of compound lava flows. Thick massive flows with relatively thin flow-top breccias form in the main lava channel with high rates of extrusion and/or steeper paleoslopes. Pillow lavas form from less voluminous flows or degassed magma where it is ponded or on gentler paleoslopes. Hyaloclastites and pillow breccias form on the top, sides and at the flow front increasing in thickness away from the vent source. The Fe-rich tholeiites were probably erupted rapidly, in deep water on the Archean ocean floor during an early shield-building stage of volcanism.