Crustal Contamination Identified in Keweenawan Osler Group Tholeiites, Ontario: A Trace Element Perspective

Abstract

In the Lake Superior area, voluminous Middle Proterozoic tholeiitic basalt with minor rhyolite and interflow sediments occupy the Midcontinent Rift and overlie Archean and older Proterozoic basement. The 3000 m-thick section of gently dipping (~10°SSE) basalts on the Black Bay Peninsula is divided into three suites based on the relationship between stratigraphy, petrography, and geochemical variations (e.g., Mg-number, $$A1_{2}O_{3}$$, La/Sm, and Gd/Yb). The Lower suite consists of augite-olivine porphyritic lavas with high Mg-number, $$SiO_{2}, TiO_{2}$$ and Gd/Yb, but low La/Sm. The overlying Central suite flows are either aphyric or plagioclase porphyritic and have lower Mg-number. $$SiO_{2}$$, La/Sm and Gd/Yb decline upward through the Central suite. The Upper suite lavas are either aphyric or plagioclase porphyritic, and element ratios plot on slightly different trends compared to the Central suite. The Lower suite has a narrow compositional range; the average of the four most primitive lavas is used...