The Keweenawan (Late Precambrian) lava flows in northern Minnesota include the volcanic series: tholeiitic olivine basalt, basaltic andesite, quartz latite, and rhyolite, which is assumed to be derived from a tholeiitic parental magma. These lavas contain calcic clinopyroxene which ranges from Ca44Mg45Fe11 to Ca43Mg12Fe45. The crystallization trend of these clinopyroxenes is characterized by marked iron enrichment and is similar to that of the associated Beaver Bay Gabbro Complex and of the Skaergaard Intrusion. The mineral assemblage of olivine + ferroaugite + quartz and the crystallization trend of the ferroaugites are characteristic of equilibrium crystallization of pyroxene during the late stage of differentiation of tholeiitic magma.

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