Diabase-Granophyre Relations in the Endion Sill, Duluth, Minnesota

The gently dipping 1,500-ft thick Endion sill intrudes Keweenawanflows at Duluth, Minnesota. In a general way, from the bottomto the top of the body, there is a gradual transition from diabasethrough intermediate rock (granodiorite) to granophyre (adamellite).These latter two rock types together constitute about 40 percent of the exposed mass. Fractional crystallization of basalticmagma produced a great thickness of basic and intermediate rocktypes which accumulated predominantly in the lower portionsof the sill, and probably in the end-stage production of anaqueous, salic, alkalic liquid. It is proposed that, due toinitial inclination of the sill, part of this salic fractionmigrated up dip, accumulated and reacted with portions of thediabase and intermediate rock, and completed crystallizationat the presently exposed level. Alternatively, much or all ofthe Endion sill granophyre may represent a separate intrusionunrelated to differentiation of the earlier diabase. In eithercase the sill is composite. Bulk compositions of cryptoperthitesindicate both granophyre and intermediate rock crystallizedat magmatic temperatures. Compositional uniformity of the clinopyroxene and lack of ironenrichment in felsic portions of the Endion sill may be theresult of accumulation of H₂O and the presumed maintenance ofnearly constant partial oxygen pressure during crystallization.This mechanism would furthermore account for inferred late magmaticsolid solution between alkali feldspar and KFe³?Si₃O₈, withsubsequent subsolidus exsolution of haematite.